1044. The compound organs of flowering plants are easily recognisable, being the roots, stems, branches, leaves, and other larger parts of plants; and also the cuticle, hairs, scales, glands, prickles, and other smaller parts, which are found on the larger. All the different parts of the flower and fruit are of course included in the compound organs; and, in short, every combination of vegetable tissue, which assumes an uniform appearance in a plant, and is, at the same time, a distinct part of it as a form, and not as a mere mass of tissue. The axis of a plant is a term that ought to be understood by the young gardener. In a seedling just begun to grow, an elongation takes place at both extremities: the root, which strikes downwards in the direction of the earth's centre, is called the descending axis; and the stem, which shoots upwards in a contrary direction, is the ascending axis. The point between the two is called the crown or collar. Besides the item and root, plants have a third movement, which acts horizontally and forms the medullary system. Thus, 'when the vital action of either spore, embryo, or bud is excited, the tissue developes in three directions: upwards, downwards, and horizontally.' (Elem. of Bot. p. 24.) The root is useful not only to fix the plant in the earth, but to act as a reservoir of starchy and glutinous matter for the support of the plant, and to absorb additional nourishment for it from the earth, by means of its spongioles. Roots are always extensions of the woody tissue, and they have no leaf-buds. A leaf bud, says Dr. Lindley, 'is a young plant produced without the agency of sexes, inclosed within rudimentary leaves called scales, and developed on the outside of a stem.' (Ibid.) 'The stem is produced by the successive development of leaf-buds, which cause a corresponding horizontal growth between them.' (Ibid. p. 28.) Stems are of four kinds: - Exogenous, which increase by successive layers on the outside of the wood ; Endogenous, which become more solid from additions in the centre; Acrogenous, which are formed by the union of the bases of leaves, and the extension of the point of the axis; and Thallogenous, where no leaves or buds exist, and the stem increases by simple elongation or dilation. 'In what are called Dictyogens, the stem has the structure of Endogens, and the root nearly that of Exogens.' (Ibid.) Nodes are the places in a stem where leaves are developed, and buds are formed ; and inter-nodes are the spaces between the nodes. Regular buds are found only in the axils of the leaves ; but sometimes leaf-buds 'are found among the tissue of plants subsequently to the developement of the stem and leaves, and, without reference to the latter, are called latent, adventitious, or abnormal.' (Ibid. p. 41.) 'The manner in which the leaves are folded in the leaf-bud varies in different species, and is called the vernation of the plant.' (Ibid. p. 42.) Leaves are of the greatest importance to plants, as it is in them that most of the chemical changes in the fluids take place; and hence, as Dr. Lindley observes, it is absolutely necessary to expose 'them to the full influence of light and air, for the purpose of securing a due execution of their natural functions, Hence, also, the impropriety of mutilating plants by the destruction of their leaves. It is not, however, to be understood that assimilation is dependent upon the mere number of leaves. Many small ill-formed leaves may he of less value to a plant than a few large healthy leaves. Secreting power is in proportion to the area of foliage, its health, and its proper exposure to light and air, especially to air in motion.' (Ibid. p. 57.)
