THE WOBK OF FLOWERS 309 



hairs in the tube of the flower constitutes an effective 

 defence. 



In many plants (especially among Pears, Plums 

 and Grapes) pollen from the same flower or even from 

 another individual of the same species is without 

 effect : pollen from another variety must be employed 

 in order to set the fruit. This is usually accomplished 

 by planting a few trees of the proper variety in the 

 orchard, so that the bees may carry the pollen. This 

 condition is known as self- sterility, and is frequently 

 brought about by hybridization or even by cultivation. 

 Climate may also assist in bringing it about ; it is 

 noticed that some varieties which are self- sterile in 

 the North are not so in the South. 



The effect of pollination and fertilization is not 

 confined to the seed, but extends to the seed -case 

 and often to surrounding parts. Thus, the pulpy part 

 of the Strawberry consists of the swollen end of the 

 stalk, which is stimulated to grow as the result of the 

 fertilization of the seeds. In the Apple the fleshy part 

 is partly the wall of the ovary and partly the calyx : 

 the fruit has five compartments, and, if the seeds in 

 only one or two of these are fertilized, the fleshy growth 

 around these compartments is stimulated more than 

 elsewhere, producing a one-sided fruit. Just how the 

 development of the seed influences that of surrounding 

 parts is not known, but it seems to be due to chemical 

 changes which take place in the developing embryo. 



