138 



PART II. ANATOMY AND HISTOLOGY. 



[35 



the pith from the cortex : it consists of two portions corresponding 

 to its mode of origin ; fascicular cambium, i.e. the cambium be- 

 longing to the vascular bundles, and the inter fascicular cambium, 

 i.e. that which is formed between the bundles in the primary 

 medullary rays (see Fig. 97). 



A cambium-ring is likewise formed in the root of these plants 

 (Fig. 109). The first indication of the formation of ji cambium- 

 layer is the division of the cells of the conjunctive parenchyma on 

 the inner surface of each bast-bundle : then those on the flanks 

 of the bast-bundles begin to divide ; and thus a number of^ arcs 

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

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



to Jhe outer ends_(protoxy- 

 lem) of the wood-bundles now 

 divide, and connect the arcs 

 of cambium. Thus a con- 

 tinuous cambium -layer is 

 formed, which has at first a 

 wavy outline, as seen in 

 transverse section, but which 

 becomes circular as the de- 

 velopment 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 continu- 

 ous 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 further, the cambium is continuous 

 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 3 con- 

 stantly 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 produces an exten- 

 sion of the organ in both these directions. Of the cells formed by 



B. 



FIG. 109. Transverse section of the stela of 

 the root of Sambucus nigra, where secondary 

 growth in thickness is commencing, r Cor- 

 tex ; ed endodermis ; pc pericycle ; xxx 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. 



