276 



BIOLOGY 



down the style until it reaches the ovule at the bottom of the 

 ovary; see Figs. 65 and 127 pt. In this ovule the female 

 gametophyte has formed, and has by this time produced what 

 corresponds to archegonia with their eggs; Fig. 127 e. The tip 

 of the pollen tube approaches the egg and finally comes in 

 contact with it. Inside of the pollen tube are nuclei which 

 represent the sperms; Fig. 127 m. As we have noticed on 

 page 257, when the fertilization of an egg occurs it is only the 

 nuclei of the cells that fuse, so that the nuclei in the pollen 



E 



FIG. 128. DEVELOPMENT OF THE POLLEN 



A, a single pollen grain or microspore; B, the cell divided into two; C, the pollen, which 

 has produced a rudimentary gametophyte atg; D, a later stage with the gametophyte g 

 etill more rudimentary; E, the pollen developing the pollen tube. The nucleus m 

 divides later into two nuclei representing sperms. 



tube represent all of the important parts of a sperm. When 

 the pollen tube comes in contact with the egg it allows these 

 nuclei to escape into the egg, where one of them fuses with the 

 nucleus of the egg, thus producing the actual sex union. 



The fertilized egg is now endowed with powers of growth 

 and begins at once to develop into a new plant. Again follow- 

 ing the comparison with the fern, we shall expect that the plant 

 which comes from the fertilized egg must be the sporophyte, 

 which in this case is, of course, the plant that produces the 

 flowers. The egg develops at once, growing quickly into a 

 tiny plant with a stem and one or two leaves. This occurs 

 while the egg is still retained in the ovary of the flower that 

 produced the spores. After a time this plant stops growing 



