Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/104

We now proceed to consider the form and internal structure of the various organs of plants.

I. ORGANS OF NUTRITION.

1. Root or Descending Axis.

Speaking generally, the root is that portion of the plant which descends into the soil. In all plants the root is at first entirely cellular. It may remain permanently so, or vessels may be formed in it. The Radicle, or young root (fig. 58, r}, is the first portion of the embryo protruded from the seed or spore (fig. 54) when germination com mences, and resembles very much in structure the young stem. Both are entirely cel lular, consisting of a central nucleus of cellular tissue covered by two or more layers of cells. But at the apex of the root a mass of cells is developed, which constitutes, what is known as the root- cap or pileorhiza. These cells extend for some distance along the sides of the root, forming a sheath, and in some cases, as Lemna, the cap becomes loosened from the root, remaining attached by a few cells at the apex only, and then it is known as the ampulla. This root-cap distinguishes structurally the root from the stem, and it serves as a protection to the apical growing-point of the root. The roots of Thallophytes, con sisting entirely of cells, do not develop a root-cap.

The dicotyledonous embryo of the Pea

laid open, e, c, the two fleshy cotyle dons, or seed-lobes, which remain un der ground when the plant sprouts; r, the radicular extremity of the axis whence the root arises; t, the axis bearing the young stalk and leaves g,

which lie in a depression of the cotyledons/.

The root is merely a prolongation downwards of the stem, and the part where they unite is the collum or neck. After wards the root is distinguished from the stem by the absence of a provision for the development of leaf -buds. It is not always easy to distinguish between a stem and a root. Many so-called roots bear at their upper part a portion called their crouni, whence leaf-buds arise. Underground stems and roots are often confounded. Some plants, as the Moutan Pasony, the Plum-tree, Pyrus japonica, and especially Anemone japonica, have a power of forming buds on what are commonly called their roots. The last- mentioned plant develops these buds on every part of its extensively ramifying root-like prolongations, which may be chopped into numerous pieces, each capable of giving rise to a new plant. Such is also the case with the annulated root of Ipecacuan. Ptoots are usually subter ranean and colourless. Externally, they have a cellular epidermal covering of a delicate texture, sometimes called epiblema, in which no stomata exist. In woody plants fibrp-vascular bundles are found in the roots, and there is an internal arrangement of tissues similar to that seen in the stem itself, but spiral vessels are rare in the root. The axis of the root gives off branches which divide into radicles or fibrils, the extremities of which, composed of delicate cellular tissue constituting the pundum vegetationis, have been erroneously called spongioles or spongelets; they are not distinct organs. Hairs are often seen on roots, but no true leaves. These hairs consist of simple elongated cells, which occur singly, and appear to serve the purpose of absorption. Roots increase principally by additions to their extremities, which are constantly renewed, so that the minute fibrils serve only a temporary purpose, and represent deciduous leaves; but in large trees which form thick roots an increase in diameter occurs in the root similar to what is seen in the stem itself. In some plants no roots are formed at all; thus in the Orchidaceous plants Epipogium Gmelini and Corallorhiza ihnata, and also in Lemna arrhiza, no roots occur. Hoots may be given off from any portion of a stem, originating as cellular prolonga tions from the inner portion of the stem, and coming off at any point of the stem, or at small lenticular points to which the name of lenticeh has been given by some. When the stem is more or less horizontal the roots given off from it pass directly into the soil; but if the stem be erect they pass for a certain distance downwards through the air, and are called aerial. The latter are well seen in the Screw- pine (Pandanus), the Banyan (Ficus indica, fig. 59), and many other species of Ficus, where they assist in support ing the stem and branches. In the Mangrove they often form the entire support of the stem, which has decayed at its lower part. In Tree-ferns they form a dense coating around, and completely concealing, the stem; such is also the case in some Dracaenas and Palms. In Epiphytes, or plants growing in the air, attached to the trunks of trees, such as Orchids of warm climates, the aerial roots produced do not reach the soil; they continue always aerial and greenish, and they possess stomata. Delicate hairs are often seen on these epiphytal roots, as well as a peculiar investment formed by the cells of the epidermis which have lost their succulent contents and are now filled with air. This layer is called velamcn radiciim, or covering of the roots. The aerial roots of the Ivy are not the nutritive roots of the plant, but are only intended for mechanical support.

Fig. 59.

Ficus indica, the Banyan tree, sending out numerous aerial roots, which reach the soil, and prop the branches.

Parasitic plants, as the Mistleto (Viscum), Broom-rape (Orobanche), and Bafflesia, send root-like processes into the substance of the plants whence they derive nourishment. In the Dodder (Cuscuta), the tissue around the roots swells Plate X. into a kind of sucker (haustorium), which is applied flat upon the other plant, and ultimately becomes concave, so as to attach the plant by a vacuum. From the bottom of the sucker the root protrudes, which penetrates the support ing body. In the case of parasitic Fungi, such as Mould, there are cellular filaments which spread among the tissues of plants, and which may be looked upon as equivalent to roots and stems united. They form the spcmw or mycelium of these plants, and in some cases cause rapid destruction of the tissues of plants, as in the disease called Dry-rot.

The forms of roots depend upon the mode in which the

axis descends and branches. The mode of branching of roots is almost universally monopodial, only in Lycopodiacese is it dichotomous. When the central axis goes deep into the ground in a tapering manner, vithout dividing, a tap root is produced. This kind of root is sometimes shortened, and becomes succulent, forming the conical root of Carrot, or the fusiform or spindle-shaped root of Kadish, or the napiform root of Turnip. In ordinary forest trees the first

root protruded continues to elongate and forms a long