Elements of Structural and Systematic Botany Part 13

The second order (_Ligulatae_) is represented by two very distinct families: the smaller club mosses (_Selaginelleae_) and the quill-worts (_Isoeteae_). Of the former the majority are tropical, but are common in greenhouses where they are prized for their delicate moss-like foliage (Fig. 74, _A_).

[Ill.u.s.tration: FIG. 74.--_A_, one of the smaller club mosses (_Selaginella_). _sp._ spore-bearing branch, 2. _B_, part of a stem, sending down naked rooting branches (_r_), 1. _C_, longitudinal section of a spike, with a single macrosporangium at the base; the others, microsporangia, 3. _D_, a scale and microsporangium, 5.

_E_, young microsporangium, 150. The shaded cells are the spore mother cells. _F_, a young macrospore, 150. _G_, section of the stem, 50. _H_, a single fibro-vascular bundle, 150. _I_, vertical section of the female prothallium of _Selaginella_, 50. _ar._ archegonium. _J_, section of an open archegonium, 300. _o_, the egg cell. _K_, microspore, with the contained male prothallium, 300.

_x_, vegetative cell. _sp._ sperm cells. _L_, young plant, with the attached macrospore, 6. _r_, the first root. _l_, the first leaves.]

The leaves in most species are like those of the larger club mosses, but more delicate. They are arranged in four rows on the upper side of the stem, two being larger than the others. The smaller branches grow out sideways so that the whole branch appears flattened, reminding one of the habit of the higher liverworts. Special leafless branches (_B_, _r_) often grow downward from the lower side of the main branches, and on touching the ground develop roots which fork regularly.

The sporangia are much like those of the ground pines, and produced singly at the bases of scale leaves arranged in a spike or cone (_A_, _sp._), but two kinds of spores, large and small, are formed. In the species figured the lower sporangium produces four large spores (macrospores); the others, numerous small spores (microspores).

Even before the spores are ripe the development of the prothallium begins, and this is significant, as it shows an undoubted relationship between these plants and the lowest of the seed plants, as we shall see when we study that group.

If ripe spores can be obtained by sowing them upon moist earth, the young plants will appear in about a month. The microspore (Fig. 74, _K_) produces a prothallium not unlike that of some of the water ferns, there being a single vegetative cell (_x_), and the rest of the prothallium forming a single antheridium. The spermatozoids are excessively small, and resemble those of the bryophytes.

The macrospore divides into two cells, a large lower one, and a smaller upper one. The latter gives rise to a flat disc of cells producing a number of small archegonia of simple structure (Fig. 74, _I_, _J_). The lower cell produces later a tissue that serves to nourish the young embryo.

The development of the embryo recalls in some particulars that of the seed plants, and this in connection with the peculiarities of the sporangia warrants us in regarding the _Ligulatae_ as the highest of existing pteridophytes, and to a certain extent connecting them with the lowest of the spermaphytes.

Resembling the smaller club mosses in their development, but differing in some important points, are the quill-worts (_Isoeteae_). They are mostly aquatic forms, growing partially or completely submerged, and look like gra.s.ses or rushes. They vary from a few centimetres to half a metre in height. The stem is very short, and the long cylindrical leaves closely crowded together. The leaves which are narrow above are widely expanded and overlapping at the base. The spores are of two kinds, as in _Selaginella_, but the macrosporangia contain numerous macrospores. The very large sporangia (_M_, _sp._) are in cavities at the bases of the leaves, and above each sporangium is a little pointed outgrowth (ligula), which is also found in the leaves of _Selaginella_. The quill-worts are not common plants, and owing to their habits of growth and resemblance to other plants, are likely to be overlooked unless careful search is made.

CHAPTER XIV.

SUB-KINGDOM VI.

SPERMAPHYTES: PHaeNOGAMS.

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 cla.s.ses 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.

CLa.s.s I.--GYMNOSPERMS (_Gymnospermae_).

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 cla.s.s, 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.

[10] In most conifers the symmetrical form of the young tree is maintained as long as the tree lives.

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 s.p.a.ce between the pith and bark is filled by a ma.s.s 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 quant.i.ty 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 a.s.sume a brownish color, growing considerably in size for a short time, and then ceasing to grow for several months.

[Ill.u.s.tration: FIG. 75.--Scotch pine (_Pinus sylvestris_). _A_, a ripe cone, . _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_), . _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 sh.e.l.l 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 s.p.a.ces smaller. The chloroplasts are numerous and nearly round in shape.

Scattered through the green tissue are several resin pa.s.sages (_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 s.p.a.ce (_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 s.p.a.ces 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 a.s.sume 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 s.p.a.ces 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 ma.s.ses 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.

[Ill.u.s.tration: 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 pa.s.sing 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 pa.s.sed 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 s.p.a.ce below it.]