CHAPTER XX. FERTILIZATION OF FLOWERS.
If we compare the flowers of different plants, we shall find almost infinite variety in structure, and this variation at first appears to follow no fixed laws; but as we study the matter more thoroughly, we find that these variations have a deep significance, and almost without exception have to do with the fertilization of the flower.
In the simpler flowers, such as those of a grass, sedge, or rush among the monocotyledons, or an oak, hazel, or plantain, among dicotyledons, the flowers are extremely inconspicuous and often reduced to the simplest form. In such plants, the pollen is conveyed from the male flowers to the female by the wind, and to this end the former are usually placed above the latter so that these are dusted with the pollen whenever the plant is shaken by the wind. In these plants, the male flowers often outnumber the female enormously, and the pollen is produced in great quantities, and the stigmas are long and often feathery, so as to catch the pollen readily. This is very beautifully shown in many grasses.
If, however, we examine the higher groups of flowering plants, we see that the outer leaves of the flower become more conspicuous, and that this is often correlated with the development of a sweet fluid (nectar) in certain parts of the flower, while the wind-fertilized flowers are destitute of this as well as of odor. Read the rest of this entry »
CHAPTER XVII. DICOTYLEDONS.
Division I.—Choripetalæ.
Nearly all of the dicotyledons may be placed in one of two great divisions distinguished by the character of the petals. In the first group, called Choripetalæ, the petals are separate, or in some degenerate forms entirely absent. As familiar examples of this group, we may select the buttercup, rose, pink, and many others.
Fig. 96.—Iulifloræ. A, male; B, female inflorescence of a willow, Salix (Amentaceæ), × ½. C, a single male flower, × 2. D, a female flower, × 2. E, cross-section of the ovary, × 8. F, an opening fruit. G, single seed with its hairy appendage, × 2.
The second group (Sympetalæ or Gamopetalæ) comprises those dicotyledons whose flowers have the petals more or less completely united into a tube. The honeysuckles, mints, huckleberry, lilac, etc., are familiar representatives of the Sympetalæ, which includes the highest of all plants.
The Choripetalæ may be divided into six groups, including twenty-two orders. The first group is called Iulifloræ, and contains numerous, familiar plants, mostly trees. In these plants, the flowers are small and inconspicuous, and usually crowded into dense catkins, as in willows (Fig. 96) and poplars, or in spikes or heads, as in the lizard-tail (Fig. 97, G), or hop (Fig. 97, I). The individual flowers are very small and simple in structure, being often reduced to the gynœcium or andræcium, carpels and stamens being almost always in separate flowers. The outer leaves of the flower (sepals and petals) are either entirely wanting or much reduced, and never differentiated into calyx and corolla.
CHAPTER XVI. CLASSIFICATION OF THE MONOCOTYLEDONS.
In the following chapter no attempt will be made to give an exhaustive account of the characteristics of each division of the monocotyledons, but only such of the most important ones as may serve to supplement our study of the special one already examined. The classification here, and this is the case throughout the spermaphytes, is based mainly upon the characters of the flowers and fruits.
The classification adopted here is that of the German botanist Eichler, and seems to the author to accord better with our present knowledge of the relationships of the groups than do the systems that are more general in this country. According to Eichler’s classification, the monocotyledons may be divided into seven groups; viz., I. Liliifloræ; II. Enantioblastæ; III. Spadicifloræ; IV. Glumaceæ; V. Scitamineæ; VI. Gynandræ; VII. Helobiæ.
Order I.—Liliifloræ.
The plants of this group agree in their general structure with the adder’s-tongue, which is a thoroughly typical representative of the group; but nevertheless, there is much variation among them in the details of structure. While most of them are herbaceous forms (dying down to the ground each year), a few, among which may be mentioned the yuccas (“bear grass,” “Spanish bayonet”) of our southern states, develop a creeping or upright woody stem, increasing in size from year to year. The herbaceous forms send up their stems yearly from underground bulbs, tubers, e.g. Trillium (Fig. 83, A), or thickened, creeping stems, or root stocks (rhizomes). Good examples of the last are the Solomon’s-seal (Fig. 83, B), Medeola (C, D), and iris (Fig. 84 A). One family, the yams (Dioscoreæ), of which we have one common native species, the wild yam (Dioscorea villosa), have broad, netted-veined leaves and are twining plants, while another somewhat similar family (Smilaceæ) climb by means of tendrils at the bases of the leaves. Of the latter the “cat-brier” or “green-brier” is a familiar representative. Read the rest of this entry »
CHAPTER XIV. SUB-KINGDOM VI. Spermaphytes: Phænogams.
The last and highest great division of the vegetable kingdom has been named Spermaphyta, “seed plants,” from the fact that the structures known as seeds are peculiar to them. They are also commonly called flowering plants, though this name might be also appropriately given to certain of the higher pteridophytes.
In the seed plants the macrosporangia remain attached to the parent plant, in nearly all cases, until the archegonia are fertilized and the embryo plant formed. The outer walls of the sporangium now become hard, and the whole falls off as a seed.
In the higher spermaphytes the spore-bearing leaves (sporophylls) become much modified, and receive special names, those bearing the microspores being commonly known as stamens; those bearing the macrospores, carpels or carpophylls. The macrosporangia are also ordinarily known as “ovules,” a name given before it was known that these were the same as the macrosporangia of the higher pteridophytes.
In addition to the spore-bearing leaves, those surrounding them may be much changed in form and brilliantly colored, forming, with the enclosed sporophylls, the “flower” of the higher spermaphytes.
As might be expected, the tissues of the higher spermaphytes are the most highly developed of all plants, though some of them are very simple. The plants vary extremely in size, the smallest being little floating plants, less than a millimetre in diameter, while others are gigantic trees, a hundred metres and more in height.
There are two classes of the spermaphytes: I., the Gymnosperms, or naked-seeded ones, in which the ovules (macrosporangia) are borne upon open carpophylls; and II., Angiosperms, covered-seeded plants, in which the carpophylls form a closed cavity (ovary) containing the ovules.
Class I.—Gymnosperms (Gymnospermæ).
The most familiar of these plants are the common evergreen trees (conifers), pines, spruces, cedars, etc. A careful study of one of these will give a good idea of the most important characteristics of the class, and one of the best for this purpose is the Scotch pine (Pinus sylvestris), which, though a native of Europe, is not infrequently met with in cultivation in America. If this species cannot be had by the student, other pines, or indeed almost any other conifer, will answer. The Scotch pine is a tree of moderate size, symmetrical in growth when young, with a central main shaft, and circles of branches at regular intervals; but as it grows older its growth becomes irregular, and the crown is divided into several main branches.[10] The trunk and branches are covered with a rough, scaly bark of a reddish brown color, where it is exposed by the scaling off of the outer layers. Covering the younger branches, but becoming thinner on the older ones, are numerous needle-shaped leaves. These are in pairs, and the base of each pair is surrounded by several dry, blackish scales. Each pair of leaves is really attached to a very short side branch, but this is so short as to make the leaves appear to grow directly from the main branch. Each leaf is about ten centimetres in length and two millimetres broad. Where the leaves are in contact they are flattened, but the outer side is rounded, so that a cross-section is nearly semicircular in outline. With a lens it is seen that there are five longitudinal lines upon the surface of the leaf, and careful examination shows rows of small dots corresponding to these. These dots are the breathing pores. If a cross-section is even slightly magnified it shows three distinct parts,—a whitish outer border, a bright green zone, and a central oval, colorless area, in which, with a little care, may be seen the sections of two fibro-vascular bundles. In the green zone are sometimes to be seen colorless spots, sections of resin ducts, containing the resin so characteristic of the tissues of the conifers.
The general structure of the stem may be understood by making a series of cross-sections through branches of different ages. In all, three regions are distinguishable; viz., an outer region (bark or cortex) (Fig. 76, A, c), composed in part of green cells, and, if the section has been made with a sharp knife, showing a circle of little openings, from each of which oozes a clear drop of resin. These are large resin ducts (r). The centre is occupied by a soft white tissue (pith), and the space between the pith and bark is filled by a mass of woody tissue. Traversing the wood are numerous radiating lines, some of which run from the bark to the pith, others only part way. These are called the medullary rays. While in sections from branches of any age these three regions are recognizable, their relative size varies extremely. In a section of a twig of the present year the bark and pith make up a considerable part of the section; but as older branches are examined, we find a rapid increase in the quantity of wood, while the thickness of the bark increases but slowly, and the pith scarcely at all. In the wood, too, each year’s growth is marked by a distinct ring (A i, ii). As the branches grow in diameter the outer bark becomes split and irregular, and portions die, becoming brown and hard.
The tree has a very perfect root system, but different from that of any pteridophytes. The first root of the embryo persists as the main or “tap” root of the full-grown tree, and from it branch off the secondary roots, which in turn give rise to others.
The sporangia are borne on special scale-like leaves, and arranged very much as in certain pteridophytes, notably the club mosses; but instead of large and small spores being produced near together, the two kinds are borne on special branches, or even on distinct trees (e.g. red cedar). In the Scotch pine the microspores are ripe about the end of May. The leaves bearing them are aggregated in small cones (“flowers”), crowded about the base of a growing shoot terminating the branches (Fig. 77, A ♂). The individual leaves (sporophylls) are nearly triangular in shape, and attached by the smaller end. On the lower side of each are borne two sporangia (pollen sacs) (C, sp.), opening by a longitudinal slit, and filled with innumerable yellow microspores (pollen spores), which fall out as a shower of yellow dust if the branch is shaken.
The macrosporangia (ovules) are borne on similar leaves, known as carpels, and, like the pollen sacs, borne in pairs, but on the upper side of the sporophyll instead of the lower. The female flowers appear when the pollen is ripe. The leaves of which they are composed are thicker than those of the male flowers, and of a pinkish color. At the base on the upper side are borne the two ovules (macrosporangia) (Fig. 77, E, o), and running through the centre is a ridge that ends in a little spine or point.
The ovule-bearing leaf has on the back a scale with fringed edge (F, sc.), quite conspicuous when the flower is young, but scarcely to be detected in the older cone. From the female flower is developed the cone (Fig. 75, A), but the process is a slow one, occupying two years. Shortly after the pollen is shed, the female flowers, which are at first upright, bend downward, and assume a brownish color, growing considerably in size for a short time, and then ceasing to grow for several months.
Fig. 75.—Scotch pine (Pinus sylvestris). A, a ripe cone, × ½. B, a year-old cone, × 1. C, longitudinal section of B. D, a single scale of B, showing the sporangia (ovules) (o), × 2. E, a scale from a ripe cone, with the seeds (s), × ½. F, longitudinal section of a ripe seed, × 3. em. the embryo. G, a germinating seed, × 2. r, the primary root. H, longitudinal section through G, showing the first leaves of the young plant still surrounded by the endosperm, × 4. I, an older plant with the leaves (l) withdrawing from the seed coats, × 4. J, upper part of a young plant, showing the circle of primary leaves (cotyledons), × 1. K, section of the same, × 2. b, the terminal bud. L, cross-section of the stem of the young plant, × 25. fb. a fibro-vascular bundle. M, cross-section of the root, × 25. x, wood. ph. bast, of the fibro-vascular bundle.
In Figure 75, B, is shown such a flower as it appears in the winter and early spring following. The leaves are thick and fleshy, closely pressed together, as is seen by dividing the flower lengthwise, and each leaf ends in a long point (D). The ovules are still very small. As the growth of the tree is resumed in the spring, the flower (cone) increases rapidly in size and becomes decidedly green in color, the ovules increasing also very much in size. If a scale from such a cone is examined about the first of June, the ovules will probably be nearly full-grown, oval, whitish bodies two to three millimetres in length. A careful longitudinal section of the scale through the ovule will show the general structure. Such a section is shown in Figure 77, G. Comparing this with the sporangia of the pteridophytes, the first difference that strikes us is the presence of an outer coat or integument (in.), which is absent in the latter. The single macrospore (sp.) is very large and does not lie free in the cavity of the sporangium, but is in close contact with its wall. It is filled with a colorless tissue, the prothallium, and if mature, with care it is possible to see, even with a hand lens, two or more denser oval bodies (ar.), the egg cells of the archegonia, which here are very large. The integument is not entirely closed at the top, but leaves a little opening through which the pollen spores entered when the flower was first formed.
After the archegonia are fertilized the outer parts of the ovule become hard and brown, and serve to protect the embryo plant, which reaches a considerable size before the sporangium falls off. As the walls of the ovule harden, the carpel or leaf bearing it undergoes a similar change, becoming extremely hard and woody, and as each one ends in a sharp spine, and they are tightly packed together, it is almost impossible to separate them. The ripe cone (Fig. 75, A) remains closed during the winter, but in the spring, about the time the flowers are mature, the scales open spontaneously and discharge the ripened ovules, now called seeds. Each seed (E, s) is surrounded by a membranous envelope derived from the scale to which it is attached, which becomes easily separated from the seed. The opening of the cones is caused by drying, and if a number of ripe cones are gathered in the winter or early spring, and allowed to dry in an ordinary room, they will in a day or two open, often with a sharp, crackling sound, and scatter the ripe seeds.
A section of a ripe seed (F) shows the embryo (em.) surrounded by a dense, white, starch-bearing tissue derived from the prothallium cells, and called the “endosperm.” This fills up the whole seed which is surrounded by the hardened shell derived from the integument and wall of the ovule. The embryo is elongated with a circle of small leaves at the end away from the opening of the ovule toward which is directed the root of the embryo.
The seed may remain unchanged for months, or even years, without losing its vitality, but if the proper conditions are provided, the embryo will develop into a new plant. To follow the further growth of the embryo, the ripe seeds should be planted in good soil and kept moderately warm and moist. At the end of a week or two some of the seeds will probably have sprouted. The seed absorbs water, and the protoplasm of the embryo renews its activity, beginning to feed upon the nourishing substances in the cells of the endosperm. The embryo rapidly increases in length, and the root pushes out of the seed growing rapidly downward and fastening itself in the soil (G, r). Cutting the seed lengthwise we find that the leaves have increased much in length and become green (one of the few cases where chlorophyll is formed in the absence of light). As these leaves (called “cotyledons” or seed leaves) increase in length, they gradually withdraw from the seed whose contents they have exhausted, and the young plant enters upon an independent existence.
The young plant has a circle of leaves, about six in number, surrounding a bud which is the growing point of the stem, and in many conifers persists as long as the stem grows (Fig. 75, K, b). A cross-section of the young stem shows about six separate fibro-vascular bundles arranged in a circle (S, fb.). The root shows a central fibro-vascular cylinder surrounded by a dark-colored ground tissue. Growing from its surface are numerous root hairs (Fig. 75, M).
For examining the microscopic structure of the pine, fresh material is for most purposes to be preferred, but alcoholic material will answer, and as the alcohol hardens the resin, it is for that reason preferable.
Cross-sections of the leaf, when sufficiently magnified, show that the outer colorless border of the section is composed of two parts: the epidermis of a single row of regular cells with very thick outer walls, and irregular groups of cells lying below them. These latter have thick walls appearing silvery and clearer than the epidermal cells. They vary a good deal, in some leaves being reduced to a single row, in others forming very conspicuous groups of some size. The green tissue of the leaf is much more compact than in the fern we examined, and the cells are more nearly round and the intercellular spaces smaller. The chloroplasts are numerous and nearly round in shape.
Scattered through the green tissue are several resin passages (r), each surrounded by a circle of colorless, thick-walled cells, like those under the epidermis. At intervals in the latter are openings—breathing pores—(Fig. 76, J), below each of which is an intercellular space (i). They are in structure like those of the ferns, but the walls of the guard cells are much thickened like the other epidermal cells.
Each leaf is traversed by two fibro-vascular bundles of entirely different structure from those of the ferns. Each is divided into two nearly equal parts, the wood (x) lying toward the inner, flat side of the leaf, the bast (T) toward the outer, convex side. This type of bundle, called “collateral,” is the common form found in the stems and leaves of seed plants. The cells of the wood or xylem are rather larger than those of the bast or phloem, and have thicker walls than any of the phloem cells, except the outermost ones which are thick-walled fibres like those under the epidermis. Lying between the bundles are comparatively large colorless cells, and surrounding the whole central area is a single line of cells that separates it sharply from the surrounding green tissue.
In longitudinal sections, the cells, except of the mesophyll (green tissue) are much elongated. The mesophyll cells, however, are short and the intercellular spaces much more evident than in the cross-section. The colorless cells have frequently rounded depressions or pits upon their walls, and in the fibro-vascular bundle the difference between the two portions becomes more obvious. The wood is distinguished by the presence of vessels with close, spiral or ring-shaped thickenings, while in the phloem are found sieve tubes, not unlike those in the ferns.
The fibro-vascular bundles of the stem of the seedling plant show a structure quite similar to that of the leaf, but very soon a difference is manifested. Between the two parts of the bundle the cells continue to divide and add constantly to the size of the bundle, and at the same time the bundles become connected by a line of similar growing cells, so that very early we find a ring of growing cells extending completely around the stem. As the cells in this ring increase in number, owing to their rapid division, those on the borders of the ring lose the power of dividing, and gradually assume the character of the cells on which they border (Fig. 76, B, cam.). The growth on the inside of the ring is more rapid than on the outer border, and the ring continues comparatively near the surface of the stem (Fig. 76, A, cam.). The spaces between the bundles do not increase materially in breadth, and as the bundles increase in size become in comparison very small, appearing in older stems as mere lines between the solid masses of wood that make up the inner portion of the bundles. These are the primary medullary rays, and connect the pith in the centre of the stem with the bark. Later, similar plates of cells are formed, often only a single cell thick, and appearing when seen in cross-section as a single row of elongated cells (C, m).
As the stem increases in diameter the bundles become broader and broader toward the outside, and taper to a point toward the centre, appearing wedge-shaped, the inner ends projecting into the pith. The outer limits of the bundles are not nearly so distinct, and it is not easy to tell when the phloem of the bundles ends and the ground tissue of the bark begins.
A careful examination of a cross-section of the bark shows first, if taken from a branch not more than two or three years old, the epidermis composed of cells not unlike those of the leaf, but whose walls are usually browner. Underneath are cells with brownish walls, and often more or less dry and dead. These cells give the brown color to the bark, and later both epidermis and outer ground tissue become entirely dead and disappear. The bulk of the ground tissue is made up of rather large, loose cells, the outer ones containing a good deal of chlorophyll. Here and there are large resin ducts (Fig. 76, H), appearing in cross-section as oval openings surrounded by several concentric rows of cells, the innermost smaller and with denser contents. These secrete the resin that fills the duct and oozes out when the stem is cut. All of the cells of the bark contain more or less starch.
The phloem, when strongly magnified, is seen to be made up of cells arranged in nearly regular radiating rows. Their walls are not very thick and the cells are usually somewhat flattened in a radial direction.
Some of the cells are larger than the others, and these are found to be, when examined in longitudinal section, sieve tubes (Fig. 76, E) with numerous lateral sieve plates quite similar to those found in the stems of ferns.
Fig. 76.—Scotch pine. A, cross-section of a two-year-old branch, × 3. p, pith. c, bark. The radiating lines are medullary rays. r, resin ducts. B, part of the same, × 150. cam. cambium cells. x, tracheids. C, cross-section of a two-year-old branch at the point where the two growth rings join: I, the cells of the first year’s growth; II, those of the second year. m, a medullary ray, × 150. D, longitudinal section of a branch, showing the form of the tracheids and the bordered pits upon their walls. m, medullary ray, × 150. E, part of a sieve tube, × 300. F, cross-section of a tracheid passing through two of the pits in the wall (p), × 300. G, longitudinal section of a branch, at right angles to the medullary rays (m). At y, the section has passed through the wall of a tracheid, bearing a row of pits, × 150. H, cross-section of a resin duct, × 150. I, cross-section of a leaf, × 20. fb. fibro-vascular bundle. r, resin duct. J, section of a breathing pore, × 150. i, the air space below it.
The growing tissue (cambium), separating the phloem from the wood, is made up of cells quite like those of the phloem, into which they insensibly merge, except that their walls are much thinner, as is always the case with rapidly growing cells. These cells (B, cam.) are arranged in radial rows and divide, mainly by walls, at right angles to the radii of the stem. If we examine the inner side of the ring, the change the cells undergo is more marked. They become of nearly equal diameter in all directions, and the walls become woody, showing at the same time distinct stratification (B, x).
On examining the xylem, where two growth rings are in contact, the reason of the sharply marked line seen when the stem is examined with the naked eye is obvious. On the inner side of this line (I), the wood cells are comparatively small and much flattened, while the walls are quite as heavy as those of the much larger cells (II) lying on the outer side of the line. The small cells show the point where growth ceased at the end of the season, the cells becoming smaller as growth was feebler. The following year when growth commenced again, the first wood cells formed by the cambium were much larger, as growth is most vigorous at this time, and the wood formed of these larger cells is softer and lighter colored than that formed of the smaller cells of the autumn growth.
The wood is mainly composed of tracheids, there being no vessels formed except the first year. These tracheids are characterized by the presence of peculiar pits upon their walls, best seen when thin longitudinal sections are made in a radial direction. These pits (Fig. 76, D, p) appear in this view as double circles, but if cut across, as often happens in a cross-section of the stem, or in a longitudinal section at right angles to the radius (tangential), they are seen to be in shape something like an inverted saucer with a hole through the bottom. They are formed in pairs, one on each side of the wall of adjacent tracheids, and are separated by a very delicate membrane (F, p, G, y). These “bordered” pits are very characteristic of the wood of all conifers.
The structure of the root is best studied in the seedling plant, or in a rootlet of an older one. The general plan of the root is much like that of the pteridophytes. The fibro-vascular bundle (Fig. 75, M, fb.) is of the so-called radial type, there being three xylem masses (x) alternating with as many phloem masses (ph.) in the root of the seedling. This regularity becomes destroyed as the root grows older by the formation of a cambium ring, something like that in the stem.
The development of the sporangia is on the whole much like that of the club mosses, and will not be examined here in detail. The microspores (pollen spores) are formed in groups of four in precisely the same way as the spores of the bryophytes and pteridophytes, and by collecting the male flowers as they begin to appear in the spring, and crushing the sporangia in water, the process of division may be seen. For more careful examination they may be crushed in a mixture of water and acetic acid, to which is added a little gentian violet. This mixture fixes and stains the nuclei of the spores, and very instructive preparations may thus be made.[11]
Fig. 77.—Scotch pine (except E and F). A, end of a branch bearing a cluster of male flowers (♂), × ½. B, a similar branch, with two young female flowers (♀), natural size. C, a scale from a male flower, showing the two sporangia (sp.); × 5. D, a single ripe pollen spore (microspore), showing the vegetative cell (x), × 150. E, a similar scale, from a female flower of the Austrian pine, seen from within, × 4. o, the sporangium (ovule). F, the same, seen from the back, showing the scale (sc.) attached to the back. G, longitudinal section through a full-grown ovule of the Scotch pine. p, a pollen spore sending down its tube to the archegonia (ar.). sp. the prothallium (endosperm), filling up the embryo sac, × 10. H, the neck of the archegonium, × 150.
The ripe pollen spores (Fig. 77, D) are oval cells provided with a double wall, the outer one giving rise to two peculiar bladder-like appendages (z). Like the microspores of the smaller club mosses, a small cell is cut off from the body of the spore (x). These pollen spores are carried by the wind to the ovules, where they germinate.
The wall of the ripe sporangium or pollen sac is composed of a single layer of cells in most places, and these cells are provided with thickened ridges which have to do with opening the pollen sac.
We have already examined in some detail the structure of the macrosporangium or ovule. In the full-grown ovule the macrospore, which in the seed plants is generally known as the “embryo sac,” is completely filled with the prothallium or “endosperm.” In the upper part of the prothallium several large archegonia are formed in much the same way as in the pteridophytes. The egg cell is very large, and appears of a yellowish color, and filled with large drops that give it a peculiar aspect. There is a large nucleus, but it is not always readily distinguished from the other contents of the egg cell. The neck of the archegonium is quite long, but does not project above the surface of the prothallium (Fig. 77, H).
The pollen spores are produced in great numbers, and many of them fall upon the female flowers, which when ready for pollination have the scales somewhat separated. The pollen spores now sift down to the base of the scales, and finally reach the opening of the ovule, where they germinate. No spermatozoids are produced, the seed plants differing in this respect from all pteridophytes. The pollen spore bursts its outer coat, and sends out a tube which penetrates for some distance into the tissue of the ovule, acting very much as a parasitic fungus would do, and growing at the expense of the tissue through which it grows. After a time growth ceases, and is not resumed until the development of the female prothallium and archegonia is nearly complete, which does not occur until more than a year from the time the pollen spore first reaches the ovule. Finally the pollen tube penetrates down to and through the open neck of the archegonium, until it comes in contact with the egg cell. These stages can only be seen by careful sections through a number of ripe ovules, but the track of the pollen tube is usually easy to follow, as the cells along it are often brown and apparently dead (Fig. 77, G).
Classification of the Gymnosperms.
There are three classes of the gymnosperms: I., cycads (Cycadeæ); II., conifers (Coniferæ); III., joint firs (Gnetaceæ). All of the gymnosperms of the northern United States belong to the second order, but representatives of the others are found in the southern and southwestern states.
The cycads are palm-like forms having a single trunk crowned by a circle of compound leaves. Several species are grown for ornament in conservatories, and a few species occur native in Florida, but otherwise do not occur within our limits.
Fig. 78.—Illustrations of gymnosperms. A, fruiting leaf of a cycad (Cycas), with macrosporangia (ovules) (ov.), × ¼. B, leaf of Gingko, × ½. C, branch of hemlock (Tsuga), with a ripe cone, × 1. D, red cedar (Juniperus), × 1. E, Arbor-vitæ (Thuja), × 1.
The spore-bearing leaves usually form cones, recalling somewhat in structure those of the horse-tails, but one of the commonest cultivated species (Cycas revoluta) bears the ovules, which are very large, upon leaves that are in shape much like the ordinary ones (Fig. 78, A).
Of the conifers, there are numerous familiar forms, including all our common evergreen trees. There are two sub-orders,—the true conifers and the yews. In the latter there is no true cone, but the ovules are borne singly at the end of a branch, and the seed in the yew (Taxus) is surrounded by a bright red, fleshy integument. One species of yew, a low, straggling shrub, occurs sparingly in the northern states, and is the only representative of the group at the north. The European yew and the curious Japanese Gingko (Fig. 78, B) are sometimes met with in cultivation.
Of the true conifers, there are a number of families, based on peculiarities in the leaves and cones. Some have needle-shaped leaves and dry cones like the firs, spruces, hemlock (Fig. 78, C). Others have flattened, scale-like leaves, and more or less fleshy cones, like the red cedar (Fig. 78, D) and Arbor-vitæ (E).
A few of the conifers, such as the tamarack or larch (Larix) and cypress (Taxodium), lose their leaves in the autumn, and are not, therefore, properly “evergreen.”
The conifers include some of the most valuable as well as the largest of trees. Their timber, especially that of some of the pines, is particularly valuable, and the resin of some of them is also of much commercial importance. Here belong the giant red-woods (Sequoia) of California, the largest of all American trees.
The joint firs are comparatively small plants, rarely if ever reaching the dimensions of trees. They are found in various parts of the world, but are few in number, and not at all likely to be met with by the ordinary student. Their flowers are rather more highly differentiated than those of the other gymnosperms, and are said to show some approach in structure to those of the angiosperms.
CHAPTER XV.
SPERMAPHYTES.
Class II.—Angiosperms.
The angiosperms include an enormous assemblage of plants, all those ordinarily called “flowering plants” belonging here. There is almost infinite variety shown in the form and structure of the tissues and organs, this being particularly the case with the flowers. As already stated, the ovules, instead of being borne on open carpels, are enclosed in a cavity formed by a single closed carpel or several united carpels. To the organ so formed the name “pistil” is usually applied, and this is known as “simple” or “compound,” as it is composed of one or of two or more carpels. The leaves bearing the pollen spores are also much modified, and form the so-called “stamens.” In addition to the spore-bearing leaves there are usually other modified leaves surrounding them, these being often brilliantly colored and rendering the flower very conspicuous. To these leaves surrounding the sporophylls, the general name of “perianth” or “perigone” is given. The perigone has a twofold purpose, serving both to protect the sporophylls, and, at least in bright-colored flowers, to attract insects which, as we shall see, are important agents in transferring pollen from one flower to another.
When we compare the embryo sac (macrospore) of the angiosperms with that of the gymnosperms a great difference is noticed, there being much more difference than between the latter and the higher pteridophytes. Unfortunately there are very few plants where the structure of the embryo sac can be readily seen without very skilful manipulation.
Fig. 79.—A, ripe ovule of Monotropa uniflora, in optical section, × 100. m, micropyle. e, embryo sac. B, the embryo sac, × 300. At the top is the egg apparatus, consisting of the two synergidæ (s), and the egg cell (o). In the centre is the “endosperm nucleus” (k). At the bottom, the “antipodal cells” (g).
There are, however, a few plants in which the ovules are very small and transparent, so that they may be mounted whole and examined alive. The best plant for this purpose is probably the “Indian pipe” or “ghost flower,” a curious plant growing in rich woods, blossoming in late summer. It is a parasite or saprophyte, and entirely destitute of chlorophyll, being pure white throughout. It bears a single nodding flower at the summit of the stem. (Another species much like it, but having several brownish flowers, is shown in Figure 115, L.)
If this plant can be had, the structure of the ovule and embryo sac may be easily studied, by simply stripping away the tissue bearing the numerous minute ovules, and mounting a few of them in water, or water to which a little sugar has been added.
The ovules are attached to a stalk, and each consists of about two layers of colorless cells enclosing a central, large, oblong cell (Fig. 79, A, E), the embryo sac or macrospore. If the ovule is from a flower that has been open for some time, we shall find in the centre of the embryo sac a large nucleus (k) (or possibly two which afterward unite into one), and at each end three cells. Those at the base (g) probably represent the prothallium, and those at the upper end a very rudimentary archegonium, here generally called the “egg apparatus.”
Of the three cells of the “egg apparatus” the lower (o) one is the egg cell; the others are called “synergidæ.” The structure of the embryo sac and ovules is quite constant among the angiosperms, the differences being mainly in the shape of the ovules, and the degree to which its coverings or integuments are developed.
The pollen spores of many angiosperms will germinate very easily in a solution of common sugar in water: about fifteen per cent of sugar is the best. A very good plant for this purpose is the sweet pea, whose pollen germinates very rapidly, especially in warm weather. The spores may be sown in a little of the sugar solution in any convenient vessel, or in a hanging drop suspended in a moist chamber, as described for germinating the spores of the slime moulds. The tube begins to develop within a few minutes after the spores are placed in the solution, and within an hour or so will have reached a considerable length. Each spore has two nuclei, but they are less evident here than in some other forms (Fig. 79).
Fig. 80.—Germinating pollen spores of the sweet pea, × 200.
The upper part of the pistil is variously modified, having either little papillæ which hold the pollen spores, or are viscid. In either case the spores germinate when placed upon this receptive part (stigma) of the pistil, and send their tubes down through the tissues of the pistil until they reach the ovules, which are fertilized much as in the gymnosperms.
The effect of fertilization extends beyond the ovule, the ovary and often other parts of the flower being affected, enlarging and often becoming bright-colored and juicy, forming the various fruits of the angiosperms. These fruits when ripe may be either dry, as in the case of grains of various kinds, beans, peas, etc.; or the ripe fruit may be juicy, serving in this way to attract animals of many kinds which feed on the juicy pulp, and leave the hard seeds uninjured, thus helping to distribute them. Common examples of these fleshy fruits are offered by the berries of many plants; apples, melons, cherries, etc., are also familiar examples.
The seeds differ a good deal both in regard to size and the degree to which the embryo is developed at the time the seed ripens.
Classification of the Angiosperms.
The angiosperms are divided into two sub-classes: I. Monocotyledons and II. Dicotyledons.
The monocotyledons comprise many familiar plants, both ornamental and useful. They have for the most part elongated, smooth-edged leaves with parallel veins, and the parts of the flower are in threes in the majority of them. As their name indicates, there is but one cotyledon or seed leaf, and the leaves from the first are alternate. As a rule the embryo is very small and surrounded by abundant endosperm.
The most thoroughly typical members of the sub-class are the lilies and their relatives. The one selected for special study here, the yellow adder-tongue, is very common in the spring; but if not accessible, almost any liliaceous plant will answer. Of garden flowers, the tulip, hyacinth, narcissus, or one of the common lilies may be used; of wild flowers, the various species of Trillium (Fig. 83, A) are common and easily studied forms, but the leaves are not of the type common to most monocotyledons.
The yellow adder-tongue (Erythronium americanum) (Fig. 81) is one of the commonest and widespread of wild flowers, blossoming in the northern states from about the middle of April till the middle of May. Most of the plants found will not be in flower, and these send up but a single, oblong, pointed leaf. The flowering plant has two similar leaves, one of which is usually larger than the other. They seem to come directly from the ground, but closer examination shows that they are attached to a stem of considerable length entirely buried in the ground. This arises from a small bulb (B) to whose base numerous roots (r) are attached. Rising from between the leaves is a slender, leafless stalk bearing a single, nodding flower at the top.
The leaves are perfectly smooth, dull purplish red on the lower side, and pale green with purplish blotches above. The epidermis may be very easily removed, and is perfectly colorless. Examined closely, longitudinal rows of whitish spots may be detected: these are the breathing pores.
Fig. 81.—A, plant of the yellow adder-tongue (Erythronium americanum), × ⅓. B, the bulb of the same, × ½. r, roots. C, section of B. st. the base of the stem bearing the bulb for next year (b) at its base. D, a single petal and stamen, × ½. f, the filament. an. anther. E, the gynœcium (pistil), × 1. o, ovary. st. style. z, stigma. F, a full-grown fruit, × ½. G, section of a full-grown macrosporangium (ovule), × 25: i, ii, the two integuments. sp. macrospore (embryo sac). H, cross-section of the ripe anther, × 12. I, a single pollen spore, × 150, showing the two nuclei (n, nʹ). J, a ripe seed, × 2. K, the same, in longitudinal section. em. the embryo. L, cross-section of the stem, × 12. fb. fibro-vascular bundle. M, diagram of the flower.
A cross-section of the stem shows numerous whitish areas scattered through it. These are the fibro-vascular bundles which in the monocotyledons are of a simple type. The bulb is composed of thick scales, which are modified leaves, and on cutting it lengthwise, we shall probably find the young bulb of next year (Fig. C, b) already forming inside it, the young bulb arising as a bud at the base of the stem of the present year.
The flower is made up of five circles of very much modified leaves, three leaves in each set. The two outer circles are much alike, but the three outermost leaves are slightly narrower and strongly tinged with red on the back, completely concealing the three inner ones before the flower expands. The latter are pure yellow, except for a ridge along the back, and a few red specks near the base inside. These six leaves constitute the perigone of the flower; the three outer are called sepals, the inner ones petals.
The next two circles are composed of the sporophylls bearing the pollen spores.[12] These are the stamens, and taken collectively are known as the “Andrœcium.” Each leaf or stamen consists of two distinct portions, a delicate stalk or “filament” (D, f), and the upper spore-bearing part, the “anther” (an.). The anther in the freshly opened flower has a smooth, red surface; but shortly after, the flower opens, splits along each side, and discharges the pollen spores. A section across the anther shows it to be composed of four sporangia or pollen sacs attached to a common central axis (“connective”) (Fig. H).
The central circle of leaves, the carpels (collectively the “gynœcium”) are completely united to form a compound pistil (Fig. 81, E). This shows three distinct portions, the ovule-bearing portion below (o), the “ovary,” a stalk above (st.), the “style,” and the receptive portion (z) at the top, the “stigma.” Both stigma and ovary show plainly their compound nature, the former being divided into three lobes, the latter completely divided into three chambers, as well as being flattened at the sides with a more or less decided seam at the three angles. The ovules, which are quite large, are arranged in two rows in each chamber of the ovary, attached to the central column (“placenta”).
The flowers open for several days in succession, but only when the sun is shining. They are visited by numerous insects which carry the pollen from one flower to another and deposit it upon the stigma, where it germinates, and the tube, growing down through the long style, finally reaches the ovules and fertilizes them. Usually only a comparatively small number of the seeds mature, there being almost always a number of imperfect ones in each pod. The pod or fruit (F) is full-grown about a month after the flower opens, and finally separates into three parts, and discharges the seeds. These are quite large (Fig. 81, J) and covered with a yellowish brown outer coat, and provided with a peculiar, whitish, spongy appendage attaching it to the placenta. A longitudinal section of a ripe seed (K) shows the very small, nearly triangular embryo (em.), while the rest of the cavity of the seed is filled with a white, starch-bearing tissue, the endosperm.
Fig. 82.—Erythronium. A, a portion of the wall of the anther, × 150. B, a single epidermal cell from the petal, × 150. C, cross-section of a fibro-vascular bundle of the stem, × 150. tr. vessels. D, E, longitudinal section of the same, showing the markings of the vessels, × 150. F, a bit of the epidermis from the lower surface of a leaf, showing the breathing pores, × 50. G, a single breathing pore, × 200. H, cross-section of a leaf, × 50. st. a breathing pore. m, the mesophyll. fb. a vein. I, cross-section of a breathing pore, × 200. J, young embryo, × 150.
A microscopical examination of the tissues of the plant shows them to be comparatively simple, this being especially the case with the fibro-vascular system.
The epidermis of the leaf is readily removed, and examination shows it to be made up of oblong cells with large breathing pores in rows. The breathing pores are much larger than any we have yet seen, and are of the type common to most angiosperms. The ordinary epidermal cells are quite destitute of chlorophyll, but the two cells (guard cells) enclosing the breathing pore contain numerous chloroplasts, and the oblong nuclei of these cells are usually conspicuous (Fig. 82, G). By placing a piece of the leaf between pieces of pith, and making a number of thin cross-sections at right angles to the longer axis of the leaf, some of the breathing pores will probably be cut across, and their structure may be then better understood. Such a section is shown in Figure 82, I.
The body of the leaf is made up of chlorophyll-bearing cells of irregular shape and with large air spaces between (H, m). The veins traversing this tissue are fibro-vascular bundles of a type structure similar to that of the stem, which will be described presently.
The stem is made up principally of large cells with thin walls, which in cross-section show numerous small, triangular, intercellular spaces (i) at the angles. These cells contain, usually, more or less starch. The fibro-vascular bundles (C) are nearly triangular in section, and resemble considerably those of the field horse-tail, but they are not penetrated by the air channel, found in the latter. The xylem, as in the pine, is toward the outside of the stem, but the boundary between xylem and phloem is not well defined, there being no cambium present. In the xylem are a number of vessels (C, tr.) at once distinguishable from the other cells by their definite form, firm walls, and empty cavity. The vessels in longitudinal sections show spiral and ringed thickenings. The rest of the xylem cells, as well as those of the phloem, are not noticeably different from the cells of the ground tissue, except for their much smaller size, and absence of intercellular spaces.
The structure of the leaves of the perigone is much like that of the green leaves, but the tissues are somewhat reduced. The epidermis of the outer side of the sepals has breathing pores, but these are absent from their inner surface, and from both sides of the petals. The walls of the epidermal cells of the petals are peculiarly thickened by apparent infoldings of the wall (B), and these cells, as well as those below them, contain small, yellow bodies (chromoplasts) to which the bright color of the flower is due. The red specks on the base of the perigone leaves, as well as the red color of the back of the sepals, the stalk, and leaves are due to a purplish red cell sap filling the cells at these points.
The filaments or stalks of the stamens are made up of very delicate colorless cells, and the centre is traversed by a single fibro-vascular bundle, which is continued up through the centre of the anther. To study the latter, thin cross-sections should be made and mounted in water. Each of the four sporangia, or pollen sacs, is surrounded on the outside by a wall, consisting of two layers of cells, becoming thicker in the middle of the section where the single fibro-vascular bundle is seen (Fig. 81, H). On opening, the cavities of the adjacent sporangia are thrown together. The inner cells of the wall are marked by thickened bars, much as we saw in the pine (Fig. 82, A), and which, like these, are formed shortly before the pollen sacs open. The pollen spores (Fig. 81, I) are large, oval cells, having a double wall, the outer one somewhat heavier than the inner one, but sufficiently transparent to allow a clear view of the interior, which is filled with very dense, granular protoplasm in which may be dimly seen two nuclei (n, ni.), showing that here also there is a division of the spore contents, although no wall is present. The spores do not germinate very readily, and are less favorable for this purpose than those of some other monocotyledons. Among the best for this purpose are the spiderwort (Tradescantia) and Scilla.
Owing to the large size and consequent opacity of the ovules, as well as to the difficulty of getting the early stages, the development and finer structure of the ovule will not be discussed here. The full-grown ovule may be readily sectioned, and a general idea of its structure obtained. A little potash may be used to advantage in this study, carefully washing it away when the section is sufficiently cleared. We find now that the ovule is attached to a stalk (funiculus) (Fig. 81, G, f), the body of the ovule being bent up so as to lie against the stalk. Such an inverted ovule is called technically, “anatropous.” The ovule is much enlarged where the stalk bends. The upper part of the ovule is on the whole like that of the pine, but there are two integuments (i, ii) instead of the single one found in the pine.
As the seed develops, the embryo sac (G, sp.) enlarges so as to occupy pretty much the whole space of the seed. At first it is nearly filled with a fluid, but a layer of cells is formed, lining the walls, and this thickens until the whole space, except what is occupied by the small embryo, is filled with them. These are called the “endosperm cells,” but differ from the endosperm cells of the gymnosperms, in the fact that they are not developed until after fertilization, and can hardly, therefore, be regarded as representing the prothallium of the gymnosperms and pteridophytes. These cells finally form a firm tissue, whose cells are filled with starch that forms a reserve supply of food for the embryo plant when the seed germinates. The embryo (Fig. 81, K, em., Fig. 82, J), even when the seed is ripe, remains very small, and shows scarcely any differentiation. It is a small, pear-shaped mass of cells, the smaller end directed toward the upper end of the embryo sac.
The integuments grow with the embryo sac, and become brown and hard, forming the shell of the seed. The stalk of the ovule also enlarges, and finally forms the peculiar, spongy appendage of the seeds already noticed (Fig. 81, J, K).
CHAPTER XII. SUB-KINGDOM V. Pteridophytes.
If we compare the structure of the sporogonium of a moss or liverwort with the plant bearing the sexual organs, we find that its tissues are better differentiated, and that it is on the whole a more complex structure than the plant that bears it. It, however, remains attached to the parent plant, deriving its nourishment in part through the “foot” by means of which it is attached to the plant.
In the Pteridophytes, however, we find that the sporogonium becomes very much more developed, and finally becomes entirely detached from the sexual plant, developing in most cases roots that fasten it to the ground, after which it may live for many years, and reach a very large size.
The sexual plant, which is here called the “prothallium,” is of very simple structure, resembling the lower liverworts usually, and never reaches more than about a centimetre in diameter, and is often much smaller than this.
The common ferns are the types of the sub-kingdom, and a careful study of any of these will illustrate the principal peculiarities of the group. The whole plant, as we know it, is really nothing but the sporogonium, originating from the egg cell in exactly the same way as the moss sporogonium, and like it gives rise to spores which are formed upon the leaves.
The spores may be collected by placing the spore-bearing leaves on sheets of paper and letting them dry, when the ripe spores will be discharged covering the paper as a fine, brown powder. If these are sown on fine, rather closely packed earth, and kept moist and covered with glass so as to prevent evaporation, within a week or two a fine, green, moss-like growth will make its appearance, and by the end of five or six weeks, if the weather is warm, little, flat, heart-shaped plants of a dark-green color may be seen. These look like small liverworts, and are the sexual plants (prothallia) of our ferns (Fig. 66, F). Removing one of these carefully, we find on the lower side numerous fine hairs like those on the lower surface of the liverworts, which fasten it firmly to the ground. By and by, if our culture has been successful, we may find attached to some of the larger of these, little fern plants growing from the under side of the prothallia, and attached to the ground by a delicate root. As the little plant becomes larger the prothallium dies, leaving it attached to the ground as an independent plant, which after a time bears the spores.
Fig. 66.—A, spore of the ostrich fern (Onoclea), with the outer coat removed. B, germinating spore, × 150. C, young prothallium, × 50. r, root hair. sp. spore membrane. D, E, older prothallia. a, apical cell, × 150. F, a female prothallium, seen from below, × 12. ar. archegonia. G, H, young archegonia, in optical section, × 150. o, central cell. b, ventral canal cell. c, upper canal cell. I, a ripe archegonium in the act of opening, × 150. o, egg cell. J, a male prothallium, × 50. an. antheridia. K, L, young antheridia, in optical section, × 300. M, ripe antheridium, × 300. sp. sperm cells. N, O, antheridia that have partially discharged their contents, × 300. P, spermatozoids, killed with iodine, × 500. v, vesicle attached to the hinder end.
In choosing spores for germination it is best to select those of large size and containing abundant chlorophyll, as they germinate more readily. Especially favorable for this purpose are the spores of the ostrich fern (Onoclea struthiopteris) (Fig. 70, I, J), or the sensitive fern (O. sensibilis). Another common and readily grown species is the lady fern (Asplenium filixfœmina) (Fig. 70, H). The spores of most ferns retain their vitality for many months, and hence can be kept dry until wanted.
The first stages of germination may be readily seen by sowing the spores in water, where, under favorable circumstances, they will begin to grow within three or four days. The outer, dry, brown coat of the spore is first ruptured, and often completely thrown off by the swelling of the spore contents. Below this is a second colorless membrane which is also ruptured, but remains attached to the spore. Through the orifice in the second coat, the inner delicate membrane protrudes in the form of a nearly colorless papilla which rapidly elongates and becomes separated from the body of the spore by a partition, constituting the first root hair (Fig. 66, B, C, r). The body of the spore containing most of the chlorophyll elongates more slowly, and divides by a series of transverse walls so as to form a short row of cells, resembling in structure some of the simpler algæ (C).
In order to follow the development further, spores must be sown upon earth, as they do not develop normally in water beyond this stage. Read the rest of this entry »
CHAPTER VIII. SUB-KINGDOM III. Fungi.
The name “Fungi” has been given to a vast assemblage of plants, varying much among themselves, but on the whole of about the same structural rank as the algæ. Unlike the algæ, however, they are entirely destitute of chlorophyll, and in consequence are dependent upon organic matter for food, some being parasites (growing upon living organisms), others saprophytes (feeding on dead matter). Some of them show close resemblances in structure to certain algæ, and there is reason to believe that they are descended from forms that originally had chlorophyll; others are very different from any green plants, though more or less evidently related among themselves. Recognizing then these distinctions, we may make two divisions of the sub-kingdom: I. The Alga-Fungi (Phycomycetes), and II. The True Fungi (Mycomycetes).
Class I.—Phycomycetes.
These are fungi consisting of long, undivided, often branching tubular filaments, resembling quite closely those of Vaucheria or other Siphoneæ, but always destitute of any trace of chlorophyll. The simplest of these include the common moulds (Mucorini), one of which will serve to illustrate the characteristics of the order.
If a bit of fresh bread, slightly moistened, is kept under a bell jar or tumbler in a warm room, in the course of twenty-four hours or so it will be covered with a film of fine white threads, and a little later will produce a crop of little globular bodies mounted on upright stalks. These are at first white, but soon become black, and the filaments bearing them also grow dark-colored.
These are moulds, and have grown from spores that are in the atmosphere falling on the bread, which offers the proper conditions for their growth and multiplication.
One of the commonest moulds is the one here figured (Fig. 32), and named Mucor stolonifer, from the runners, or “stolons,” by which it spreads from one point to another. As it grows it sends out these runners along the surface of the bread, or even along the inner surface of the glass covering it. They fasten themselves at intervals to the substratum, and send up from these points clusters of short filaments, each one tipped with a spore case, or “sporangium.”
For microscopical study they are best mounted in dilute glycerine (about one-quarter glycerine to three-quarters pure water). After carefully spreading out the specimens in this mixture, allow a drop of alcohol to fall upon the preparation, and then put on the cover glass. The alcohol drives out the air, which otherwise interferes badly with the examination. Read the rest of this entry »
CHAPTER VII. Class III.—The Red Algæ (Rhodophyceæ).
These are among the most beautiful and interesting members of the plant kingdom, both on account of their beautiful colors and the exquisitely graceful forms exhibited by many of them. Unfortunately for inland students they are, with few exceptions, confined to salt water, and consequently fresh material is not available. Nevertheless, enough can be done with dried material to get a good idea of their general appearance, and the fruiting plants can be readily preserved in strong alcohol. Specimens, simply dried, may be kept for an indefinite period, and on being placed in water will assume perfectly the appearance of the living plants. Prolonged exposure, however, to the action of fresh water extracts the red pigment that gives them their characteristic color. This pigment is found in the chlorophyll bodies, and usually quite conceals the chlorophyll, which, however, becomes evident so soon as the red pigment is removed.
The red seaweeds differ much in the complexity of the plant body, but all Read the rest of this entry »
CHAPTER VI. THE BROWN ALGAE (Phæophyceae).
Fig. 24.—Forms of diatoms. A, Pinnularia. i, seen from above; ii, from the side. B, Fragillaria (?). C, Navicula. D, F, Eunotia. E, Gomphonema. G, Cocconeis. H,Diatoma. All × 300.
These plants are all characterized by the presence of a brown pigment, in addition to the chlorophyll, which almost entirely conceals the latter, giving the plants a brownish color, ranging from a light yellowish brown to nearly black. One order of plants that possibly belongs here (Diatomaceæ) are single celled, but the others are for the most part large seaweeds. The diatoms, which are placed in this class simply on account of the color, are probably not closely related to the other brown algæ, but just where they should be placed is difficult to say. In some respects they approach quite closely the desmids, and are not infrequently regarded as related to them. They are among the commonest of organisms occurring everywhere in stagnant and running water, both fresh and salt, forming usually, slimy, yellowish coatings on stones, mud, aquatic plants, etc. Like the desmids they may be single or united into filaments, and not infrequently are attached by means of a delicate gelatinous stalk (Fig. 25).
Fig. 25.—Diatoms attached by a gelatinous stalk. × 150
They are at once distinguished from the desmids by their color, which is always some shade of yellowish or reddish brown. The commonest forms, e.g. Navicula (Fig. 24, C), are boat-shaped when Read the rest of this entry »
Green Algæ—Continued.
Order III.—Pond Scums (Conjugatæ).
The Conjugatæ, while in some respects approaching the Confervaceæ in structure, yet differ from them to such an extent in some respects that their close relationship is doubtful. They are very common and familiar plants, some of them forming great floating masses upon the surface of every stagnant pond and ditch, being commonly known as “pond scum.” The commonest of these pond scums belong to the genus Spirogyra, and one of these will illustrate the characteristics of the order. When in active growth these masses are of a vivid green, and owing to the presence of a gelatinous coating feel slimy, slipping through the hands when one attempts to lift them from the water. Spread out in water, the masses are seen to be composed of slender threads, often many centimetres in length, and showing no sign of branching.
Fig. 18.—A, a filament of a common pond scum (Spirogyra) separating into two parts. B, a cell undergoing division. The cell is seen in optical section, and the chlorophyll bands are omitted, n, nʹ, the two nuclei. C, a complete cell. n, nucleus. py. pyrenoid. D, E, successive stages in the process of conjugation. G, a ripe spore. H, a form in which conjugation takes place between the cells of the same filament. All × 150.
For microscopical examination the larger species are preferable. When one of these is magnified (Fig. 18, A, C), the unbranched filament is shown to be made up of perfectly cylindrical cells, with rather delicate walls. The protoplasm is Read the rest of this entry »
CHAPTER IV. SUB-KINGDOM II. Algæ
In the second sub-kingdom of plants is embraced an enormous assemblage of plants, differing widely in size and complexity, and yet showing a sufficiently complete gradation from the lowest to the highest as to make it impracticable to make more than one sub-kingdom to include them. They are nearly all aquatic forms, although many of them will survive long periods of drying, such forms occurring on moist earth, rocks, or the trunks of trees, but only growing when there is a plentiful supply of water.
All of them possess chlorophyll, which, however, in many forms, is hidden by the presence of a brown or red pigment. They are ordinarily divided into three classes—I. The Green Algæ (Chlorophyceæ); II. Brown Algæ (Phæophyceæ); III. Red Algæ (Rhodophyceæ).
Class I.—Green Algæ.
The green algæ are to be found almost everywhere where there is moisture, but are especially abundant in sluggish or stagnant fresh water, being much less common in salt water. They are for the most part plants of simple structure, many being unicellular, and very few of them plants of large size.
We may recognize five well-marked orders of the green algæ—I. Green slimes (Protococcaceæ); II. Confervaceæ; III. Pond scums (Conjugatæ); IV. Siphoneæ; V. Stone-worts (Characeæ).
Order I.—Protococcaceæ.
The members of this order are minute unicellular plants, growing either in water or on the damp surfaces of stones, tree trunks, etc. The plants sometimes grow isolated, but usually the cells are united more or less regularly into colonies.
A common representative of the order is the common green slime, Protococcus(fig 11), which forms a dark green slimy coating over stones, tree trunks, flower pots, etc. Owing to their minute size the structure can only be made out with the microscope.
(fig 11 A, C), which forms a dark green slimy coating over stones, tree trunks, flower pots, etc. Owing to their minute size the structure can only be made out with the microscope.
Scraping off a little of the material mentioned into a drop of water upon a slide, and carefully separating it with needles, a cover glass may be placed over the preparation, and it is ready for examination. When magnified, the green film is found to be composed of minute globular cells of varying size, which may in places be found to be united into groups. With a higher power, each cell (fig.11, A) is seen to have a distinct cell wall, within which is colorless protoplasm. Careful examination shows that the chlorophyll is confined to several roundish bodies that are not usually in immediate contact with the wall of the cell. These green masses are called chlorophyll bodies (chloroplasts). Toward the centre of the cell, especially if it has first been treated with iodine, the nucleus may be found. The size of the cells, as well as the number of chloroplasts, varies a good deal.
With a little hunting, specimens in various stages of division may be found. The division takes place in two ways. In the first (fig.11, B), known as fission, a wall is formed across the cell, dividing it into two cells, which may separate immediately or may remain united until they have undergone further division. In this case the original cell wall remains as part of the wall of the daughter cells. Fission is the commonest form of cell multiplication throughout the vegetable kingdom.
The second form of cell division or internal cell division is shown at C. Here the protoplasm and nucleus repeatedly divide until a number of small cells are formed within the old one. These develop cell walls, and escape by the breaking of the old cell wall, which is left behind, and takes no part in the process. The cells thus formed are sometimes provided with two cilia, and are capable of active movement.
Internal cell division, as we shall see, is found in most plants, but only at special times.
Closely resembling Protococcus, and answering quite as well for study, are numerous aquatic forms, such as Chlorococcum (fig 12). These are for the most part destitute of a firm cell wall, but are imbedded in masses of gelatinous substance like many Cyanophyceæ. The chloroplasts are smaller and less distinct than in Protococcus. The cells are here oval rather than round, and often show a clear space at one end.
Fig. 12.—Chlorococcum, a plant related to Protococcus, but the naked cells are surrounded by a colorless gelatinous envelope. A, motionless cells. B, a cell that has escaped from its envelope and is ciliated, × 300. Read the rest of this entry »
