The cambium in the root, which is generally found in those plants which have a cambium in the stem, always arises in the conjunctive tissue internal to the primary phloem, and forms new (secondary) phloem in contact with the primary and secondary xylem internally. In roots which thicken but slightly, and whose cambium usually appears late, it is confined to these regions. If the development of secondary tissues is to proceed further, arcs of cambium are formed in the pericycle external to the primary xylem, and the two sets of cambial arcs join, forming a continuous, wavy line in transverse section, with bays opposite the primary phloems and promontories opposite the primary xylems. Owing to the resistance offered by the first formed secondary xylem, the bays are pushed outwards as growth proceeds, and the wavy line becomes a circle. Opposite the primary xylems the cambium either (a) forms parenchyma on both sides, making a broad, secondary (principal) ray, which interrupts the vascular ring and is divided at its inner extremity by the islets of primary xylem, or (b) forms secondary xylem and phloem, completing the vascular ring. In either case narrow secondary rays are formed at intervals, just as in the stem. Thus the structure of an old thickened root approximates to that of an old thickened stem, and so far as the vascular tissues are con cerned, can often be distinguished from the latter only by the position and orientation of the primary xylem.
The tissues above considered all owe their origin ultimately to the growing points or apical meristems de fined in the embryonic plant-body. To the great variety of apical meristems found in vascular plants reference alone can here be made. It must suffice to note that in most Pteridophytes there is a single large apical cell at the end of each stem- and root axis, that it usually has the form of a tetrahedron, that its base occupies the surface of the growing point, and that its apex points inwards. By definite and regular divisions of such a cell a mass of still undifferentiated cells is produced immediately behind the growing point, which is continuously occupied by a residual apical cell. From these undifferentiated cells the tissues above considered sooner or later arise by further growth, divisions and differenti ation. In most Phanerogams the apical (or primary) meristems, instead of consisting of single apical cells or of a definite group of initials, are stratified, i.e., there is more than one layer of initials. 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 the 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 irregularities of division and displacement of the cells by irregular tensions, destroying the obviously layered ar rangement. 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. The separation of layers in the apical meri stem of the root is usually much more obvious than in the stem. The outermost layer 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 the Monocotyledons ; and the plerome, which is nearly always sharply separated from the periblem, gives rise to the vascular system. i There is generally some definite progression in the actual dif ferentiation of the individual tissues, so that, for example, com paratively simple xylem and phloem elements may be recognized in certain portions of a stem or root before the differentiation of xylem and phloem is accomplished in the stele as a whole. Such first formed xylem and phloem elements are known respectively as protoxylem and protophloem, and serve for the initial con duction of food-materials, while the later formed elements of the primary xylem and phloem are called respectively metaxylem and metaphloem.
The branches of the stem arise by multiplication of the cells of the epidermis and cortex at a given point, thus forming a protuberance at the end of which an apical meristem is organized, The vascular system is connected in various ways with that of the parent axis by the differentiation of bundles 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 outward path for which is prepared by digestion of the surround ing tissues. The connections of the stele of such a root with that of the parent axis are made across the pericycle of the latter. The cortex of the new root is never connected with that of the parent, but with its pericycle. Adventitious roots, arising from the stem, usually take origin in the pericycle, but sometimes from other parts of the conjunctive tissue.
Thus it will be seen that throughout the vascular plants there are definite and orderly methods of production and distribution of the tissues destined to play their parts in the major or minor processes of life, and that such organisms are well fitted for life on land. (J. McL. T.)