Regeneration 243 



A somewhat similar regeneration of vascular strands is to be seen in 

 leaves where some of the veins have been cut. New strands are differen- 

 tiated in the mesophyll cells which connect the severed ends. Freundlich 

 ( 1908 ) studied the origin of these xylem strands, and Kaan-Albest ( 1934 ) 

 followed the differentiation of new sieve tubes (Fig. 9-5). The latter do 

 not arise, as do those of the xylem, by conversion of whole parenchyma 

 cells, but small cells are cut out of the sides of these larger elements and 

 join up with one another, end to end, from one cell to the next. These 



Fig. 9-5. Impatiens. Sieve-tube connections developing between phloem bundles, one 

 of which has been severed. ( From Kaan-Albest. ) 



phloem strands in their development suggest the fiber strands of Luffa 

 (p. 197). 



What are the factors, one may ask, that impel the ^differentiation of 

 a vascular system when intercommunication among its parts has been 

 interrupted? Auxin may be diffusing from the end of a cut bundle, but 

 how this operates to convert a series of parenchyma cells into a vascular 

 strand is difficult to understand. Here is differentiation in very simple ex- 

 pression. Doubtless the same general factors are involved as in normal 

 development but the process takes place here on a greatly enlarged scale 

 where it can be more readily studied than in the very small-celled tissues 



