Elements of Structural and Systematic Botany Part 15

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 const.i.tute 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 "_Andrcium_." 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_).

[12] The three outer stamens are shorter than the inner set.

The central circle of leaves, the carpels (collectively the "gyncium") 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.

[Ill.u.s.tration: 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 dest.i.tute 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 s.p.a.ces 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 princ.i.p.ally of large cells with thin walls, which in cross-section show numerous small, triangular, intercellular s.p.a.ces (_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 s.p.a.ces.

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 s.p.a.ce 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 s.p.a.ce, 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 ma.s.s 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 sh.e.l.l 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 XVI.

CLa.s.sIFICATION 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 cla.s.sification here, and this is the case throughout the spermaphytes, is based mainly upon the characters of the flowers and fruits.

The cla.s.sification 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 cla.s.sification, the monocotyledons may be divided into seven groups; viz., I. _Liliiflorae_; II. _Enantioblastae_; III. _Spadiciflorae_; IV. _Glumaceae_; V. _Scitamineae_; VI. _Gynandrae_; VII. _Helobiae_.

ORDER I.--_Liliiflorae_.

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 gra.s.s," "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 (_Dioscoreae_), 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 (_Smilaceae_) climb by means of tendrils at the bases of the leaves. Of the latter the "cat-brier" or "green-brier" is a familiar representative.

[Ill.u.s.tration: FIG. 83.--Types of _Liliiflorae_. _A_, _Trillium_, .

_B_, single flower of Solomon's-seal (_Polygonatum_), 1. _C_, upper part of a plant. _D_, underground stem (rhizome) of Indian cuc.u.mber root (_Medeola_), . _E_, a rush (_Juncus_), 1. _F_, a single flower, 2. _A-D_, _Liliaceae_; _E_, _Juncaceae_.]

The flowers are for the most part conspicuous, and in plan like that of the adder's-tongue; but some, like the rushes (Fig. 83, _E_), have small, inconspicuous flowers; and others, like the yams and smilaxes, have flowers of two kinds, male and female.

[Ill.u.s.tration: FIG. 84.--Types of _Liliiflorae_. _A_, flower of the common blue-flag (_Iris_), (_Iridaceae_). _B_, the petal-like upper part of the pistil, seen from below, and showing a stamen (_an._).

_st._ the stigma, . _C_, the young fruit, . _D_, section of the same, 1. _E_, diagram of the flower. _F_, part of a plant of the so-called "gray moss" (_Tillandsia_), (_Bromeliaceae_). _G_, a single flower, 2. _H_, a seed, showing the fine hairs attached to it, 1. _I_, plant of pickerel-weed (_Pontederia_), (_Pontederiaceae_). _J_, a single flower, 1. _K_, section of the ovary, 4.]

The princ.i.p.al family of the _Liliiflorae_ is the _Liliaceae_, including some of the most beautiful of all flowers. All of the true lilies (_Lilium_), as well as the day lilies (_Funkia_, _Hemerocallis_) of the gardens, tulips, hyacinths, lily-of-the-valley, etc., belong here, as well as a number of showy wild flowers including several species of tiger-lilies (_Lilium_), various species of _Trillium_ (Fig. 83, _A_), Solomon's-seal (_Polygonatum_) (Fig. 83, _B_), bellwort (_Uvularia_), and others. In all of these, except _Trillium_, the perigone leaves are colored alike, and the leaves parallel-veined; but in the latter the sepals are green and the leaves broad and netted-veined. The fruit of the _Liliaceae_ may be either a pod, like that of the adder's-tongue, or a berry, like that of asparagus or Solomon's-seal.

Differing from the true lilies in having the bases of the perigone leaves adherent to the surface of the ovary, so that the latter is apparently below the flower (inferior), and lacking the inner circle of stamens, is the iris family (_Iridaceae_), represented by the wild blue-flag (_Iris versicolor_) (Fig. 84, _A_, _E_), as well as by numerous cultivated species. In iris the carpels are free above and colored like the petals (_B_), with the stigma on the under side. Of garden flowers the gladiolus and crocus are the most familiar examples, besides the various species of iris; and of wild flowers the little "blue-eyed gra.s.s" (_Sisyrinchium_).

[Ill.u.s.tration: FIG. 85.--_Enantioblastae_. _A_, inflorescence of the common spiderwort (_Tradescantia_), (_Commelyneae_). _B_, a single stamen, showing the hairs attached to the filament, 2. _C_, the pistil, 2.]

The blue pickerel-weed (_Pontederia_) is the type of a family of which there are few common representatives (Fig. 84, _I_, _K_).

The last family of the order is the _Bromeliaceae_, all inhabitants of the warmer parts of the globe, but represented in the southern states by several forms, the commonest of which is the so-called "gray moss"

(_Tillandsia_) (Fig. 84, _F_, _H_). Of cultivated plants the pineapple, whose fruit consists of a fleshy ma.s.s made up of the crowded fruits and the fleshy flower stalks, is the best known.

ORDER II.--_Enantioblastae_.

The second order of the monocotyledons, _Enantioblastae_, includes very few common plants. The most familiar examples are the various species of _Tradescantia_ (Fig. 88), some of which are native, others exotic.

Of the cultivated forms the commonest is one sometimes called "wandering-jew," a trailing plant with zigzag stems, and oval, pointed leaves forming a sheath about each joint. Another common one is the spiderwort already referred to. In this the leaves are long and pointed, but also sheathing at the base. When the flowers are showy, as in these, the sepals and petals are different, the former being green. The flowers usually open but once, and the petals shrivel up as the flower fades. There are four families of the order, the spiderwort belonging to the highest one, _Commelyneae_.

ORDER III.--_Spadiciflorae_.

The third order of the monocotyledons, _Spadiciflorae_, is a very large one, and includes the largest and the smallest plants of the whole sub-cla.s.s. In all of them the flowers are small and often very inconspicuous; usually, though not always, the male and female flowers are separate, and often on different plants. The smallest members of the group are little aquatics, scarcely visible to the naked eye, and of extremely simple structure, but nevertheless these little plants produce true flowers. In marked contrast to these are the palms, some of which reach a height of thirty metres or more.

The flowers in most of the order are small and inconspicuous, but aggregated on a spike (spadix) which may be of very large size. Good types of the order are the various aroids (_Aroideae_), of which the calla (_Richardia_) is a very familiar cultivated example. Of wild forms the sweet-flag (_Acorus_), Jack-in-the-pulpit (_Arisaema_) (Fig. 86, _A_, _D_), skunk-cabbage (_Symplocarpus_), and wild calla may be noted. In _Arisaema_ (Fig. 86, _A_) the flowers are borne only on the base of the spadix, and the plant is dicious. The flowers are of the simplest structure, the female consisting of a single carpel, and the male of four stamens (_C_, _D_). While the individual flowers are dest.i.tute of a perigone, the whole inflorescence (cl.u.s.ter of flowers) is surrounded by a large leaf (spathe), which sometimes is brilliantly colored, this serving to attract insects. The leaves of the aroids are generally large and sometimes compound, the only instance of true compound leaves among the monocotyledons (Fig. 86, _B_).

[Ill.u.s.tration: FIG. 86.--Types of _Spadiciflorae_. _A_, inflorescence of Jack-in-the-pulpit (_Arisaema_, _Aroideae_). The flowers (_fl._) are at the base of a spike (spadix), surrounded by a sheath (spathe), which has been cut away on one side in order to show the flowers, .

_B_, leaf of the same plant, . _C_, vertical section of a female flower, 2. _D_, three male flowers, each consisting of four stamens, 2. _E_, two plants of a duck-weed (_Lemna_), the one at the left is in flower, 4. _F_, another common species. _L_, _Trisulea_, 1.

_G_, male flower of _E_, 25. _H_, optical section of the female flower, showing the single ovule (_ov._), 25. _I_, part of the inflorescence of the bur-reed (_Sparganium_), with female flowers, (_Typhaceae_). _J_, a single, female flower, 2. _K_, a ripe fruit, 1. _L_, longitudinal section of the same. _M_, two male flowers, 1. _N_, a pond-weed (_Potomogeton_), 1 (_Naiadaceae_). _O_, a single flower, 2. _P_, the same, with the perianth removed, 2.

_Q_, fruit of the same, 2.]

Probably to be regarded as reduced aroids are the duck-weeds (_Lemnaceae_) (Fig. 86, _F_, _H_), minute floating plants without any differentiation of the plant body into stem and leaves. They are globular or discoid ma.s.ses of cells, most of them having roots; but one genus (_Wolffia_) has no roots nor any trace of fibro-vascular bundles. The flowers are reduced to a single stamen or carpel (Figs.

_E_, _G_, _H_).

The cat-tail (_Typha_) and bur-reed (_Sparganium_) (Fig. 86, _I_, _L_) are common representatives of the family _Typhaceae_, and the pond-weeds (_Naias_ and _Potomogeton_) are common examples of the family _Naiadeae_. These are aquatic plants, completely submerged (_Naias_), or sometimes partially floating (_Potomogeton_). The latter genus includes a number of species with leaves varying from linear (very narrow and pointed) to broadly oval, and are everywhere common in slow streams.

The largest members of the group are the screw-pines (_Pandaneae_) and the palms (_Palmae_). These are represented in the United States by only a few species of the latter family, confined to the southern and southwestern portions. The palmettoes (_Sabal_ and _Chamaerops_) are the best known.

Both the palms and screw-pines are often cultivated for ornament, and as is well known, in the warmer parts of the world the palms are among the most valuable of all plants. The date palm (_Phnix dactylifera_) and the cocoanut (_Cocos nucifera_) are the best known. The apparently compound ("pinnate" or feather-shaped) leaves of many palms are not strictly compound; that is, they do not arise from the branching of an originally single leaf, but are really broad, undivided leaves, which are closely folded like a fan in the bud, and tear apart along the folds as the leaf opens.