Chapter XL. 



Adaptations of Plants to their Surroundings. 



if 



It has already been remarked that plants which resemble each 

 other in external form must not for that reason alone be con- 

 sidered as related. Indeed, quite the opposite is often true, and 

 plants that are outwardly very dissimilar are found upon careful 

 examination to bear the marks of kinship. Thus, the locust 

 tree and the pea vine are really connected with each other much 

 more intimately than are locusts and ashes, although the latter 

 are similar in size, in habit of growth and even in the production 

 of such special types of starch-making organs as pinnately-com- 

 pounded leaves. Plants may, however, for purposes of investi- 

 gation be arranged in groups according to their adaptations to 

 the various conditions of life and growth. Such adaptational 

 groups will include plants of widely different genealogy, but 

 throughout there will be discovered certain similarities of struc- 

 ture and habit. Thus, for example, among aquatic plants there 

 are often striking likenesses in structure between comparatively 

 unrelated forms, and it is in many instances a puzzling problem 

 to determine just which structural resemblances are indicative 

 of true relationship and which are indicative of similar adapta- 

 tions to similar conditions. The submerged leaves, for illustra- 

 tion, of the water crowfoot or water buttercup are in general 

 appearance not particularly different from the submerged leaves 

 of the bur-marigold, in both plants those leaves produced un- 

 derneath the surface of the pond in which they live are dis- 

 sected into fine, threadlike portions. Again, the quillworts, 

 plants belonging to the fern alliance, bear the same type of 

 cylindrical hollow leaves that is distinctive of the water lobe- 

 lia a member of one of the highest families of flowering plants. 

 Or, passing to plants of a different habit of growth, it may be 

 noted that the curious, compressed forms of vegetation charac- 

 teristic of cacti are almost exactly reproduced in members of 



28 



