164 



GENERAL BOTANY 



Stigma, 



formation of a pollen tube. The single nucleus of the pollen 

 grain (a) divides into two nuclei (&), around one of which a 

 cell is organized which is to form the two male gametes, the 

 other nucleus being left free in the cytoplasm. The newly 

 organized cell divides to form the two male gametes within the 

 pollen tube (<?), and the free nucleus becomes what is called the 



tube nucleus, which is concerned 

 with the growth of the pollen 

 tube. The above changes may 

 be completed while the pollen is 

 resting on the stigmatic surface 

 after pollination, but the male 

 gametes may not be completely 

 organized until the pollen tube 

 begins to grow down through 

 the style just before fertilization. 

 While the above changes are 

 going on in the pollen the female 

 gamete, or egg cell, is being 

 formed in the ovule. Sections 

 of young ovules and of mature 

 ovules at the period of fertiliza- 

 tion (Fig. 86, a, I) show that the 

 body of the ovule is covered 

 by two integuments made up of 

 layers of cells which inclose the 

 central body of the ovule except 

 at one point, where the ovule coats do not quite come together. 

 This failure of the ovule coats to meet leaves a pore, called the 

 micropyle, leading to a large central cavity within the mature 

 ovule (Fig. 86, 6) called the embryo sac. Within this embryo 

 sac the egg apparatus is formed, which is composed of the egg 

 cell, or gamete, and two associated cells, called the synergids. 

 Two free nuclei, the polar nuclei, may also be seen near the 

 center or at one end of the sac at this time. The end of the sac 

 opposite the egg apparatus also contains three cells, the antipo- 

 dals, with which we are not now concerned. After the male 



FIGS. 85. Germination of pollen 

 and pollination 



A, pollen germination and the male 



gametes ; B, diagrammatic figure of the 



pistil and anthers, with growth of pollen 



tube through the style to an ovule 



