440 PLANT PHYSIOLOGY 



generative nucleus of the pollen exerts a chemical stimulus that 

 leads to further development of the egg nucleus. 



Fertilization not only results in the development of the embryo 

 and the endosperm but also in a number of other changes par- 

 ticularly in the ovule and other parts of the flower. The integu- 

 ments of the ovule commence to grow and finally form the seed 

 coat. The walls of the ovary develop and form the pericarp, 

 while parts of the receptacle also are frequently involved in its 

 formation. At the same time, the other organs of the flower, 

 stamens, corolla, and often the calyx also, die and either drop off 

 or dry up. 



Since most of the parts of the flower are not directly involved 

 in the process of fertilization, these changes must be regarded 

 as a peculiar effect on the part of the developing embryo, or 

 embryos, in case of polycarpic fruits. There is every reason 

 to suppose that it is a matter of chemical stimulation. The 

 fertilized egg probably secretes substances, which diffuse into the 

 surrounding tissues and produce changes in their growth or 

 senescence. 



It should be noted that in certain plants withering of some 

 of the floral organs occurs even before fertilization has taken 

 place or at the time the pollen tubes begin to penetrate into the 

 tissue of the stigma. This phenomenon is evident particularly 

 in the orchids, where the nonpollinated flowers remain fresh 

 for a long time but wither rapidly as soon as the pollen is trans- 

 ferred to the stigma. Fitting's experiments (1909) have shown 

 that the same effect is produced when pollen that is incapable of 

 producing fertilization is used. This seems to indicate quite 

 clearly that special substances diffuse from the pollen and cause 

 the withering of certain parts of the flower. 



In some cases, there may arise chemical stimuli that will 

 cause the growth of the ovary even when fertilization has not 

 taken place and the ovule does not develop. Under such 

 conditions, the plant will bear seedless fruits, which are character- 

 istic of some cultivated plants such as the bananas, certain 

 varieties of grapes, pears, and apples. This phenomenon is 

 known as ''parthenocarpy," 



Usually, not all of the fruit set will ripen. Frequently, a 

 considerable number soon cease to develop and are shed. Such a 

 shedding of the fruit is especially frequent in fruit trees, in 



