278 REPRODUCTION 



case it is designated perisperm ; some seeds may contain both 

 endosperm and perisperm. 



After fertilisation has been accomplished, the style and stigma 

 of the carpels and also the corolla of most conspicuous flowers, 

 wither and fall off. The stimulus of the sexual act incites the 

 ovule to grow, and a similar influence is transmitted to the 

 tissues of the ovary-wall, which also grow and expand to allow 

 the development of the seeds within : the gynsecium of the flower 

 becomes converted into a. fruit. 



Moreover, the act of fertilisation frequently induces growth 

 and change in the receptacle and flower-stalk, as in the apple, 

 pear, and strawberry. 



Some cultivated plants, such as varieties of cucumber, grape, 

 pine-apple, orange, and banana, produce 'seedless fruits,' the 

 walls of the ovaries developing extensively apart from any seed 

 production. However, in the tomato, melon, plum, and the 

 majority of plants, fruits either do not develop at all or drop 

 off long before they reach normal size, when fertilisation does 

 not take place. 



That the development of the seed influences the growth of 

 the fruit may be seen by watching the development of an 

 apple flower in which some of the five stigmas present have 

 been pollinated and others left : the ' fruit ' from such an 

 incompletely pollinated flower becomes somewhat one-sided 

 and unsymmetrical in form, for only the carpels corresponding 

 to the pollinated stigmas produce seeds, and it will be found 

 that the part of the ' fruit ' in which the seeds are present grflws 

 more rapidly than the seedless part. 



Tomatoes and strawberries also develop into one-sided, 

 irregular fruits when pollination is incomplete. 



Only one pollen-grain is necessary to fertilise a single ovule, 

 and more pollen is always produced by flowers than is absolutely 

 necessary for the impregnation of all the ovules within their 

 carpels. There is however some evidence to believe that when 



