36 



PLANT STUDIES 



framework of veins is variously arranged. The large veins 

 which enter the hlude send off smaller hranches, 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 

 shownljy a ""skel- 

 eton" leaf, one 

 which has been so 

 treated that all 

 the green sub- 

 stance has disap- 

 peared, and only 

 the network of 

 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 

 a.ny veins (see 

 Jugs. X\i, 20). 



:!U. Significance 

 of leaf veins. — It 

 is clear that the 

 frajiiewoi-k nC \-cins is doing at least two tilings for the 

 blade: ( I ) il nieelia.nieally siipiiDi'ts the s]iread mxt green suli- 

 stane.e ; anil (".') it I'undui.'ts material to ^ind IVoiu the screen 

 substan(_;o. So complete is the network (.)f \eins that this 



Fig. 20. A ](\if of hawthorn, showing a short petiole, and 

 a broad lootlicd blade with a conspiciiouB network of 

 ^■einH. Note tln^ relation between tlie veins and the 

 tei'tli. — After STRAsnrunEii. 



