REPRODUCTION IN PLANTS 77 



be omitted until after the cellular structure of plants has been 

 considered.) When the pollen grains are first formed in the 

 anther, each consists of a single cell. Later the nucleus of 

 this cell divides and forms two nuclei, one of which is the 

 generative nucleus. The generative nucleus then divides 

 and forms two sperm nuclei. The ovule is more complex 

 in its structure, being composed of many cells of different 

 kinds. But here, as is the case with the pollen grain, there is 

 one important cell that is essential in the process of reproduc- 

 tion, and this is known as the egg-cell (Figs. 27 and 29, A). 



91. The formation of an embryo. When the pollen 

 grain germinates and forms the tube, the sperm nucleus is 

 carried by the tube down through the stigma and style into 

 the cavity of the ovary, and finally through the micropyle of 

 the ovule, until one of the sperm nuclei comes to lie beside 

 the nucleus of the egg-cell. The two nuclei now unite in the 

 process of fertilization to form a fertilized egg-cell. The 

 nucleus of this cell then divides and later the cell-body, thus 

 forming two distinct cells. Each of these divides to form two 

 cells, and the four cells thus produced give rise to eight, then 

 sixteen, thirty-two, and so on, until a many-celled structure 

 is developed which is a miniature plant called the embryo. 

 This embryo, together with other parts of the ovule, consti- 

 tutes the seed. Some of the cells of the embryo will later 

 form the roots, others the stem, and still others the leaves 

 of the plant (Fig. 29, A-E). 



Hence, the new plant formed by this method of reproduc- 

 tion is clearly descended from two different parents, one 

 parent flower furnishing in its pistil the egg-cell and the 

 other in its stamen the fertilizing pollen. We may, therefore, 

 give the following as a general definition of the process we 

 are studying : Fertilization is the union of the nucleus of 



