6oo SECONDARY CHANGES. 



In addition to the phenomena of secondary cambial formation above described, there 

 occurs in the Beet the above-mentioned phenomenon of growth of the older paren- 

 chyma, even at a distance from the active cambium : this process influences the definitive 

 structure. Even in young roots, in which the formation of the innermost zone of 

 increase has begun, it may not unfrequently be seen that the connecting cells (comp. 

 P- 351), which separate the primary plate of xylem from the secondary strands of wood, 

 are transversely extended, divide, and thus give rise to a band of parenchyma, which 

 intervenes between the strands of secondary wood and the primary plate, and separates 

 the two parts one from another. This phenomenon may appear on both or only one 

 face of the plates. With the further growth of the root the intervening band may 

 increase to a width of 2°"". It is continuous externally into the simultaneouily 

 grotuing parenchyma around it, especially of the chief medullary rays. Also the other 

 internal parenchyma, both that belonging to the xylem and that which forms the greater 

 part of the zones of bast, has a growth accompanied by slow cell division, and continuing 

 long after the appearance of the next outer cambial ring: the limits of this growth 

 have not been exactly ascertained. In consequence of the growth of the zones of bast: 

 in a radial direction, the successive rings of wood are separated more and more from one 

 another; and the increase in volume of the parenchyma in the individual ring of wood 

 separates the non-parenchymatous elements of it one from another, both in the direction 

 of the radius and of the periphery; especially the meshes, which are formed by the 

 bundles, are widened by dilatation of the medullary rays. It is uncertain whether 

 meanwhile the non-parenchymatous elements of the wood also grow, as in the cortical 

 bundles of Cycas (Sect. 195). 



4. In the stem of Phytolacca dioica the beginning of the secondary thickening is 

 naturally different from that in the roots. The first cambial ring appears normally 

 in the primary ring composed of the bundles of the leaf-trace (comp. p. 249), together 

 with intermediate bundles. The periphery of this, marked by the outer margins of the 

 primary phloem-bundles, is immediately surrounded externally by some 2-3 layers of 

 parenchyma ; further outwards there follows an almost closed ring of bast fibres, next 

 to which is the large-celled parenchyma of the outer cortex. The ring of bast-fibres is 

 to be regarded as the limit of the layer of primary bast. The layers of parenchyma just 

 mentioned, which adjoin it internally, show for the future, at least for the most part, 

 growth by dilatation like the outer cortex. But the parenchymatous cells, which succeed 

 them internally, and which accordingly belong to the margin of the phloem-bundle, 

 begin, when the increase derived from the first cambial ring has gone on for some time, 

 to extend greatly in a radial direction, and to divide tangentially. These processes 

 arise at certain not exactly defined points of the periphery, and extend laterally from 

 them over medullary rays and bundles, so as to form an annular layer showing the above 

 phenomena. As far as has been investigated, this is derived from only one, or at most 

 here and there from two layers of cells. After several successive tangential divisions 

 the division stops at the inner side of the layer in question, their products grow into 

 parenchymatous cells much extended in a radial direction : these retain with considerable 

 accuracy the arrangement in the radial series in which they arose. In the outer part of 

 the annular zone the tangential division continues, the beginnings of corresponding 

 groups of wood and bast, i.e. of vascular bundles, alternating with medullary rays, 

 appear arranged in a ring, which, like the first, increases normally by means of a normal 

 cambium. At the outer limit of this second outer ring there arises later, in the same way 

 as it arose, a third ring, and a layer of parenchyma separating it from the second; the 

 same process may repeat itself through higher orders of rings. In each ring the formative 

 activity of the cambium ceases almost simultaneously with the appearance of the next 

 outer ring. Thus arise the often described concentric rings of vascular bundles, 

 separated from one another by broad zones of parenchyma, the number of which 

 annually formed may rise in a strong branch or stem to six and more. The rings 

 are in this case often still more incomplete and irregular than in the root of Beta, 



