HOW PLANTS ARE INFLUENCED 



337 



192. Cross-section of a leaf of Ole- 

 ander, showing the stomata (s) 

 sunken in depressions, which are 

 filled with hairs. 



Ver} T different in appearance 

 are the plants which inhabit 

 streams and lakes, such as the 

 Water-lily, Spatterdock, 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 other 



submerged parts) of water- 

 plants obtain air from the 

 water (see pages 193 and 283 

 for experiments) ; the leaves IV 

 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. 

 head leaves above water. The 

 figure also shows a heart - 



Arrowhead, producing ribbon-like 

 leaves below the surface (w) of the 

 water and an arrowhead leaf above 

 the surface; the heart-shaped leaf 

 (h) forms a transition between the 

 two forms. 



