NATURE AND OFFICE OF THE FLOWER 223 



322. The Course of Floral Evolution. With this concep- 

 tion of the nature of the flower we can readily see that the 

 less specialized its organs are and the more nearly they 

 approach in structure and arrangement to the condition of 

 an undifferentiated branch, the more primitive and unde- 

 veloped the type to which it belongs. On the other hand, 

 if the parts are highly specialized and widely differentiated 

 from the crude branch, a proportionately high stage of 

 floral evolution is indicated. 



323. Office of the Flower. The one object of the flower 

 is the production of fruit and seed, and all its wonderful 

 specializations and variations of form and color tend either 

 directly or indirectly to that end. 



324. Fertilization. It was stated in Section 286 that no 

 seed can be developed unless some of the pollen reaches 

 the stigma, but even this is not sufficient unless the process 

 known as fertilization takes place. The exact nature 

 of this process it is not easy to explain without going 

 into details beyond the scope of this work, but a good 

 general idea of fertilization may be obtained by refer- 

 ring to Figure 443 in connection with a study of the 

 pollinated pistil of some large flower, like the hollyhock 

 or hibiscus. 



325. The Pollen Tubes. Obtain if possible the flower 

 of a Syrian hibiscus (okra will answer nearly as well) that 

 has begun to close up, or to change color, and compare 

 the stigma with that of a freshly opened flower. What dif- 

 ference do you observe in the pollen grains adherent to 

 each ? The yellow, withered look of the former is due to 

 the fact that they have begun to germinate on the moist 

 surface of the stigma ; that is, to send clown little tubes 

 into its substance (Fig. 443, z), and the nourishment con- 

 tained in the grain is being used up, just as the endosperm 

 of the seed is used up when the embryo begins to germi- 

 nate. (The germination of pollen, however, means some- 

 thing very different from the germination of the seed, and 



