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acute point. They may be, not improperly, considered as very compound hardened hairs. They have no connection with the woody tissue, by which character they are obviously distinguished from spines, of which mention will be made under the head of branches; but are a developement of the epiphlæum of the bark. According to Dutrochet (Mémoires, i. 174.), it is exclusively by the base where the epiphloum and prickle are in contact, that the developement takes place of cells to increase the prickle in size. In the Rose the prickle is formed in one year, and afterwards dies. In Xanthoxylon juglandifolium it is the produce of two or three years' growth, according to the last mentioned author. Prickles are found upon all parts of a plant, except the stipules and stamens. They are very rarely found upon the corolla, as in Solanum Hystrix; their most usual place is upon the stem, as in Rosa, Rubus, &c.
SECT. II. Of the Stem, or Ascending Axis.
WHEN a plant first begins to grow from the seed, it is a little body called an embryo, with two opposite extremities, of which the one lengthens in the direction of the earth's centre, and the other, taking a direction exactly the contrary, extends upwards into the air. This disposition to develope in two diametrically opposite directions is found in all seeds, properly so called, there being no known exception to it; and the tendency is moreover so powerful, that, as we shall hereafter see (Book II.), the most powerful external influence is rarely sufficient to overcome it. The result of this developement is the axis, or centre, round which the leaves and other appendages are arranged. That part which forces its way downwards constantly avoiding light, and withdrawing from the influence of the air, is the descending axis, or the root; and that which seeks the light, always striving to expose itself to the air, and expanding itself to the utmost extent of its nature to the solar rays, is the ascending axis, or the stem, The only exception to this is when the embryo first begins to grow. At that time the first part of the axis formed below
the cotyledons belongs to the stem, and it is only after the first joint of the stem, however minute and short it may be, is completed, that a root is formed. This will be more particularly explained hereafter; see Section XIV. of this chapter. As the double elongation just mentioned exists in all plants, it follows that all plants must necessarily have, at an early period of their existence at least, both stem and root; and that, consequently, when plants are said to be rootless, or stemless, such expressions are not to be considered physiologically correct.
The STEM has received many names; such as caudex ascendens, caudex intermedius, culmus, stipes, truncus, and truncus ascendens. It consists of bundles of vascular and woody tissue, embedded in cellular substance in various ways, and the whole enclosed within an epidermis. The manner in which these parts are arranged with respect to each other will be explained hereafter. The more immediate subject of consideration must be those organs which are common to all stems.
1. Of its Parts.
Where the stem and root, or the ascending and descending axis diverge, there commences in many plants a difference of anatomical structure, and in all a very essential physiological dissimilarity; as will be hereafter seen. This portion of the axis is called the neck or collum, (coarcture of Grew, nœud vital of Lamarck, limes communis, or fundus plantæ, of Jungius,) and has been thought by some to be the seat of vegetable vitality; an erroneous idea, of which more will be said in the next book. At first it is a space that we have no difficulty in distinguishing, so long as the embryo, or young plant, has not undergone any considerable change; but in process of time it is externally obliterated; so that in trees of a few years' growth its existence becomes a matter of theory, instead of being actually evident to our senses.
Immediately consequent upon the growth of a plant is the formation of leaves. The point of the stem from whence these arise is called the node (geniculum, Jungius), and the space
between two nodes is called an internode (merithallus, Du Petit Thouars). In internodes the arrangement of the vascular and woody tissue, of whatever nature it may be, of which they are composed, is nearly parallel, or, at least, experiences no horizontal interruption. At the nodes on the contrary, vessels are sent off horizontally into the leaf; the general developement of the axis is momentarily arrested while this horizontal communication is effecting, and all the tissue is more or less contracted. In many plants this contraction, although it always exists, is scarcely appreciable; but in others it takes place in so remarkable a degree as to give their stems a peculiar character; as, for instance, in the Bamboo, in which it causes diaphragms that continue to grow and harden, notwithstanding the powerfully rapid horizontal distention to which the stems of that plant are subject. In all cases, without exception, a leaf-bud or buds is formed at a node immediately above the base of the leaf; generally such a bud is either sufficiently apparent to be readily recognised by the naked eye, or, at least, it becomes apparent at some time or other; but in certain plants, as Heaths, the buds are often never discoverable; nevertheless, they always exist, in however rudimentary a state, as is proved by their occasional developement under favourable or uncommon circumstances. By some writers nodes, upon which buds are obviously formed, are called compound, or artiphyllous; and those in which no apparent buds are discoverable, are named simple, or pleiophyllous; they are also said to be divided, when they do not surround the stem, as in the apple and other alternate-leaved genera; or entire, when they do surround it, as in grasses and umbelliferous plants: they are further said to be pervious, when the pith passes through them without interruption; or closed, when the canal of the pith is interrupted, as if by a partition. Pervious and divided, and closed and entire nodes usually accompany each other. For other remarks upon this subject, see Link's Elementa, and the Appendix to this volume.
All the divisions of a stem are in general terms called branches (rami); but it is occasionally found convenient to designate particular kinds of branches by special names. Thus, the twigs, or youngest shoots, are called ramuli, or
branchlets, and by the older botanists flagella; the assemblage of branches which forms the head of a forest tree is called the coma: cyma is sometimes used to express the same thing, but improperly. Shoots which have not completed their growth have received the name of innovations, a term usually applied in mosses. When such a shoot is covered with scales upon its first appearance, as the Asparagus, it is called turio: by the old botanists all such shoots were named asparagi. When a shoot is long and flexible, it receives the name of vimen. This word, however, is seldom used; its adjective being employed instead: thus, we say, rami viminei, or caulis vimineus; and not vimen. From this kind of branch, that called a virgate stem, caulis virgatus, differs only in being less flexible. A young slender branch of a tree or shrub is sometimes named virgultum. When the branches diverge nearly at right angles from the stem, they are said to be brachiate. Small stems, which proceed from buds formed at the neck of a plant without the previous production of a leaf, are called cauliculi.
Link calls a stem which proceeds straight from the earth to the summit, bearing its branches on its sides, as Pinus, a caulis excurrens, and a stem which at a certain distance above the earth breaks out into irregular ramifications, a caulis deliquescens.
From the constitution and ramifications of their branches, plants are divided into trees, shrubs, and herbs. If the branches are perennial, and supported upon a trunk, a tree (arbor) is said to be formed; for a small tree, the term arbusculus is sometimes employed. When the branches are perennial, proceeding directly from the surface of the earth without any supporting trunk, we have a shrub (frutex or arbustum, Lat.), which occasionally, when very small, receives the diminutive name of fruticulus. If a shrub is low, and very much branched, it is often called dumosus (subst. dumus). The suffrutex, or under-shrub, differs from the shrub, in perishing annually, either wholly or in part; and from the herb, in having branches of a woody texture, which frequently exist more than one year: such is the Mignionette (Reseda odorata) in its native country, or in the state in which it is known in
gardens as the Tree Mignionette. The under-shrub is exactly intermediate between the shrub and the herb. All plants producing shoots of annual duration from the surface of the earth are called herbs.
Some botanists distinguish two sorts of stems, the characters of which are derived from the mode of growth. When a stem is never terminated by a flower bud, nor has its growth stopped by any other organic cause, as in Veronica arvensis, and all perennial and arborescent plants, it is said to be indeterminate; but when a stem has its growth uniformly stopped at a particular period of its existence by the production of a terminal bud, or by some such cause, it is called determinate. The capitate and verticillate species of Mint owe their differences to causes of this nature; the stem of the former being determinate, the latter indeterminate.
The point whence two branches diverge is called the axil, or, in old botanical language, the ala.
Leaf-buds (Gemma, Linn.), being the rudiments of young branches, are of great importance in regard to the general structure of a plant. They consist of scales imbricated over
each other, the outermost being the hardest and thickest, and surrounding a minute cellular axis, or growing point, which is in direct communication with the woody and cellular tissue of the stem. In other words, they may be said to be growing points covered with rudimentary leaves for their protection,