IV.] VARIOUS THEORIES, &c. 103 



But the most important contribution to the dis- 

 cussion in 1883, was Westermaier's paper,^ in which 

 an entirely new departure was made, in that, for the 

 first time,^ attention was called to the r61e of the 

 living parenchymatous cells of the wood and 

 medullary rays. 



Westermaier accepts Zimmermann's criticism as 

 putting the Boehm-Hartig theory out of court, and 

 forthwith calls attention to the wood-parenchyma and 

 medullary rays as integral parts of the tissues 

 concerned in the ascent of the water. He then puts 

 the question, Can these living cells raise the water 

 osmotically ? If so we need no longer be troubled 

 with the resistance of the membranes, or the heights 

 of the columns. 



Two points are to be noticed : (i) The living cells 

 of the wood-parenchyma are in communication with 

 the tracheal system by numerous pits ; and (2) turgid 

 parenchyma allows water to escape by exfiltration into 

 dead contiguous elements. 



It may perhaps be assumed that cells lower down 



^ *' Zur Kenntniss der osmotischea Leistungen des lebenden Paren- 

 chym's " Be? der dmt, hot, Gesellsch. B 1. 1883, p. 371. 



2 This is not strictly accurate, as Knight {FJni. Ttans i8oi,p. 344) 

 had suggested the co-operation of medullary rays, but of comse his 

 points of view were different. 



