Flowers. 1 69 



and the further development of the ovary, and, in some 

 cases, of the receptacle and calyx. If the egg is not fer- 

 tilized, the whole flower soon drops off ; but once fertiliza- 

 tion is achieved, the ovary persists and develops into the 

 mature fruit. Evidently there is here a transmission of a 

 stimulus from the fertilized egg to all parts of the ovary 

 and even to other structures of the flower. 



122. Time required for Fertilization. The distance trav- 

 ersed by the pollen tube varies from one to a few milli-' 

 meters in the case of the smaller flowers to about forty 

 centimeters through the long styles of Indian corn, com- 

 monly known as the silk. The time which elapses between 

 the deposition of the pollen on the stigma and the fusion 

 of the sperm with the egg may therefore vary from a few 

 hours to several days. In the case of the pine, for other 

 reasons, the interval extends through an entire year. 



123. Relative Value of Asexual and Sexual Reproduction. 

 If we consider the relative cost to the plant of the 

 asexual and the sexual methods of reproduction, we can 

 see that the sexual method is much the more expensive in 

 materials and energy. The calyx and corolla are costly 

 contrivances, and the production of nectar and aromatic 

 substances for the allurement of insects, and of styles and 

 elaborate stigmatic surfaces, has been at the cost of much 

 valuable material; but, added to this, most of the pollen 

 does not reach the stigma and never takes part in fertiliza- 

 tion, and is therefore an entire loss. 



The struggle for existence among plants is too great to 

 permit any extravagance that does not bring with it some 

 amply compensating benefits ; and we might therefore con- 

 clude that the expensive and elaborate contrivances which 

 we find attending the sexual method result in some great 

 advantage. It will perhaps add to the clearness of the 



