196 Plant Life and Evolution 



various modifications. Where the plant is com- 

 pletely submersed, the exterior tissues are quite des- 

 titute of a cuticle, and the stomata are entirely ab- 

 sent; but if any parts emerge above the water, such 

 for instance as the upper surface of the water-lily 

 leaf or the aerial leaves of the arrow-head, the epi- 

 dermis shows the usual cuticle and stomata. Sub- 

 mersed leaves are generally either narrow or 

 finely dissected, and the contrast between the sub- 

 mersed and aerial leaves of the same species is 

 often very striking. Thus in the water-shield 

 (Cabomba), and some species of water crowfoot, 

 the aerial leaves are quite entire, or only slightly 

 lobed, while the submersed leaves are finely divided 

 into very narrow segments. How far the peculiar 

 form of submersed leaves is directly due to the 

 physical properties of the surrounding medium, and 

 how much is to be attributed to adaptation to light 

 and food conditions, has not been satisfactorily 

 demonstrated. The more direct exposure to light 

 and to the action of CO 2 and free oxygen dissolved 

 in the water, are probably important factors con- 

 cerned with the form of these submersed leaves. 



Comparatively few woody plants are aquatics, and 

 where they have roots completely submerged they 

 may show some interesting modifications, usually 

 associated with the aeration of the roots. The curi- 

 ous growths from the roots of the southern cypress, 

 known as " cypress-knees," and the aerial roots of 

 the mangrove, are undoubtedly aerating organs. 



