VARIATIONS IN STRUCTURE 



277 



of perfectly distinct kinds, all alike as to function, but all 

 different as to structure. The kinds of flowers are not 

 only more easily distinguished from one another than the 

 kinds of leaves and other organs, but flowers of the same 

 kind of plant do 



not vary as the it- 



leaves and other 

 organs of the same 

 kind of plant vary. 

 (See Figure 100.) 

 On different kinds 

 of plants, flowers 

 vary much more 

 than leaves, but 

 on the same kind 

 they vary much 

 less. On dry, 

 sandy soil a wild 

 rose has small 

 leaves, short 

 stems, and many 

 thorns ; on rich 

 soil it has larger 

 leaves, longer 

 stems, and fewer 

 thorns. But 

 wherever it grows 

 the flower is practically the same. It is a general princi- 

 ple that surrounding conditions affect the structure of 

 nutritive organs more than they affect the structure of 

 reproductive organs. Evidently, then, reproductive or- 

 gans, being less variable, are more useful in classification 



Fig. 100. — Variation in the dandelion. Both plants 

 are mature. The smaller one was grown from a 

 portion of the larger one, but under less favorable 

 conditions. The stems and leaves are very different 

 in the two specimens, but the flowers are about the 

 same. 



