CH. IV] 



XYLEM, PHLOEM. 



59 



by delicate ropes of protoplasm, by which means there is 

 continuity of the living element from cell to cell through- 

 out the sieve-tube. 



Another point of difference between the xylem vessels 

 and the sieve-tubes (phloem vessels) is the character of 

 their walls : the walls of the xylem vessels are no longer 

 simple cellulose, but have suffered a change known as 

 lignification. They are firmer and more resisting than the 

 soft sieve-tube walls, but the difference between the two 

 is not merely one of texture, they are chemically different. 

 The xylem vessels no longer give the reaction of cellulose 

 with Schulze's solution, which colours them yellow, while 

 the sieve-tube walls still give the purple colour charac- 

 teristic of cellulose. 



corf. 



vase bimd. 



LONGITUDINAL SECTION THROUGH THE STEM OF Helianthus 

 tuberosus, THE JERUSALEM ARTICHOKE. 



corf, cortex. 



, epidermis. 



p, parenchyma of cortex. 



/, pericycle fibres. 



cb, cambium. 



sp, spiral vessels. 



vase, bund, vascular bundle. 



c, collenchyma. 

 &. s t endodermis. 

 .<?. t y sieve tube. 



d, d, dotted or pitted vessels. 

 p. p, parenchyma of pith. 



