CAMBIUM Secondary Tissues.—In most of the modern Pteridophytes, Monocotyledons, and in annual plants among the Dicotyledons, there is no further growth of much structural importance after the tissues above considered have been differentiated. But in nearly all perennial Dicotyledons, and in all dicotyledonous and gymnospermous trees and shrubs, certain layers of cells retain the power of division, although they may be for long mature, and are apparently fixed elements of a tissue. By this power of growth and division they become the originators of new or secondary tissues, which bring in their train many modifications in the plant body. Primary tissues which are thus awakened to new divisional activity become, like the growing points of stems and roots, meri stems or the initials of tissues, but while the apical meristems of stems and roots give rise to the primary tissues above considered, these new meristems are secondary meristems since they give rise to secondary tissues. There are two chief secondary meri stems, the cambium and the phellogen.
The cambium typically continues the formation of xylem and phloem, thus adding to the conductive tissue already produced. It is merely the cycle or band of parenchymatous tissue lying between the primary xylem and primary phloem. Its cell walls are generally thin, and as its cells enlarge by special nourishment, they are divided principally by tangential walls, thus producing radial rows of cells. The primary xylem remains stationary, but as the cell rows thus formed increase in number and the in dividual cells enlarge, the primary phloem and all the external tissues move outwards. The tracts of cambium between the strands of xylem and phloem are called fascicular while those which cross the primary medullary rays from bundle to bundle are called interfascicular cambium. The cells of the inter fascicular cambium likewise divide mainly by tangential walls, and with the same frequency as the fascicular cambium. Thus radially within the interfascicular cambium the primary medullary rays are left stationary, while the surrounding tissue is pushed out wards. There is usually one cylindrical layer of actively dividing cells in the cambial cylinder, and it may be imagined that at any moment it has given rise by tangential division to two such layers. The products of the interfascicular cambium may first
be considered. The inner products differentiate as parenchyma tous cells which are added to the medullary rays; the outer prod ucts retain the power of division. Thus the cambium is moved slightly outwards and its cells divide as before by tangential walls into outer and inner layers. The inner layer thus formed differentiates as a parenchymatous addition to the medullary ray, and the outer layer retains the power of division. Thus a general outward movement of the cambium proceeds, leaving behind or within layers of cells which radially augment the medullary rays.
From time to time it is the outer layer of cells produced by cambial activity which becomes differentiated as parenchymatous cells, while the inner layer remains cambial. In this way the outward progress of the cambium is temporarily delayed but is not arrested. After a long period of cambial activity the inter fascicular cambium has moved outwards to a considerable dis tance, having greatly augmented the medullary ray within and radially, and to a lesser extent, but also radially, without. It will be apparent that the interfascicular cambium is not a fixed or permanent layer of cells, but that the power of division is steadily transferred outwards, and the locus of the cambium is changed with time. It will be further evident that with each step in the outward movement of the cambium the circumference of the cambium must increase. The division of the cambial cells is accordingly not accomplished solely by tangential walls, but from time to time radial divisions also occur. Thus the cambial cylinder is widened and its integrity is maintained as it moves outwards. But while in the vast majority of higher plants the additions to the medullary rays thus produced are uniformly parenchymatous, and these primary medullary rays increase outwards as wedges, there is some complication in the products of cambial division between the primary xylem and phloem. The fascicular cambium is generally moved outwards with the same rapidity as the inter fascicular cambium, so that the cylindrical form of the cambium is maintained.