192 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[ 35. 



of cambium are formed, extending from the inner surface of each 

 bast-bundle to the pericycle. The pericycle-cells lying externally 

 to the outer ends (protoxylem) of the wood-bundles now divide, 

 and connect the arcs of cambium. ' Thus a continuous cambium- 

 layer is formed, which has at first a wavy outline, as seen in 

 transverse section, but which' becomes circular as the development 

 of the secondary tissue proceeds. 



The cambium-layer of the primary root is continuous with that 

 of the primary stem ; hence, in a plant in which stem and root 

 grow in thickness, there is a continuous layer of merismatic tissue 

 extending from one end of it to the other ; for the cambium of 

 the branches of both stem and root is continuous with that of the 



primary members ; and fur- 

 ther, the cambium is con- 

 tinuous with the merismatic 

 tissue of the growing-points 

 of the primary stem and root 

 and of their branches. 



The cells of the cambium- 

 ring, in the stem and root 

 alike, constantly undergo 

 both tangential and radial 

 division, so that the number 

 of the cells increases in the 

 radial direction as well as 

 in the circumferential ; the 

 growth of these cells pro- 

 duces an extension of the 

 organ in both these direc- 

 tions. Of the cells formed 

 by tangential division, those 

 lying on the inner side of the 

 cambium, are transformed into the elements of the wood (Fig. 

 143 B W), those on the outer side, into the elements of the bast, 

 while the cells of the intermediate zone continue to be capable of 

 dividing. The activity of the cambium thus gives rise to secondary 

 wood and secondary bast, as distinguished from the primary con- 

 stituents of the bundle, which existed previously to, and indepen- 

 dently of, the activity of the cambium. The primary wood of the 

 bundle is thus the innermost part of it } and the primary bast the 

 most external. . 



8, 



FIG. 144. Transverse section of the stele of 

 the root of Sambucus nigra, where secondary 

 growth in thickness is commencing, r Cor- 

 tex ; ed endodermis ; pc pericycle : x x x the 

 three groups of protoxylem ; p p p the three 

 groups of phloem ; c dividing cells of the con- 

 junctive tissue forming part of the developing 

 cambium-ring. 



