HOW PLANTS A BIS INFLUENCED 



337 



192. Cross-section of a leaf of Ole- 

 ander, showing the stomata (s) 

 sunken in depressions, which are 

 filled with hairs. 



Very different in appearance 

 are the plants which inhabit 

 streams and lakes, such as the 

 Water-lily, Sjjatterdock, Ar- 

 rowhead, etc., which are typical 

 water-plants, with broad thin 

 leaves, devoid of hairy cover- 

 ings and provided with enor- 

 mous air-spaces, especially in 

 the leaf -stalk. These great air- 

 spaces are necessary to convey 

 air down to the roots and other 

 submerged parts of the plant. 

 The submerged 

 leaves (and 



submerged parts) of water- 

 plants obtain air from the 

 water (see pages 193 and 283 

 for experiments) ; the leaves i/v 

 are usually split up into fine 

 divisions by which a greater 

 surface is secured. Fig. 193 

 shows the behavior of the 

 Arrowhead, which produces 

 ribbon-like leaves under water 



and the characteristic Arrow- 193. Arrowhead, producing rlbbon-liUe 



leaves below the surface {w) of the 



head leaves above water. The S'TuS'eTSeTJirt'.'lhrr^dYel? 



figure also shows a heart- {Mtorr^s^a transition between the 



