Page:EB1911 - Volume 21.djvu/771

Rh from the outer sides of these initial cells. In most of the Phanerogams the apical (or primary) meristem, instead of consisting of a single apical cell or a group of initials, is stratified—i.e. there is more than one layer of initials (fig. 22). Throughout the Angiosperms the epidermis of the shoot originates from separate initials, which never divide tangentially, so that the young shoot is covered by a single layer of dividing cells, the dermatogen. Below this are the initials of the cortex and central cylinder. Whether these are always in layers which remain separate is not known, but it is certain that in many cases such layers cannot be distinguished. This, however, may be due to irregularity of division and displacement of the cells by irregular tensions destroying the obvious layered arrangement. In some cases there is a perfectly definite line of separation between the young cylinder (plerome) and young cortex (periblem), the latter having one or more layers of initials at the actual apex. This clear separation between periblem and plerome is mostly found in plants whose stem-apex forms a naked cone, the leaves being produced relatively late, so that the stele of the young stem is obvious above the youngest leaf-traces (fig. 22). Where the leaves are developed early, they often quite overshadow the actual apex of the stem, and, the rapid formation of leaf-tissue disturbs the obviousness of, and perhaps actually destroys, the stratified arrangement of the shoot initials. In this case also,

the differentiation of leaf-bundles, which typically begins at the base of the leaf and extends upwards into the leaf and downwards into the stem, is the first phenomenon in the development of vascular tissue, and is seen at a higher level than the formation of a stele. The latter is produced (except in cases of complete astely where a cylinder is never formed) after a number of leaf-traces have appeared on different sides of the stem so as to form a circle as seen in transverse section, the spaces intervening between adjacent bundles becoming bridged by small-celled tissue closing the cylinder. In this tissue fresh bundles may become differentiated, and what remains of it becomes the rays of the fully-formed stele. Many cases exist which are intermediate between the two extreme types described. In these the stele becomes obvious in transverse section at about the same level as that at which the first leaf-traces are developed. Where a large-celled pith is developed this often becomes obvious very early, and in some cases it appears to have separate initials situated below those of the hollow vascular cylinder. In some cases where there is apparently a well-marked plerome at the apex, this is really the young pith, the distinction between the stelar and cortical initials, if it exists, being, as is so often the case, impossible to make out. The young tissue of the stelar cylinder, in the case of the modified siphonostele characteristic of the dicotyledonous stem, differs from the adjoining pith and cortex in its narrow elongated cells, a difference produced by the stopping of transverse and the increased frequency of longitudinal divisions. This is especially the case in the young vascular bundles themselves (desmogen strands). The protoxylem and protophloem are developed a few cells from the inner and outer margins respectively of the desmogen strand, the desmogenic tissue left over giving rise to the segments of endocycle and pericycle capping the bundle. Differentiation of the xylem progresses outwards, of the phloem inwards, but the two tissues never meet in the centre. Sometimes development stops altogether, and a layer of undifferentiated parenchyma (the mesodesm) is left between them; or it may continue indefinitely, the central cells keeping pace by their tangential division with the differentiation of tissue on each side. In this case the formation of the primary bundle passes straight over into the formation of secondary tissue by a cumbium, and no line can be drawn between the two processes. The differentiation of the stelar stereom, which usually takes the form of a sclerized pericycle, and may extend to the endocycle and parts of the rays, takes place in most cases later than the formation of the primary vascular strand. In the very frequent cases where the bundles have considerable individuality, the fibrous “pericyclic” cap very clearly has a common origin from the same strand of tissue as the vascular elements themselves. In such cases it is part of the peridesm or sheath of elongated narrow-celled tissue surrounding the individual bundle.

The separation of layers in the apical meristem of the root is usually very much more obvious than in that of the stem. The outermost is the calyptrogen, which gives rise to the root-cap, and in Dicotyledons to the piliferous layer as well. The periblem, one cell thick at the apex, produces the cortex, to which the piliferous layer belongs in Monocotyledons; and the plerome, which is nearly always sharply separated from the periblem, gives rise to the vascular cylinder. In a few cases the boundaries of the different layers are not traceable. The protoxylems and the phloem strands are developed alternately, just within the outer limit of the young cylinder. The differentiation of metaxylem follows according to the type of root-stele, and, finally, any stereom there may be is developed. Differentiation is very much more rapid—i.e. the tissues are completely formed much nearer to the apex, than is the case in the stem. This is owing to the elongating region (in which protoxylem and protophloem alone are differentiated) being very much shorter than in the stem. The root hairs grow out from the cells of the piliferous layer immediately behind the elongating region.

The branches of the stem arise by multiplication of the cells of the epidermis and cortex at a given spot, giving rise to a protuberance, at the end of which an apical meristem is established. The vascular system is connected in various ways with that of the parent axis by the differentiation of bundle-connexions across the cortex of the latter. This is known as exogenous branch-formation. In the root, on the other hand, the origin of branches is endogenous. The cells of the pericycle, usually opposite a protoxylem strand, divide tangentially and give rise to a new growing-point. The new root thus laid down burrows through the cortex of the mother-root and finally emerges into the soil. The connexions of its stele with that of the parent axis are made across the pericycle of the latter. Its cortex is never in connexion with the cortex of the parent, but with its pericycle. Adventitious roots, arising from stems, usually take origin in the pericycle, but sometimes from other parts of the conjunctive.

In most of the existing Pteridophytes, in the Monocotyledons, and in annual plants among the Dicotyledons, there is no

further growth of much structural importance in the tissues after differentiation from the primary meristems. But in nearly all perennial Dicotyledons, in all dicotyledonous and gymnospermous trees and shrubs,