356 TEXTBOOK OF PLANT PHYSIOLOGY 



At the same time the other organs of the flower, stamens, corolla, 

 and calyx, 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, most of these changes must be re- 

 garded as a peculiar effect on the part of the developing embryo 

 or embryos, in cases of polycarpelate fruits. There is every 

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

 The fertilized egg probably secretes certain hormones, which diffuse 

 into the surrounding tissues and produce their growth. 



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 non-pollinated flowers remain fresh for a long time, but 

 wither rapidly as soon as the pollen is transferred to the stigma. 

 Fitting's experiments have shown that the same effect is produced 

 when either dead pollen or that taken from a plant which is incapa- 

 ble of producing fertilization is used. This seems to indicate quite 

 clearly that special substances diffuse from the pollen which cause 

 the withering of certain parts of the flower. 



In some cases, there may arise chemical stimuli which will 

 cause the growth of the ovary even when fertilization has not 

 taken place and the ovule does not develop. Under such condi- 

 tions, the plant will bear seedless fruits, which are characteristic 

 of some cultivated plants; the bananas, certain varieties of grapes, 

 pears, apples, and other fruits. This phenomenon is known as 

 "parthenocarpy." 



103. The Role and Methods of Plant Propagation. — Fertiliza- 

 tion gives rise to an entirely new organism formed by the union of 

 the male and the female gametes. This organism, though very 

 similar to its parents, possesses a somewhat different combination 

 of hereditary units. This is particularly evident in cases of hybrid- 

 ization, when considerable differences exist between the paternal 

 and maternal organisms. Besides sexual reproduction, there exists 

 another common method of multiplication in the plant world. 

 Various parts of the plant may separate from the parent body and 

 grow into a new organism with all the organs peculiar to it. This 

 method of multiplication is known as "vegetative propagation." 



Vegetative propagation is merely a special type of growth of an 

 organism. Certain parts lose their connection with other parts and 



