BRANCHING 297 



cells in regard to each salt, and on the relative perme- 

 ability of the cytoplasmic membranes to the different 

 salts. 



Structure of the Older Parts of the Root. In ex- 

 ceptional cases, as has already been said, root hairs 

 may persist indefinitely during the life of the root, 

 but normally their life is quite short, and they die 

 and peel off, with the piliferous layer, behind the 

 limited root-hair region. The surface of the root is 

 then formed by the outer cells of the cortex, and the 

 outer walls of these cells, now in contact with the soil, 

 are cutinised and no longer absorb water from the soil. 

 Meanwhile the metaxylem vessels of the vascular 

 cylinder are now completely lignified, and the conduct- 

 ing capacity of the primary structure has reached its 

 fullest extent. Secondary changes may now occur : 



(i) Branching. Branch roots arise in this region, by 

 the division of the cells of the pericycle, usually opposite 

 a protoxylem strand. A new apical meristem is 

 established, covered by a root cap, and the tip of 

 the new root grows out through the endodermis and 

 cortex of the mother root (Fig. 47, d), partly by digesting 

 the cells, partly by mechanical pressure, and emerges 

 into the soil. When a root is branching freely a 

 fairly close-set row of branches may grow out from 

 opposite each protoxylem of the mother root, so that 

 if the mother root is tetrarch, four such rows of branches 

 will appear. The vascular tissues of the new root 

 establish connexions with the corresponding tissues of 

 the mother root ; thus the xylem strands connect 

 with the xylem opposite which the new root has arisen 

 by the differentiation of tracheids in the intervening 

 tissue, the phloems by the formation of sieve tubes 

 connecting with the sieve tubes of the two phloem 



