82 STT7DY OF COMMON PLANTS. 



that in the course of an indefinite period of time have 

 gradually taken on shapes manifestly correlated with the 

 visits of insects or other agents by which pollen is carried 

 from one flower to another. 



Many flowers have undergone the suppression of one or 

 more parts. In some cases a whole whorl is wanting, as 

 in the anemone, which is destitute of a corolla ; 

 or several whorls may be lacking, as in the wil- 

 lows, the flowers of which are reduced to a single whorl. 

 Frequently, however, a part of a whorl only is wanting, 

 and in such cases it often happens that a rudiment, or 

 trace, of the missing parts remains to indicate a former 

 condition. In the common toad-flax, for example, there are 

 four perfect stamens and a trace of the fifth ; some of the 

 mints now have but two stamens, although five was the 

 original number ; and many plants, as the lupine and its 

 allies, otherwise on the plan of five, have the ovary reduced 

 to a single carpel. 



The symmetry of the flower is interfered with, not only 

 by the suppression, but also by the multiplication of parts, 

 Multiplioa- SO that it not infrequently happens that the 

 tion. original plan, in some one whorl at least, is no 



longer recognizable. The very numerous stamens of the 

 cacti will serve as an illustration. 



The changes described are of great interest as indicating 

 actual steps in the developmental history of flowers. They 

 help us to see, if not fully yet in pai-t, how such extraor- 

 dinary structures as those of a milkweed flower or an 

 orchid have come to be what they are.^ 



' Lack of space renders it necessary to refer the student to a much 

 more extended discussion of the subject than can here he undertaken. 

 Cf. Gray, Structural Botany, pp. 179-209, which has been followed in 

 the main in the brief resume just given. 



