36 



PLANT RELATIONS. 



fiamework of veins is variously arranged. The large veins 

 which enter the blade send off smaller branches, and these 

 send off still smaller ones, until the smallest veinlets are 



invisible, and the 

 framework is a 

 close network of 

 branching veins. 

 This is plainly 

 shown by a "skel- 

 eton " leaf, one 

 which has been so 

 treated that all 

 the green sub- 

 stance has disap- 

 peared, and only 



veins remains. It 

 will be noticed 

 that in some 

 leaves the veins 

 and veinlets are 

 very prominent, 

 in others only 

 the main veins 

 are prominent, 

 while in some it 

 is hard to detect 

 any veins (see 

 Figs. 25, 20). 



29. Significance 

 of leaf veins. — It 

 is clear that the 

 framework of veins is doing at least two things for the 

 blade: (1) it mechanically supports the spread out green sub- 

 stance ; and (2) it conducts material to and from the green 

 substance. So complete is the network of veins that this 



Fig. 26. A leaf of hawthorn, showing a short petiole, and 

 a hroad toothed blade with a conspicuous network of 

 veins. Note the relation between the veins and the 

 teeth.— After Strasburger. 



