Page:Encyclopædia Britannica, Ninth Edition, v. 4.djvu/105

primary root-axis, whence secondary axes come off. In other plants, especially Monocotyledons, the primary axis soon dies and the secondary axes take its place. When the descending axis is very short, and at once divides into thin, nearly etpal, fibrils, the root is called fibrous, as in many Grasses (fig. GO) ; when the fibrils are thick and suc culent the root is fasciculated, as in Ranunculus Ficaria, rlate VII. Asphodelus luteus, and CEnanthe crocata ; when some of the fibrils arc developed in the form of tubercules, the root is tubercular, as in Orchis (fig. 61) ; when the fibrils enlarge in certain parts only, the root is nodulose, as in Spiraea Filipendula, or moniliform, as in Pelargonium triste, or anmdated, as in Ipecacuan. Some of these so-called roots are formed of a stem and root combined, and when cut in pieces they give rise to buds and new plants. In some cultivated plants, as Turnip, the central root is some times injured, so as to end abruptly, and it then divides into numerous branches, resembling a fasciculated root. This gives rise to the disease called Fingers and Toes, which is very injurious to the crop. Anbury is a disease where a clubbing of the root takes place. The mode in which the fibres of roots are produced and developed gives

origin to different forms of rhizotaxis, or root-arrangement.

Fig. 60.

Fig. Cl. Fig. G2. Fio. GO Fibrous root of a Grass. Numerous fibrils coming off from one point. FIG. Gl. Orchis, showing tuberculcs or tuberous roots, which contain a gummy matter called bassorin. Fio. 62. An epiphytic Orchid with pseudo-bulbs.

Roots either fix the plant in the soil or attach it to other bodies. They absorb nourishment by a process of imbibition or endosmose through their cellular extremities. The elongation of the roots by their extremities enables them to accommodate themselves to the soil, and allows the ex tremities of the rootlets to extend deeply without being injured. Hoots, in their lateral extension, bear usually a relation to the horizontal spreading of the branches, so as to fix the plant firmly, and to allow fluid nutritive sub stances to reach the absorbing extremities. As has been already stated, the structure of perennial roots is identical with that of the stem. Thus in Dicotyledons we find a pith, medullary rays, zones of wood, cambium layers, and bark, although no medullary sheath is present. In Mono cotyledons we have fibro-vascular bundles distributed in a matrix of cellular tissue. The young primary root in Monocotyledons differs from that in Dicotyledons in that it rises deeply within the embryonal tissue, and on germina tion this tissue is ruptured and forms a sheath, around the base of the roots, called coleorhiza. Amongst Monocotyledons the primary roots usually soon die, and secondary roots are formed in abundance. In vascular Dicotyledonous plants the structure of the root is similar to that of the stem. In Thallogens the roots consist merely of simple or branching filamentous hair-like structures. In some large tropical Sea weeds the root-like bodies develop to a large extent, but serve only as fixing organs, and take no share in nourishing the plant.

2. Stem or Ascending Axis.

A stem may be defined as an axis bearing leaves. Stum. Structurally it differs from a root in having no develop ment of cells forming a cap over the growing point. Under the term caulome (stem structure) are included all those parts of a plant morphologically equivalent in bearing leaves. The stem generally ascends, seeking air and light, and has therefore been termed the ascending axis. Stems have usually considerable firmness and solidity, but some times they are weak, and either lie prostrate on the ground, thus becoming procumbent, or climb on plants and rocks by means of rootlets, like the Ivy, being then called scandent, or twist round other plants in a spiral manner like Wood bine, when they are volubile. Twining plants turn either from right to left, as the French Bean, Convolvulus, Passion flower, Dodder, Periploca, and Gourd ; or from left to right Plate X. as Honeysuckle, Twin-ing Polygonum, Hop, and Tamus. Bryony tendrils twine from right to left, and left to right, alternately. In warm climates twining plants (lianas) often form thick woody stems ; while in temperate regions they are generally herbaceous. Exceptions, however, occur in the case of the Clematis, Honeysuckle, and Vine ; the twining stem of the vine has been called sarmentum. Some stems are developed more in diameter than in height, and present a peculiar shortened and thickened aspect, as Testudinaria or Tortoise-plant, Cyclamen, Melocactus, Echinocactus, and other Cactaceae ; while in many Orchids (fig. 62) the stem assumes an oval or rounded form, and is called a pseudo-bulb.

Names are given to plants, according to the nature and duration of their stems. Herbs, or herbaceous plants, have stems which die down annually. In some of them the whole plant perishes after flowering ; in others, the lower part of the stem forming the croiun of the root remains, bearing buds from which the stem arises next season. In what are called biennial herbs, the whole plant perishes after two years, while in perennial herbs the crown is capable of producing stems for many years, or new annual products are repeatedly added many times, if not indefinitely, to the old stems. The short permanent stem of herbaceous plants is covered partially or completely by the soil, so as to protect the buds. Plants producing permanent woody stems are called trees and shrubs. The latter are less than five times the height of a man, and produce branches from or near the ground ; while the former have conspicuous trunks, which attain at least five times the height of a man. Shrubby plants of small stature are called cinder-shrubs or bushes. The limits between those different kinds of stem are not always well defined ; and there are some plants occupying an intermediate posi tion between shrubs and trees, to which the name of arborescent shrubs is occasionally given. The stem receives the name of caulis in ordinary herbaceous plants which do not form a woody stem, culm in grasses, truncus in trees, caudex or stock in Palms and in some Ca-cti, and stipe in Ferns. The term haulm is probably a corruption of culm ; it is used by farmers to designate the stem of grasses and the herbaceous stems of plants. The stem is not always conspicuous. Plants with a distinct stem are called caulescent ; those in which it is inconspicuous are acaulescent, as the Primrose, Cowslip, Gentian, and Dandelion. A similar term is given in ordinary language to plants whose stems are buried in the soil, such as Cyclamen or Sowbread. Some plants are truly stemless, and consist only of expansions of cellular tissue representing stem and leaf, called a thallus, and hence are denominated Thallogens, or Thallophytes.

Stems have a provision for a symmetrical arrangement Leaf-buds