18 



A TEXTBOOK OF BOTANY 



[Ch. Ill, 2 



some typical leaf, e.g. from one of our common trees, be 

 held up against the light and examined with a hand lens, 

 one can see very clearly that the ultimate meshes of 



the network of vein- 

 lets inclose little 

 polygonal areas of 

 pure chlorenchyma, 

 into which often, 

 though not always, 

 extend free tips of 

 the tiniest veinlets 

 (Fig. 2). This ulti- 

 mate relation of 

 veinlets and chlor- 

 enchyma is impor- 

 tant, as will later 

 appear. The small- 

 est veinlets are 

 buried within the 

 leaf blade, but the 

 larger ones and the 

 veins which are pro- 

 gressively thicker 

 towards the leaf 

 base, swell gradually 

 out from the blade 

 on its under side 

 until they become 

 many times thicker 

 than the ever uni- 

 form chlorenchyma. 

 Third is the epi- 

 dermis, a very thin 

 and transparent 

 layer by which all leaves are covered, and which often displays 

 a shining surface when viewed obliquely towards the light. 



Fig. 2. — The vein systems of English Ivy 

 (above) and Silver Poplar; reduced. (From 

 The Phantom Bouquet, by Edward Parrish, 1865.) 



The pictures were drawn from specimens 

 " skeletonized " by removal of the chloren- 

 chyma. A magnifying lens should be used to 

 render visible the ultimate veinlets. 



