LEAVES: AQUATIC PLANTS 293 



Similarly the slender petiole, when it is present, allows the 

 leaf to move readily in the wind and so reduces resistance. 

 Perhaps the quivering leaf of the aspen with its delicately 

 balanced, laterally compressed petiole represents a higher 

 degree of efficiency in this direction. 



§ 5. Aquatic Plants 



In water plants the mechanical system is much modified. 

 In still water the whole weight of the shoot is borne by the 

 medium in which it floats ; the plant does not sink to the 

 bottom, chiefly because the presence of air bubbles in its 

 tissues makes its specific gravity the same, or nearly the same, 

 as that of the water. The root system is, as we have seen, 

 often relatively feeble. 



Plants with floating leaves, and those which live in 

 running water, show special features. If the leaf is to float 

 it must be non-wettable, and this is secured by a strong 

 development of cuticle which gives the leaves of pondweeds, 

 water lilies, and the amphibious persicary, their character- 

 istic poHshed appearance. In addition to this, the margins 

 of the leaf may be turned up, a feature very noticeable in 

 Victoria regia, the leaf of which is large enough and suffi- 

 ciently buoyant to support the weight of a baby. Movement 

 of the surface water involves strains which are met by the 

 leathery texture of the floating leaf, sometimes reinforced 

 by sclerotic cells, as in the water lily. The long lax petioles 

 allow the leaf to follow changes in the water level. 



Leaves of plants growing in running water are frequently 

 much dissected ; the fine segments are extremely pliable 

 and turn readily with the current. Such is the case in 

 Ranunculus fluitans and Myriophyllum spicatum. Here 

 we see again the tearing action of the current, in this case 

 of water, lessened by lowered resistance. The dissected 

 type of leaf is, however, also common in submerged leaves 

 of plants of still water, and it may be related to other require- 

 ments, such as the ready absorption of carbon dioxide and of 

 salts through a larger surface, or to the absorption of light. 



