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 ma.s.ses (_x_) alternating with as many phloem ma.s.ses (_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]

[11] See the last chapter for details.

[Ill.u.s.tration: FIG. 77.--Scotch pine (except _E_ and _F_). _A_, end of a branch bearing a cl.u.s.ter of male flowers (?), . _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_).

CLa.s.sIFICATION OF THE GYMNOSPERMS.

There are three cla.s.ses of the gymnosperms: I., cycads (_Cycadeae_); II., conifers (_Coniferae_); III., joint firs (_Gnetaceae_). 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.

[Ill.u.s.tration: FIG. 78.--Ill.u.s.trations of gymnosperms. _A_, fruiting leaf of a cycad (_Cycas_), with macrosporangia (ovules) (_ov._), .

_B_, leaf of _Gingko_, . _C_, branch of hemlock (_Tsuga_), with a ripe cone, 1. _D_, red cedar (_Juniperus_), 1. _E_, _Arbor-vitae_ (_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 j.a.panese _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-vitae_ (_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.

CLa.s.s II.--ANGIOSPERMS.

The angiosperms include an enormous a.s.semblage 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.

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 dest.i.tute 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.

[Ill.u.s.tration: 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 synergidae (_s_), and the egg cell (_o_). In the centre is the "endosperm nucleus" (_k_). At the bottom, the "antipodal cells" (_g_).]

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 "synergidae." 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).

[Ill.u.s.tration: FIG. 80.--Germinating pollen spores of the sweet pea, 200.]

The upper part of the pistil is variously modified, having either little papillae 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.

CLa.s.sIFICATION OF THE ANGIOSPERMS.

The angiosperms are divided into two sub-cla.s.ses: 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-cla.s.s 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.

[Ill.u.s.tration: FIG. 81.--_A_, plant of the yellow adder-tongue (_Erythronium americanum_), ?. _B_, the bulb of the same, . _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, . _f_, the filament. _an._ anther. _E_, the gyncium (pistil), 1.

_o_, ovary. _st._ style. _z_, stigma. _F_, a full-grown fruit, .

_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_, _n'_). _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.]