98 STRUCTUEAL BOTANY 



always arise from the pericycle. The bark of our trees 

 and large shrubs arises essentially in the same way as 

 that of the Wallflower, though there are many differences 

 of detail. 1 



h. Secondary Thickening in the Root 



The root, like the stem, grows indefinitely in thick- 

 ness, owing to the formation of new tissues from a 

 cambium. This secondary growth begins very early 

 in the root, just after the stage shown in transverse 

 section in Fig. 28, so that in the older roots 

 much the greater part of the tissue is of cambial 

 origin. The first formation of the cambium in the 

 root is rather different from that in the stem, owing 

 to the different arrangement of the primary tissues. 

 It has already been mentioned that the two phloem 

 groups are separated from the xylem by parenchy- 

 matous cells belonging to the conjunctive tissue. It 

 is in these cells that the tangential divisions begin. 

 The cambium, consequently, at first forms two bands 

 of actively dividing cells, lying on either side of the 

 xylem-plate and immediately within the two phloem 

 groups (see p. 89, Fig. 34, C, g). These cells give rise 

 to new xylem on the inside and new phloem on the 

 outside' in the usual way, but the cambium does not as 

 yet form a complete ring. It is completed in this way. 

 The pericycle, which was at first a single layer of 

 cells, divides and becomes two or more cells in thick- 

 ness, at least at the places opposite the two protoxylem 



1 In popular language the word "bark" is commonly used to denote 

 all tissues outside the cambium, and thus includes the phloem. This 

 wider use of the term is adopted in some botanical w rks. 



