84 UNIVERSALITY OF DEGENERATIVE EVOLUTION 



former are very seldom provided with stomata, 

 and the latter only exhibit stomata on their 

 upper surfaces (see page 26, note on Ranunculus 

 and Stratiotes). The conducting apparatus too, i.e. 

 the wood and the roots, has greatly diminished in 

 size owing to the diminution of transpiration. 



3. Very little light penetrates to submerged 

 leaves, and oxygen is only partially soluble in 

 water. The blade has therefore become much 

 smaller. Very little importance, however, is at- 

 tached to this last modification, as the size of the 

 blade varies greatly in different specimens, whereas 

 the other modifications are similar in all. 



(a) In the species already mentioned Sagittaria 

 (p. 72) and Nymphaea (p. 76) the first leaves 

 which are formed are submerged, with petioles either 

 short or entirely absent, and very slender blades. 

 In Vallisneria and many species of Potamogeton there 

 are only slender submerged leaves. 



All these leaves augment their surface contact 

 with the water by ridding themselves of the deeper 

 layers of assimilative cells. Very little light could 

 penetrate to these cells, and any oxygen formed 

 would be difficult to get rid of. 



(b) In other plants, especially in the Dicotyledons, 

 another kind of modification takes place for the 

 same end ; the blade is divided into very thin seg- 

 ments which in some cases present a hair-like 

 appearance. 



The Ranunculus is a good example of this. 



