520 SIGNIFICANCE OF CAMBIUM 



of the tissue of an adult individual. The cambial initials are of two 

 distinct shapes and sizes: (1) numerous, large, much elongated (paral- 

 lel to the long axis of the stem or root) elements; and (2) scattered 

 aggregations of small, more or less isodiametric cells which divide to 

 form the horizontal sheets of radially disposed parenchyma, the 

 so called medullary rays. The principal divisions in both types of 

 initials are periclinal or parallel to tangents to the circumference of 

 the stem or root. In other words, the large cells divide in a tangen- 

 tial longitudinal plane, which is a division plane of maximal area, 

 whereas the ray initials form partitions that commonly are surfaces 

 of minimal area. 



The tangential diameter of the cambial initials increases to a cer- 

 tain extent during the earlier stages of the enlargement of stems 

 and roots, but falls far short of being sufficient to compensate for 

 the rapid increase in the periphery of the cambium. Nageli inferred 

 from this that the elongated initials must divide periodically in a ra- 

 dial longitudinal plane. He even elaborated formulas for computing 

 the frequency of such divisions during a given increase in the radius 

 of a stem. However, as is not infrequently the case with a priori 

 mathematical deductions concerning complex biological phenomena, 

 Nageli's generalization is supported by few, if any, of the actual facts. 

 Although the hypothetical radial longitudinal divisions are described 

 and figured in many botanical text-books, I have been unable to find 

 them in any of the gymnosperms and less highly specialized dicoty- 

 ledons that I have studied. The cells slowly elongate, sHding by 

 one another, until they have reached a certain size. They then divide 

 by means of a more or less oblique transverse partition into two short 

 halves, which in turn elongate and divide.^ Thus, the increase in the 

 periphery of the cambium is due primarily not to radial longitudinal 

 divisions of the large initials accompanied by lateral enlargement of 

 the products of such divisions, but to the formation of transverse par- 

 titions, followed by elongation or longitudinal "sliding growth. "^ 



^ It should be kept clearly in mind, in this connection, that during the process 

 of elongation, between successive transverse divisions, the cells continue to 

 divide in the tangential longitudinal plane. 



^ This phenomenon of sliding growth is of considerable practical significance 

 in the study of the properties and utilization of timber. 



