Elements of Structural and Systematic Botany Part 16

Although these plants reach such a great size, an examination of the stem shows that it is built on much the same plan as that of the other monocotyledons; that is, the stem is composed of a ma.s.s of soft, ground tissue through which run many small isolated, fibro-vascular bundles. A good idea of this structure may be had by cutting across a corn-stalk, which is built on precisely the same pattern.

ORDER IV.--_Glumaceae_.

The plants of this order resemble each other closely in their habit, all having long, narrow leaves with sheathing bases that surround the slender, distinctly jointed stem which frequently has a hard, polished surface. The flowers are inconspicuous, borne usually in close spikes, and dest.i.tute of a perigone or having this reduced to small scales or hairs. The flowers are usually surrounded by more or less dry leaves (glumes, paleae) which are closely set, so as to nearly conceal the flowers. The flowers are either hermaphrodite or unis.e.xual.

[Ill.u.s.tration: FIG. 87.--Types of _Glumaceae_. _A_, a sedge, _Carex_ (_Cyperaceae_). ?, the male; ?, the female flowers, . _B_, a single male flower, 2. _C_, a female flower, 2. _D_, fruiting spike of another _Carex_, . _E_, a single fruit, 1. _F_, the same, with the outer envelope removed, and slightly enlarged. _G_, section of _F_, 3. _em._ the embryo. _H_, a bulrush, _Scirpus_ (_Cyperaceae_), . _I_, a single spikelet, 2. _J_, a single flower, 3. _K_, a spikelet of flowers of the common orchard gra.s.s, _Dactylis_ (_Gramineae_), 2. _L_, a single flower, 2. _M_, the base of a leaf, showing the split sheath encircling the stem, 1. _N_, section of a kernel of corn, showing the embryo (_em._), 2.]

There are two well-marked families, the sedges (_Cyperaceae_) and the gra.s.ses (_Gramineae_). The former have solid, often triangular stems, and the sheath at the base of the leaves is not split. The commonest genera are _Carex_ (Fig. 87, _A_, _G_) and _Cyperus_, of which there are many common species, differing very little and hard to distinguish. There are several common species of _Carex_ which blossom early in the spring, the male flowers being quite conspicuous on account of the large, yellow anthers. The female flowers are in similar spikes lower down, where the pollen readily falls upon them, and is caught by the long stigmas. In some other genera, _e.g._ the bulrushes (_Scirpus_) (Fig. 87, _H_), the flowers are hermaphrodite, _i.e._ contain both stamens and pistils. The fruit (Fig. 87, _F_) is seed-like, but really includes the wall of the ovary as well, which is grown closely to the enclosed seed. The embryo is small, surrounded by abundant endosperm (Fig. 87, _G_). Very few of the sedges are of any economic importance, though one, the papyrus of Egypt, was formerly much valued for its pith, which was manufactured into paper.

The second family, the gra.s.ses, on the contrary, includes the most important of all food plants, all of the grains belonging here. They differ mainly from the sedges in having, generally, hollow, cylindrical stems, and the sheath of the leaves split down one side; the leaves are in two rows, while those of the sedges are in three.

The flowers (Fig. 87, _L_) are usually perfect; the stigmas, two in number and like plumes, so that they readily catch the pollen which is blown upon them. A few, like the Indian corn, have the flowers unis.e.xual; the male flowers are at the top of the stem forming the "ta.s.sel," and the female flowers lower down forming the ear. The "silk" is composed of the enormously lengthened stigmas. The fruits resemble those of the sedges, but the embryo is usually larger and placed at one side of the endosperm (_N_, _em._).

While most of the gra.s.ses are comparatively small plants, a few of them are almost tree-like in their proportions, the species of bamboo (_Bambusa_) sometimes reaching a height of twenty to thirty metres, with stems thirty to forty centimetres in diameter.

ORDER V.--_Scitamineae_.

[Ill.u.s.tration: FIG. 88.--_Scitamineae_. _A_, upper part of a flowering plant of Indian shot (_Canna_), much reduced in size (_Cannaceae_).

_B_, a single flower, . _C_, the single stamen (_an._), and petal-like pistil (_gy._), 1. _D_, section of the ovary, 2. _E_, diagram of the flower. The place of the missing stamens is indicated by small circles. _F_, fruit, . _G_, section of an unripe seed.

_em._ embryo. _p_, perisperm, 2.]

The plants of this order are all inhabitants of the warmer parts of the earth, and only a very few occur within the limits of the United States, and these confined to the extreme south. They are extremely showy plants, owing to their large leaves and brilliant flowers, and for this reason are cultivated extensively. Various species of _Canna_ (Fig. 88) are common in gardens, where they are prized for their large, richly-colored leaves, and cl.u.s.ters of scarlet, orange, or yellow flowers. The leafy stems arise from thick tubers or root stocks, and grow rapidly to a height of two metres or more in the larger species. The leaves, as in all the order, are very large, and have a thick midrib with lateral veins running to the margin. The young leaves are folded up like a trumpet. The flowers are irregular in form, and in _Canna_ only a single stamen is found; or if more are present, they are reduced to petal-like rudiments. The single, perfect stamen (Fig. 88, _C_, _an._) has the filament broad and colored like the petals, and the anther attached to one side. The pistil (_gy._) is also petal-like. There are three circles of leaves forming the perigone, the two outer being more or less membranaceous, and only the three inner petal-like in texture. The ovary (_o_) is inferior, and covered on the outside with little papillae that afterward form short spines on the outside of the fruit (_F_).

The seeds are large, but the embryo is very small. A section of a nearly ripe seed shows the embryo (_em._) occupying the upper part of the embryo sac which does not nearly fill the seed and contains no endosperm. The bulk of the seed is derived from the tissue of the body of the ovule, which in most seeds becomes entirely obliterated by the growth of the embryo sac. The cells of this tissue become filled with starch, and serve the same purpose as the endosperm of other seeds.

This tissue is called "perisperm."

Of food plants belonging to this order, the banana (_Musa_) is much the most important. Others of more or less value are species of arrowroot (_Maranta_) and ginger (_Zingiber_).

There are three families: I. _Musaceae_ (banana family); II. _Zingiberaceae_ (ginger family); and III. _Cannaceae_ (_Canna_, _Maranta_).

ORDER VI.--_Gynandrae_.

By far the greater number of the plants of this order belong to the orchis family (_Orchideae_), the second family of the order (_Apostasieae_), being a small one and unrepresented in the United States. The orchids are in some respects the most highly specialized of all flowers, and exhibit wonderful variety in the shape and color of the flowers, which are often of extraordinary beauty, and show special contrivances for cross-fertilization that are without parallel among flowering plants.

[Ill.u.s.tration: FIG. 89.--_Gynandrae_. _A_, inflorescence of the showy orchis (_Orchis spectabilis_), 1 (_Orchideae_). _B_, a single flower, with the upper leaves of the perianth turned back to show the column (_x_). _sp._ the spur attached to the lower petal or lip. _o_, the ovary, 1. _C_, the column seen from in front. _an._ the stamen.

_gy._ the stigmatic surface, 1. _D_, the two pollen ma.s.ses attached to a straw, which was inserted into the flower, by means of the viscid disc (_d_): i, the ma.s.ses immediately after their withdrawal; ii, iii, the same a few minutes later, showing the change in position. _E_, diagram of the flower; the position of the missing stamens indicated by small circles.]

The flowers are always more or less bilaterally symmetrical (zygomorphic). The ovary is inferior, and usually twisted so as to turn the flower completely around. There are two sets of perigone leaves, three in each, and these are usually much alike except the lower (through the twisting of the ovary) of the inner set. This petal, known as the "lip" or "labellum," is usually larger than the others, and different in color, as well as being frequently of peculiar shape. In many of them it is also prolonged backward in a hollow spur (see Fig. 89, _B_). In all of the orchids except the lady's-slippers (_Cypripedium_) (Fig. 90, _B_), only one perfect stamen is developed, and this is united with the three styles to form a special structure known, as the "column" or "gynostemium" (Fig. 89, _B_, _C_). The pollen spores are usually aggregated into two or four waxy ma.s.ses ("pollinia," sing. pollinium), which usually can only be removed by the agency of insects upon which all but a very few orchids are absolutely dependent for the pollination of the flowers.

[Ill.u.s.tration: FIG. 90.--Forms of _Orchideae_. _A_, putty-root (_Aplectrum_), 1. _B_, yellow lady's-slipper (_Cypripedium_), .

_C_, the column of the same, 1. _an._ one of the two perfect stamens. _st._ sterile, petal-like stamen. _gy._. stigma. _D_, _Arethusa_, . _E_, section of the column, 1: _an._ stamen. _gy._ stigma. _F_, the same, seen from in front. _G_, _Habenaria_, 1. _H_, _Calopogon_, 1. In the last the ovary is not twisted, so that the lip (_L_) lies on the upper side of the flower.]

In the lady-slippers there are two fertile stamens, and a third sterile one has the form of a large triangular shield terminating the column (Fig. 90, _C_, _st._).

The ovules of the orchids are extremely small, and are only partly developed at the time the flower opens, the pollen tube growing very slowly and the ovules maturing as it grows down through the tissues of the column. The ripe seeds are excessively numerous, but so fine as to look like dust.

The orchids are mostly small or moderate-sized plants, few of them being more than a metre or so in height. All of our native species, with the exception of a few from the extreme south, grow from fibrous roots or tubers, but many tropical orchids, as is well known, are "epiphytes"; that is, they grow upon the trunks and branches of trees.

One genus, _Vanilla_, is a twining epiphyte; the fruit of this plant furnishes the vanilla of commerce. Aside from this plant, the economical value of the orchids is small, although a few of them are used medicinally, but are not specially valuable.

Of the five thousand species known, the great majority are inhabitants of the tropics, but nevertheless there are within the United States a number of very beautiful forms. The largest and showiest are the lady's-slippers, of which we have six species at the north. The most beautiful is the showy lady's-slipper (_Cypripedium spectabile_), whose large, pink and white flowers rival in beauty many of the choicest tropical orchids. Many of the _Habenarias_, including the yellow and purple fringed orchids, are strikingly beautiful as are the _Arethuseae_ (_Arethusa_, _Pogonia_, _Calopogon_). The last of these (Fig. 90, _H_) differs from all our other native orchids in having the ovary untwisted so that the labellum lies on the upper side of the flower.

A number of the orchids are saprophytic, growing in soil rich in decaying vegetable matter, and these forms are often nearly or quite dest.i.tute of chlorophyll, being brownish or yellowish in color, and with rudimentary leaves. The coral roots (_Corallorhiza_), of which there are several species, are examples of these, and another closely related form, the putty-root (_Aplectrum_) (Fig. 90, _A_), has the flowering stems like those of _Corallorhiza_, but there is a single, large, plaited leaf sent up later.

ORDER VII.--_Helobiae_.

The last order of the monocotyledons is composed of marsh or water plants, some of which recall certain of the dicotyledons. Of the three families, the first, _Juncagineae_, includes a few inconspicuous plants with gra.s.s-like or rush-like leaves, and small, greenish or yellowish flowers (_e.g._ arrow-gra.s.s, _Triglochin_).

The second family (_Alismaceae_) contains several large and showy species, inhabitants of marshes. Of these the water-plantain (_Alisma_), a plant with long-stalked, oval, ribbed leaves, and a much-branched panicle of small, white flowers, is very common in marshes and ditches, and the various species of arrowhead (_Sagittaria_) are among the most characteristic of our marsh plants.

The flowers are unis.e.xual; the female flowers are usually borne at the base of the inflorescence, and the male flowers above. The gyncium (Fig. 91, _B_) consists of numerous, separate carpels attached to a globular receptacle. The sepals are green and much smaller than the white petals. The leaves (_F_) are broad, and, besides the thickened, parallel veins, have numerous smaller ones connecting these.

[Ill.u.s.tration: FIG. 91.--Types of _Helobiae_. _A_, inflorescence of arrowhead (_Sagittaria_), with a single female flower, (_Alismaceae_). _B_, section through the gyncium, showing the numerous single carpels, 3. _C_, a ripe fruit, 3. _D_, a male flower, 1.

_E_, a single stamen, 3. _F_, a leaf of _Sagittaria variabilis_, ?. _G_, ditch-moss (_Elodea_), with a female flower (_fl._), .

(_Hydrocharideae_). _H_, the flower, 2. _an._ the rudimentary stamens. _st._ the stigma. _I_, cross-section of the ovary, 4. _J_, male inflorescence of eel-gra.s.s (_Vallisneria_), 1. _K_, a single expanded male flower, 12. _st._ the stamen. _L_, a female flower, 1. _gy._ the stigma.]

The last family is the _Hydrocharideae_. They are submersed aquatics, or a few of them with long-stalked, floating leaves. Two forms, the ditch-moss (_Elodea_) (Fig. 91, _G_, _I_) and eel-gra.s.s (_Vallisneria_) are very common in stagnant or slow-running water. In both of these the plants are completely submersed, but there is a special arrangement for bringing the flowers to the surface of the water. Like the arrowhead, the flowers are unis.e.xual, but borne on different plants. The female flowers (_H_, _L_) are comparatively large, especially in _Vallisneria_, and are borne on long stalks, by means of which they reach the surface of the water, where they expand and are ready for pollination. The male flowers (Fig. 91, _J_, _K_) are extremely small and borne, many together, surrounded by a membranous envelope, the whole inflorescence attached by a short stalk. When the flowers are ready to open, they break away from their attachment, and the envelope opens, allowing them to escape, and they immediately rise to the surface where they expand and collect in great numbers about the open female flowers. Sometimes these are so abundant during the flowering period (late in summer) that the surface of the water looks as if flour had been scattered over it. After pollination is effected, the stem of the female flower coils up like a spring, drawing the flower beneath the water where the fruit ripens.

The cells of these plants show very beautifully the circulation of the protoplasm, the movement being very marked and continuing for a long time under the microscope. To see this the whole leaf of _Elodea_, or a section of that of _Vallisneria_, may be used.

CHAPTER XVII.

DICOTYLEDONS.

The second sub-cla.s.s of the angiosperms, the dicotyledons, receive their name from the two opposite seed leaves or cotyledons with which the young plant is furnished. These leaves are usually quite different in shape from the other leaves, and not infrequently are very thick and fleshy, filling nearly the whole seed, as may be seen in a bean or pea. The number of the dicotyledons is very large, and very much the greater number of living spermaphytes belong to this group. They exhibit much greater variety in the structure of the flowers than the monocotyledons, and the leaves, which in the latter are with few exceptions quite uniform in structure, show here almost infinite variety. Thus the leaves may be simple (undivided); _e.g._ oak, apple; or compound, as in clover, locust, rose, columbine, etc. The leaves may be stalked or sessile (attached directly to the stem), or even grown around the stem, as in some honeysuckles. The edges of the leaves may be perfectly smooth ("entire"), or they may be variously lobed, notched, or wavy in many ways. As many of the dicotyledons are trees or shrubs that lose their leaves annually, special leaves are developed for the protection of the young leaves during the winter.

These have the form of thick scales, and often are provided with glands secreting a gummy substance which helps render them water-proof. These scales are best studied in trees with large, winter buds, such as the horsechestnut (Fig. 92), hickory, lilac, etc. On removing the hard, scale leaves, the delicate, young leaves, and often the flowers, may be found within the bud. If we examine a young shoot of lilac or buckeye, just as the leaves are expanding in the spring, a complete series of forms may be seen from the simple, external scales, through immediate forms, to the complete foliage leaf. The veins of the leaves are almost always much-branched, the veins either being given off from one main vein or midrib (feather-veined or pinnate-veined), as in an apple leaf, or there may be a number of large veins radiating from the base of the leaf, as in the scarlet geranium or mallow. Such leaves are said to be palmately veined.

[Ill.u.s.tration: FIG. 92.--End of a branch of a horsechestnut in winter, showing the buds covered by the thick, brown scale leaves, 1.]

Some of them are small herbaceous plants, either upright or prostrate upon the ground, over which they may creep extensively, becoming rooted at intervals, as in the white clover, or sending out special runners, as is seen in the strawberry. Others are woody stemmed plants, persisting from year to year, and often becoming great trees that live for hundreds of years. Still others are climbing plants, either twining their stems about the support, like the morning-glory, hop, honeysuckle, and many others, or having special organs (tendrils) by which they fasten themselves to the support. These tendrils originate in different ways. Sometimes, as in the grape and Virginia creeper, they are reduced branches, either coiling about the support, or producing little suckers at their tips by which they cling to walls or the trunks of trees. Other tendrils, as in the poison ivy and the true ivy, are short roots that fasten themselves firmly in the crevices of bark or stones. Still other tendrils, as those of the sweet-pea and clematis, are parts of the leaf.

The stems may be modified into thorns for protection, as we see in many trees and shrubs, and parts of leaves may be similarly changed, as in the thistle. The underground stems often become much changed, forming bulbs, tubers, root stocks, etc. much as in the monocotyledons. These structures are especially found in plants which die down to the ground each year, and contain supplies of nourishment for the rapid growth of the annual shoots.

[Ill.u.s.tration: FIG. 93.--_A_, base of a plant of shepherd's-purse (_Capsella bursa-pastoris_), . _r_, the main root. _B_, upper part of the inflorescence, 1. _C_, two leaves: i, from the upper part; ii, from the base of the plant, 1. _D_, a flower, 3. _E_, the same, with sepals and petals removed, 3. _F_, petal. _G_, sepal.

_H_, stamen, 10. _f_, filament. _an._ anther. _I_, a fruit with one of the valves removed to show the seeds, 4. _J_, longitudinal section of a seed, 8. _K_, the embryo removed from the seed, 8.

_l_, the first leaves (cotyledons). _st._ the stem ending in the root.

_L_, cross-section of the stem, 20. _fb._ fibro-vascular bundle.

_M_, a similar section of the main root, 15. _N_, diagram of the flower.]

The structure of the tissues, and the peculiarities of the flower and fruit, will be better understood by a somewhat careful examination of a typical dicotyledon, and a comparison with this of examples of the princ.i.p.al orders and families.

One of the commonest of weeds, and at the same time one of the most convenient plants for studying the characteristics of the dicotyledons, is the common shepherd's-purse (_Capsella bursa-pastoris_) (Figs. 93-95).