50

GROWTH BY TERMINAL BUDS.

other Monocotyledonous plants, the stems have no power of forming lateral buds, from which branches can alone be produced (see page 56). These plants, which frequently rise to the height of one hundred and fifty feet or more, therefore, grow simply by the development of a terminal bud, which when it unfolds crowns the summit with a tuft of leaves, which are commonly of a great size. Monocotyledonous stems are consequently almost uniformly cylindrical from below upward, being of the same diameter throughout, instead of conical as in trees of exogenous growth. In such plants, therefore, the destruction of the terminal bud necessarily leads to their death, as they are then deprived of all further mode of increase. In some Monocotyledonous trees, however, more than one bud is developed, so that the stem is forked above; each branch again develops two other buds at its apex in like manner, and this mode of growth is continued with the successive branches, which are therefore also forked. In others we have lateral buds formed, and as the lower part of such stems receives more fibro-vascular bundles than the upper they are necessarily larger in their diameter at their base, and thus these are conical or taper upward like those of Dicotyledonous plants.

Some Monocotyledonous stems present an anomalous structure; thus, in most Grasses the stem is hollow, except at the nodes, where the leaves arise, at which points partitions are formed across the cavity, by which it is divided into a number of separate portions. Such stems when examined at their first development present the usual Endogenous structure, but, in consequence of their growth in diameter taking place more rapidly than new matter can be deposited in their interior, the central tissue becomes ruptured, and they soon become hollow.

There is nothing in the internal structure of Endogenous stems by which we can ascertain the age of Monocotyledonous trees.

ACROGENOUS STEMS.

51

C. Acrogenous or Acotyledonous Stem.-The simplest form of stem presented by Acotyledonous plants is that seen in Liverworts (Fig. 7), and in Mosses. In such a stem we have no vessels, but the whole is composed of ordinary parenchyma, with occasionally a central cord of slightly elongated cells with somewhat thickened walls. In the stem of Club-mosses and Horse-tails, we have the simplest forms of Acrogenous stems, which contain the peculiar fibro-vascular bundles (simultaneous) which are their especial characteristics. The composition of these bundles and their mode of growth have been already described (see page 40). The vessels found in the fibro-vascular bundles of the Lycopodiaceæ are spiral, and in those of the Equisetaceæ annular. All Acotyledonous stems grow by additions to their apex, and hence the term Acrogenous, or summit-growers, which is applied to them.

In the Ferns we have the Acrogenous stem in the highest state of development. The Ferns of this country are, however, but insignificant specimens of such plants, for in them the stem merely runs along the surface of the ground, or burrows beneath it, sending up its leaves, or fronds, as they are commonly called, into the air,

which die down yearly. In warm regions, and more especially in the tropics, we find such plants much more highly developed. Here the stem rises sometimes as much as forty feet, and bears on its summit a tuft of fronds. In their general appearance Tree-ferns resemble Monocotyledonous trees. They bear their foliage at the summit, produce no lateral branches, and are of uniform diameter from near their base to their apex.

FIG. 94.-Rhizome of Male Fern (Aspidium filixmas), marked externally by rhomboidal scars, which present dark-colored projections, c.

The outside of the stem of a Fern is marked with a number of scars, which have a more or less rhomboidal outline

(Fig. 94); and present on their surface darker-colored spots, c, which appearance is produced by the rupture of the fibro-vascular bundles proceeding to the leaves, by the separation of which organs the scars are produced.

Upon making a transverse section of a Tree-fern, we observe, as we have already briefly noticed (see page 37), the following parts: Thus in the center, when young, a mass of parenchyma (Fig. 87, m), the cells of which have thin walls; but in old stems this central parenchyma is destroyed, so that the stem becomes hollow. Toward the outside of this parenchyma, and just within the rind, we find the so-called wood, which is arranged in the form of irregular, sinuous, or wavy plates, v, v, v. These masses of wood have generally openings between them, by means of which the parenchyma beneath the rind and that of the center of the stem communicate; but in other cases they touch each other at their margins, and thus form a continuous circle within the rind. These woody masses consist of simultaneous fibro-vascular bundles (page 39), the vessels of which are chiefly scalariform in their character; these are situated in the center of the bundles, where they may be readily distinguished by their pale color (Fig. 87, v, v, v). The whole is surrounded by a single layer of cells, the walls of which are more or less lignified and

M

FIG. 95.---Vertical section of the forked stem of

a Tree-fern.

dark colored, thus constituting the tis

sue termed sclerenchyma, and forming what has been called the bundle-sheath. The tissues external to the fibro-vascular bundles constitute collectively what has been termed the rind (Fig. 87, e).

Some ferms become forked at their apex (Fig. 95) by the division of the terminal bud into two, from each of which a branch is formed (see page 58). But such branches are very different from those of dicotyledonous stems, which are produced from lateral buds, for,

BUDS AND RAMIFICATION.

53

as they arise simply from the splitting of one bud into two, the diameter of the two branches combined is only equal to that of the trunk. As Acotyledonous stems

only grow by the development of a terminal bud, the destruction of that bud necessarily leads to their death. There is nothing in the internal structure or external appearance of such stems by which we can ascertain their age.

2. Buds and Ramification.-We have already seen that the presence of leaves and leaf-buds is the essential characteristic by which a stem may be distinguished from a root. The leaves will be treated of hereafter, but we have now to describe the nature of leaf-buds, and the mode in which branches are formed.

A. Leaf-buds or Buds.-Under ordinary circumstances we have developed in the axil of every leaf a little more or less conical body called a leaf-bud, or simply a bud (Fig. 96, a, a). In like manner, the apex of a stem, as well as of all its divisions

which are capable of further elongation, is also terminated by a similar bud (Fig. 98). In a Dicotyledonous plant each bud, whether lateral or terminal, is produced by an elongation of the parenchymatous system of the stem or one of its divisions, and consists at first of a minute conical

central parenchymatous

b

mass (Fig. 97, i), which is FIG. 96.-Branch of Oak with alternate

connected with the pith, a ;

around this spiral and other

leaves and leaf-buds in their axils. a, a. Buds. b, b. Leaves.

vessels and wood-cells are soon developed, also in connection with similar parts of the wood, b, b; and on the outside

of these, in a parenchymatous mass which ultimately becomes the bark, we have little conical cellular projections developed, which are the rudimentary leaves. As growth

FIG. 97.-Longitudinal section of the end of a twig of the Horse-chestnut (Esculus hippocastanum), before the bursting of the bud. (After Schleiden.)___a. The pith. b, b. The wood. c, c. The bark. d, d. Scars of leaves of former

years. e, e. The fibro-vascular bundles of those leaves. ff. The axillary buds of those leaves, with their scales and fibro-vascular bundles, g. Terminal bud

proceeds these parts become more evident, and a little more or less conical body is ultimately produced at the apex of the stem or branch (Fig. 98), or laterally in the axil of the leaves, c, and the

formation of the bud is completed. In like manner the buds of Monocotyledons and Acotyledons are connected with both the parenchymatous and fibro-vascular systems of their stems.

The buds of temperate and cold climates have generally certain protective or

gans developed on their

outer surface in the form of modified leaves, which are commonly called scales or tegmenta. These are usually of a hardened texture, and

of the twig ending in a rudimentary flowering panicle.h, h. Scars formed by the falling off of the lowest scales of the bud, and above these may be seen the closed scales with their fibro-vascular bundles. i. Parenchyma leading from the pith, a, into the axillary buds, are sometimes covered with ff. FIG. 98.-A shoot one year old of the Horse-chestnut, with terminal bud, a resinous secretion, as in a. Scar produced by the falling off of

the bud-scales of the previous year. b, b. the Horse-chestnut; or with

Scars caused by the falling off of the

petioles of the leaves of the present year, a dense coating of soft hairs, with buds, c, in their axils. as in some Willows. Buds

thus protected are termed scaly; these scales have only a temporary duration, falling off as soon as the growth of the bud commences in the spring. In the buds of tropical regions, and those of herbaceous plants which are not exposed to the influence of winter, such protective organs