Page:EB1911 - Volume 21.djvu/770

ANATOMY] richly branched tubes which penetrate the other tissues of the plant mainly in a longitudinal direction. They possess a delicate

layer of protoplasm, with numerous small nuclei lining the walls, while the interior of the tube (corresponding with the cell-vacuole) contains a fluid called latex, consisting of an emulsion of fine granules and drops of very various substances suspended in a watery medium in which various other substances (salts, sugars, rubber-producers, tannins, alkaloids and various enzymes) are dissolved. Of the suspended substances, grains of caoutchouc, drops of resin and oil, proteid crystals and starch grains may be mentioned. Of this varied mixture of substances some are undoubtedly plastic (i.e. of use in constructing new plant-tissue), others are apparently end-products of metabolism, in other words excreta, though they are not actually cast out from the plant-body. The relation of the laticiferous tissue to the assimilating cells under which they often end, and the fact that where this tissue is richly developed the conducting parenchyma of the bundles, and sometimes also the sieve-tubes, are poorly developed, as well as various other facts, point to the conclusion that the laticiferous system has an important function in conducting plastic substances, in addition to acting as an excretory reservoir. As a secondary function we may recognize, in certain cases, the power of closing wounds, which results from the rapid coagulation of exuded latex in contact with the air. The use of certain plants as rubber-producers (notably Hevea brasiliensis, the Para rubber-tree) depends on this property. The trees are regularly tapped and the coagulated latex which exudes is collected and worked up into rubber. Opium is obtained from the latex of the opium poppy (Papaver somniferum), which contains the alkaloid morphine. Laticiferous tissue is of two kinds: (1) laticiferous cells (coenocytes) (fig. 19) which branch but do not anastomose, and the apices of which keep pace in their growth with that of the other tissues of the plant (Apocynaceae, most Euphorbiaceae, &c.); (2) laticiferous vessels (fig. 20) which are formed from rows of meristematic cells, the walls separating the cells breaking down, so that a network of laticiferous tubes arises (Papaveraceae, Hevea, &c.). In some cases (Allium, Convolvulaceae, &c.) rows of cells with latex-like contents occur, but the walls separating the individual cells do not break down.

The body of a vascular plant is developed in the first place by repeated division of the fertilized egg and the growth of

the products of division. The body thus formed is called the embryo, and this develops into the adult plant, not by continued growth of all its parts as in an animal, but by localization of the regions of cell-division and growth, such a localized region being called a growing-point. This localization takes place first at the two free ends of the primary axis, the descending part of which is the primary root, and the ascending the primary shoot. Later, the axis branches by the formation of new growing-points, and in this way the complex system of axes forming the body of the ordinary vascular plant is built up. In the flowering plants the embryo, after developing up to a certain point, stops growing and rests, enclosed within the seed. It is only on “germination” of the latter that the development of the embryo into the free plant is begun. In the Pteridophytes, on the other hand, development from the egg is continuous.

The triple division of tissues is laid down in most cases at a very early period of development—in the flowering plants usually before the resting stage is reached. In many Pteridophytes the first leaf is formed very early, and the first vascular strand is developed at its base, usually becoming continuous with the cylinder of the root; the strand of the second leaf is formed in a similar way and runs down to join that of the first, so that the stem stele is formed by the joined bases of the leaf-traces. In other cases, however, a continuous primitive stele is developed, extending from the primary stem to the primary root, the leaf-traces arising later. This is

correlated with the comparatively late formation and small development of the first leaves. The evidence scarcely admits of a decision as to which of these methods is to be regarded as primitive in descent. In the seed-forming plants (Phanerogams) one or more primary leaves (cotyledons) are already formed in the resting embryo. In cases where the development of the embryo is advanced at the resting period, traces run from the cotyledons and determine the symmetry of the stele of the primitive axis, the upper part of which often shows stem-structure, in some respects at least, and is called the hypocotyledonary stem or hypocotyl, while the lower part is the primary root (radicle). In other cases the root structure of the stele continues up to the cotyledonary node, though the hypocotyl is still to be distinguished from the primary root by the character of its epidermis. On germination of the seed the radicle first grows out, increasing in size as a whole, and soon adding to its tissues by cell division at its apical growing-point. The hypocotyl usually elongates, by its cells increasing very greatly in the longitudinal direction both in number and size, so that the cotyledons are raised into the air as the first foliage-leaves. Further growth in length of the stem is thenceforward confined to the apical growing point situated between the cotyledons. In other cases this growing-point becomes active at once, there being little or no elongation of the hypocotyl and the cotyledon or cotyledons remaining in the seed.



(After Sachs. From Vines' Text-Book of Botany, by permission.) 20.—Laticiferous Vessels from the cortex of the root Scorzonera hispanica, tangential section.

A, Slightly magnified. B, A small portion highly magnified. The structure of the growing-points or apical meristems varies much in different cases. In most Pteridophytes there is a single

large apical cell at the end of each stem and root axis. This usually has the form of a tetrahedron, with its base occupying the surface of the body of the axis and its apex pointing towards the interior. In the stem, segments are successively cut off from the sides of the tetrahedron, and by their subsequent division the body of the stem is produced. In the root exactly the same thing occurs, but segments are cut off also from the base of the tetrahedron, and by the division of these the root-cap is formed (fig. 21). In both stem and root early walls separate the cortex from the stele. The epidermis in the stem and the surface layer of the root soon becomes differentiated from the underlying tissue. In some Pteridophyte stems the apical cell is wedge-shaped, in others prismatic; in the latter case segments are cut off from the end of the prism turned towards the body of the stem. In other cases, again, a group of two or four prismatic cells takes the place of the apical cell. Segments are then cut off