on the surface. They also grow by additions within their extremities, and each is protected by a pileorhiza, and has at its base a coleorhiza. They have, under ordinary circumstances, a similar structure to that of true roots.

Adventitious roots generally, like true roots, have no leaves or buds, and when subterranean have no epidermis. furnished with stomata; hence, when derived from Dicotyledons, they are distinguished from the stem by the same characters as that of the true root. The adventitious roots of Monocotyledons and Acotyledons have a similar structure to their respective stems, as will be afterward noticed.

Some adventitious roots, such as those called aërial, require a more particular notice. The simplest forms of aërial roots are seen in the Ivy, and some other climbing plants, where they are essentially intended for mechanical support. But in many other plants the aërial roots which are given off by the stems or branches descend to the ground, and, fixing themselves there, not only act as mechanical supports, but also assist the true root in obtaining food.

Epiphytes. In some plants none but aërial roots are produced, and as these never reach the soil they can not obtain any food from it, but must therefore draw their food entirely from the air in which they are developed; hence the name of air-plants which is applied to them. They are also called epiphytes, because they usually grow upon other plants. The roots of such plants are commonly green, and possess a true epidermis-that is, one with stomata; in which particulars, therefore, aërial roots present exceptions to what is generally observed in other

roots.

Parasites.-There are also plants which not only grow upon others, but which, instead of sending their roots into the air and deriving their food from it, as is the case with the epiphytes, send them into the tissues of the plants

upon which they grow, and obtain nutriment from them. The plant which they thus penetrate and feed upon is termed their host. These parasites are of various natures ; some have green foliage, while many others are pale, or brownish, or possess other tints than green.

Parasitical plants vary in the degree of their parasitism; the greater number are, so far as their roots are concerned, entirely dependent upon the plants on which they grow for their food. Others obtain their food at first by means of ordinary roots contained in the soil; but, after having arrived at a certain age, these perish, and they then derive their food entirely from roots which penetrate the plants upon which they grow; others, again, continue throughout their life to derive a portion of their food by means of roots imbedded in the soil.

When green, they obtain a portion of their food, like ordinary plants, from the air; but, if of other colors than green, all their food is derived from the plants on which they grow. It must also necessarily happen that parasites, by living partially or entirely upon those on which they are placed, frequently injure, and even destroy them, and in this way great damage is done to Clover, Flax, and other crops in this country and elsewhere.

Besides the parasites just described, there is also another class of plants called saprophytes, which, while agreeing with ordinary parasites in deriving their food from already formed organic material, differ from this latter class in growing on dead organic substances, and therefore assimilating such matter which is in a state of decomposition or decay.

The Roots of Dicotyledonous, Monocotyledonous, and Acotyledonous Plants have certain distinctive characters, which may be briefly summed up as follows:

1. The Root of Dicotyledonous Plants is formed, as we have seen (page 61), by the direct elongation of the

ROOT-DEVELOPMENT.

67

radicle of the embryo from the formation of new tissue just within its apex. Such a mode of root-development has been called exorhizal, and a root thus formed is termed a true root. It follows from this mode of development that the root of a Dicotyledonous plant generally grows downward, for some distance at least, and hence forms a main trunk or descending axis (Fig. 112, r), from which branches are given off in various directions. Such a root is termed a tap-root, and we have seen (page 62) that it has commonly no pith or medullary sheath. There are many plants, however, such as the Horse-chestnut, I where the pith is prolonged downward for some distance into the root.

-

2. The Root of Monocotyledonous Plants. In these plants the radicle does not itself, except in rare cases, become prolonged to form the root, but gives off above its base one or more branches of equal size, which separately pierce the radicular extremity of the embryo, and become the roots (Fig. 111, r); each of these roots is surrounded at its base, where it pierces the integuments, with a kind of cellular collar termed the coleorhiza, co. Such a mode of root-development has been termed endorhizal. The roots of Monocotyledo

nous plants are therefore commonly FIG. 112.-Lower part of the adventitious or secondary.

It rarely happens that the plants of this class have tap-roots, but they have instead a variable number of roots of nearly equal size, termed compound.

C.

stem and root of the common Stock. r. The taproot with its branches. The base of the root or point of union between the stem and root, which has been termed the neck. t. The stem. f,f. Leaves. b, b. Buds in process of development into branch

es.

Aërial roots are very common in Monocotyledonous plants. In its internal structure the root of a Monocotyle

68

FORMS OF ROOTS.

don corresponds to that of the stem in the same class of plants.

3. The Root of Acotyledonous Plants.-Such plants have no true seeds containing an embryo, but are propagated by spores, from which roots are developed in a very irregular manner; and hence this mode of root-development has been called heterorhizal. Such roots are therefore adventitious; and resemble those of Monocotyledonous plants in being compound. Aërial roots are also very common in Acotyledons; indeed, in Tree-ferns as in many Palms, these roots are so abundant at the base of the stem, that they sometimes double, triple, or still further increase its normal thickness, and hence give to the lower part of such stems a conical form. The internal structure of the root of Acotyledons in all essential characters resembles that of the stem in the same class of plants.

Forms of Roots.-When some of the fibers of fibrous roots become enlarged, the expansions so formed are called tubercules (not tubers), and these tubercules receive names descriptive of the forms assumed. They may be hand-shaped, or palmate, in tufts, or fasciculate, ring-like, or annulate, irregularly enlarged at the end, or nodulose. The enlargement of a tap-root may be conical, as in the carrot, fusiform, as in the radish, or napiform, as in the common turnip. Familiarity with root-forms is best gained by observations in the field, the garden or the market.

SECTION 3. THE LEAF OR PHYLLOME.

I. General Description and Parts of the Leaf. -The leaf may be defined as a lateral development of the stem or branch; it is formed of similar structures, and these are in direct connection one with the other. The leaf is therefore an appendicular organ of the stem, but it differs from it in the order of its development; in the stem or branch the apex is the youngest part, while in

THE LEAF OR PHYLLOME.

69

the leaf the apex is first formed and is gradually pushed outward by the formation of the other parts between it and the stem; in many plants the leaves are thick and fleshy, when they are said to be succulent. In the scales of the bud, the thin membranous coverings of tunicated bulbs and corms, the fleshy scales of bulbs, etc., they are colorless, or of a brownish color, and of simple structure; they are then termed scales or cataphyllary leaves, the ordinary leaves being called foliage leaves.

The leaf has commonly only two surfaces, but when succulent it has frequently more than two. The terms upper and lower are applied to the two surfaces of ordinary leaves, because in by far the greater number of plants such leaves are placed horizontally, so that one surface is turned upward, and the other downward. There are certain leaves, however, which are placed vertically, in which case the margins are turned upward and downward instead of the surfaces. The angle formed by the union of the upper surface of the leaf with the stem is called the axil, and everything which arises out of that point is said to be axillary to the leaf; or, if from the stem above, or below the axil, it is extra-axillary; or, as more generally described when above, supra-axillary, if below, infra-axillary.

Leaves commonly fall off after they have performed their functions, but their duration varies in different plants. When the leaf lasts throughout the season, it is deciduous; or if beyond a single season, or until new leaves are formed, so that the plant is never without leaves, it is persistent, and the plant is called evergreen.

When a leaf separates from the stem, without leaving any trace of its existence, except a scar (Fig. 98, b, b) at the point of disruption, it is said to be articulated; or if it decays gradually upon it, so that portions remain upon the stem for some time, it is non-articulated.

The petiole is frequently more or less contracted at