476 THE STRUCTURE OF ECONOMIC PLANTS 



vessel." When the vessel segment reaches its full diameter, it 

 still contains its protoplast, the longitudinal walls are thin, and 



!»» 



m 



it 

 It 



I 



^ 



• 



« 



G 



E 



Fig. 144. Stages in development and disintegration of vessel end wall : A-D, stages 

 showing thickened end wall and formation of spiral secondary thickenings ; E, H, rim be- 

 tween two mature vessel elements shown at two different focal planes; F, G, I, stages in 

 disintegration of end wall; /, end wall of vessel elements which in mature state communicate 

 through small perforation ; K, median line in thickened end wall ; L, end wall in face view ; 

 M, end wall broken in preparation. (After Esau, Hilgardia.^ 



