XL] FORMS OF LEAVES, ADAPTIVE 101 



has primarily arisen, like the same forms in the 

 Nymphceacece and Sagittaria, from an aquatic 

 habit. 



The reader will conclude that all the forms 

 of leaves submerged or floating represent so 

 many adaptations to their environment. This 

 is particularly well seen in the formation of the 

 linear ribbon-like form of monocotyledons and 

 the prevailing dissected type of dicotyledons. 

 For these can be more or less produced at will. 

 Thus M. Costantin has shown that if a leaf of 

 Sagittaria which has begun to form a sagittate 

 blade, by developing the two basal extensions as 

 of a spear near the surface, be suddenly plunged 

 into deep water, the form is instantly changed ; 

 the blade now develops an elongated point, 

 attempting, in fact, to return to the deep-water 

 ribbon form. I have, however, a specimen in 

 which all the three points appear to have been 

 affected ; and although the blade is sagittate, 

 the points have grown out into elongated and 

 narrow prolongations. 



As another instance of immediate adaptation, 

 I have found grasses growing completely sub- 

 merged, the leaves of which, instead of being 

 of the ordinary short - lengthed type, had the 

 elongated ribbon form, while the anatomy was 

 the same as that of ordinary and normal ribbon- 

 like types of submerged leaves. 



Similarly with Ranunculus heterophyllus, leaves 



