POLLINATION AND FERTILIZATION 357 



life habits of the male gametophyte quite as remarkable as those 

 of the female. These peculiarities are concerned with two dis- 

 tinct processes necessary to insure the development of seeds, 

 namely, pollination and fertilization. 



The pollen grain is a microspore developed in groups of four 

 (tetrads) in pollen mother cells in essentially the same manner 

 as the spores are developed in all bryophytes and pteridophytes 

 (Fig. 302, B). The pollen grain forms a very much reduced 

 male, gametophyte, which is represented by the protoplasmic 

 contents of the pollen grain and pollen tube, It would be use- 

 less for the male gametophyte to form and discharge sperms 

 which could not possibly reach the embryo sac imbedded in the 

 nucellus of the ovule. So the sperm-forming habits of the 

 pteridophytes, bryophytes, and the algae are generally given up, 

 although curiously they still persist, as will be described later, 

 in the cycads and Ginkgo (Sec. 348). The sperms are repre- 

 sented by two sperm nuclei developed by each male gameto- 

 phyte and discharged from the tip of the pollen tube. 



The pollen tube is an outgrowth from the pollen grain. Its 

 purpose is to carry the sperm, nuclei to the embryo sac, where 



one of the two may unite with the egg nucleus and fertilize the 



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egg. In one of the two subdivisions of seed plants called the 

 gymnosperms (meaning naked seeds) the pollen grains are 

 applied directly to the ovules, and the pollen tube need only 

 grow through the tissue of the nucellus (megasporangium) to 

 reach the embryo sac. In the other large group called the angio- 

 sperms (meaning seeds inclosed in a vessel) the pollen tubes 

 must penetrate a case (the pistil) which contains the ovules 

 before they can reach the ovules themselves. There is a special 

 receptive surface, called the stigma, upon this structure, where 

 the pollen grains find moisture and other conditions favorable 

 for their germination. 



Pollination is the application of the pollen to the ovule or to the 

 stigma. This application is effected in various ways, sometimes 

 by the wind, sometimes by other chance processes, but many 



