FLOWERS 119 



a protective wall, which is so characteristic in structure that 

 its species can often be determined. Bee-keepers can deter- 

 mine the plants supplying the honey flow by examining the 

 pollen on the bees. 



Pollination is the transfer of pollen from the anther to the 

 stigma. Wind and insects are the chief agents of pollination, 

 although some flowers have structures in which no outside 

 agency is necessary. Cross-pollination refers to a transfer 

 from the anthers of one to the stigma of another plant, while 

 self-pollination includes all cases of transfer on the same 

 plant. 



The pollen grain germinates when it lights on the stigma 

 and by the action of its enzymes grows a tube through the 

 style into the ovule. During the development of the pollen 

 tube the nucleus, which contains the chromatin, or the 

 heredity carriers, divides and finally forms two male sex cells 

 called sperms. These are released in the embryo sac in the 

 region of the female sex cell or egg. It is important that the 

 pollen tube grows well in the style. Studies have shown that 

 in cross-pollination there are many cases of incompatible 

 pollen, resulting in failure to grow properly in the style. The 

 rate of growth in the styles has been studied by splitting 

 them after pollination. 



While the pollen grain is germinating the ovule in the 

 ovary develops into a many-celled structure in the center of 

 which are three important cells: the egg cell, and two polar 

 cells or bodies. The egg cell has a nucleus which bears the 

 chromatin of the female plant. 



Fertilization results when a male sex cell fuses with a 

 female sex cell. This fertilized egg cell is the beginning of 

 the new plant called the embryo. It contains the heredity 

 carriers or genes of both sex cells. The second male sex cell 

 fuses with two polar bodies, and becomes the endosperm. 

 This explains the origin of the embryo and endosperm of the 

 corn kernel (Fig. 1). Fertilization results in a powerful 



