250 General Botany 



of the sperms moves to the egg and fuses with it, forming the 

 fertihzed egg. In more general terms, the male gamete unites 

 with the female gamete and forms a zygote. This is the beginning, 

 or first cell, of the embryo. Fertilization is the essential part of 

 sexual reproduction both in plants and animals, and it marks the 

 actual beginning of a new individual. After fertilization the 

 zygote may develop into a new plant or animal of the same kind 

 as its parents. It should be clear that the sperms from one pollen 

 tube fertilize the egg in only one ovule, and that to fertilize all 

 the eggs in a pistil, as many pollen tubes must grow down through 

 the style as there are ovules in the ovulary below. 



The endosperm. At the same time that the egg is fertilized, 

 the second sperm from the pollen tube unites with the fusion 

 nucleus at the center of the embryo sac and forms the endosperm 

 nucleus. The endosperm nucleus is therefore a nucleus made up 

 of three nuclei, and on this account the process by which it is 

 formed is called triple fusion. This fusion is followed by rapid 

 cell division and the formation of a soft tissue filling the rapidly 

 enlarging embryo sac. The tissue thus formed is the endosperm, 

 and into it pass large amounts of food from the plant. In the 

 grains and some other kinds of seeds the endosperm occupies 

 most of the space within the seed coat. 



The embryo. The zygote, or fertilized egg, starts growth and 

 cell division at once. As the mass of cells enlarges, it grows 

 farther and farther into the endosperm, from which its food 

 materials are derived. It finally takes on the form of the embryo, 

 or young plant, that we find inside the seed. As development 

 proceeds, the growth is slowed down and finally ceases until the 

 seed germinates. Sometimes, as in the bean, all the contents 

 of the endosperm are consumed and all that remains of it in 

 the mature seed is a thin layer of cells around the embryo. 

 In other seeds the endosperm partly or wholly surrounds the 

 embryo with a thick layer of cells, as in the castor bean, corn, 

 and lotus. 



