348 GENERAL SCIENCE 



the pollen on the stigma; this is self-pollination. Bees 

 and other insects may carry the pollen from one flower 

 to the stigma of another flower of the same kind; this 

 is cross-pollination. The wind may blow the pollen about 

 and some may fall on the proper stigmas. Corn is 

 pollinated by the wind. 



The stigmas are covered with a sweet, sticky sub- 

 stance which prevents the pollen from falling off and also 

 causes the pollen grains to start to grow or germinate 

 much like seed, but they only form a growth corresponding 

 to the first root of a germinating seed. The pollen 

 grain forms a root-like tube without branches, which 

 grows down through the stigma and style and into the 

 ovary. In the ovary are ovules, each one of which con- 

 tains an embryo sac, and in this sac is an egg cell which 

 cannot grow unless it is fertilized by the sperm cell of a 

 pollen grain. This sperm cell of the pollen grain goes 

 down the pollen tube, and when the pollen tube enters 

 an ovule in the ovary, the sperm cell joins with the egg 

 cell to form a cell known as a fertilized egg. Each pollen 

 grain can fertilize only one egg cell, so each pistil needs 

 as many pollen grains on its stigma as there are ovules 

 in the ovary; each ovule has one egg cell. In order to 

 provide for this, some plants grow many thousands of 

 pollen grains for each ovule in the ovary. In the case 

 of flowers that are pollinated by the wind or by insects, 

 much of the pollen goes to waste and never falls on a 

 stigma. 



After the egg cell is fertilized, it immediately starts to 

 grow and form a young plant with root and top like that 

 in a bean or in a grain of corn. This is an embryo plant 

 or immature plant. While the embryo plant is being 

 formed, the parent plant stores food around it. When 



