Moreover, of the inner products of division in the fascicular cambium many differentiate as segments of a vessel, tracheids, fibres or parenchyma, which may be variously grouped, but which collectively constitute the secondary xylem. In many instances sooner or later one or more cambial cells, within which radially such elements have differentiated, produce exclusively radial rows of parenchyma. These rows have in general the characters of medullary rays, but are obviously purely of secondary origin, and continue neither inwards to the pith nor outwards to the cortex like the primary rays. They are indeed secondary rays of purely cambial origin. Of the outer products of division in the fascicular cambium many differentiate as sieve-tubes and compan ion cells, or as fibres or parenchyma arranged in various patterns and collectively constituting secondary phloem. With this tissue may also be associated minor secondary rays, reaching inwards to the cambium, often the direct radial continuations of similar rays in the xylem, but not reaching the cortex without. It will be appar ent that the tangential dimension of each tract of fasicular cam bium is steadily increased as the cambium moves outwards, so that the secondary wood increases as an outward widening wedge, while the secondary phloem appears as an inward widening wedge, the two wedges meeting at the cambium. It is not to be supposed that at any moment uniform behaviour is maintained by all the cambial cells in a longitudinal row, so that while at one level, and at various points in the fascicular cambium, secondary medullary rays are being formed in both xylem and phloem, immediately above and below this level, fibres, vessel-segments, sieve-tubes, wood-parenchyma and phloem-parenchyma are being produced. The secondary medullary rays are accordingly of no great depth, and may well be described as narrow bands of parenchyma placed radially in the xylem and phloem. Similarly the behaviour of the interfascicular cambium is not uniform at all levels or at all stages in its outward progress, for while for a time it may produce parenchyma exclusively, locally its products may be secondary phloem and secondary xylem. Thus of ter a time an island of secondary wood and phloem is seen in a cross section of the stem, dividing the primary ray into two branches, which, when followed outwards, reach the cortex. Nor are the primary rays of great depth, though on an average they are much deeper than the secondary rays, for the downward course of both xylem and phloem is tortuous, and fusion of strands occurs above and below the rays, which are accordingly of lenticu lar outline when seen in a tangential section of the stem.
In those plants whose annual activity is interrupted by a regular winter or dry season the limit of each year's increment of secondary wood, and to a less obvious degree of secondary bast, is marked by a more or less distinct line which is produced by the sharp contrast between the elements formed in the late summer of one year and those produced in the spring of the next. It is believed that in relation to the large and rapid supply of water and other substances to the unfolding leaves in spring and early summer, large vessels are produced by the cambium in the spring, while as summer advances and the need for water diminishes, the vessels formed are narrower, and as the supply diminishes still further, more abundant fibres, with greatly reduced cavities, arise. Thus in a single season of cambial
activity a rough outward progression may be traced in the secondary wood of that season from wide vessels to narrow fibres, beyond which the transition to the wide vessels of the succeeding spring is sudden. Each zone thus recognized in a cross section of the stem is called an annual ring, and the lines of separation of successive rings mark the temporary halting places of the cambium at the end of successive seasons.
The older wood of large trees, forming a cylinder in the centre of the trunk, frequently under goes marked changes in character. The wood-parenchyma and medullary rays die, and the walls of all the cells often become greatly hardened, owing to the deposit in them of special sub stances. Wood thus altered is called heart-wood or duramen, as distinguished from the younger sap-wood or alburnum, which is nearer the cambium, carries on the active function of conduction, and retains its parenchymatous cells in life. The heart-wood ceases to be of any use to the tree except as a support, and owing to its dryness and hardness is much used for industrial purposes.
It will be obvious that secondary increase of xylem and phloem involves an outward movement of the primary phloem and of all the elements which surround the latter, and that the cortex, and particularly the epidermis, must accommodate for this movement, either yielding passively to a limit and being injured or destroyed by the outward pressure, or being to some extent modified so as to persist. In most cases the epidermis is unable to maintain the active radial division of its cells necessary for such expansion as occurs, is soon stretched to its limit, dies and is destroyed. Extensive radial division in the cortical cells is progressively less essential as the inner cortex is followed inwards, and as a rule, by a combination of radial and tangential divisions, the integrity of the cortex is maintained. Towards the periphery of the cortex, and commonly by modifica tion of its outer layer, a secondary meristem, similar to the cambium, arises and produces external and internal secondary tissues. This meristem is the phellogen, and the whole of the tissue it gives rise to is known as periderm. The phellogen derives its name from the fact that its external product is the character istic tissue known as cork. This consists typically of closely fitting layers of cells which quickly die, and when mature, have completely suberised walls which collectively serve to replace the epidermis as an external protective layer, when the former is ruptured. The outer layers of the cork are constantly being destroyed, but new layers are in progress of formation within. The internal tissue formed by the phellogen is known as phello derm; it augments the cortex, and typically consists of living parenchymatous cells to some extent capable of further division. Indeed, an inward succession of phellogens may arise in the phelloderm and in even more deeply seated living layers. In certain cases the epidermis becomes the phellogen. To the great activity of the phellogen, and to the power of formation of a series of phellogens in deep layers, the value of the cork oak as a source of cork is due.