

362 



THE SPERMATOPHYTES 



termed the pollen chamber (Fig. 295, D,p). The pollen grains 

 germinate in the pollen chamber, forming male gametophytes, 

 whose development disorganizes much of the tissue at the tip 



of the nucellus, so that the 

 pollen grain end of the male 

 gametophytes finally hang 

 down just above the em- 

 bryo sac. 



348. The gametophytes 

 of the cycads. The embryo 

 sac of the cycads is said to 

 develop from one of a group 

 of four cells in the interior 

 of the nucellus. Such a 

 group is undoubtedly a 

 tetrad, and each of the four 

 cells corresponds to a mega- 

 spore, but only one produces 

 a female gametophyte, and 

 thus becomes an embryo sac. 

 The nucleus of the em- 

 bryo sac (megaspore nucleus) 

 gives rise 'to a great many 

 hundred nuclei, and the 

 amount of protoplasm in- 

 creases very greatly until 

 the embryo sac occupies the 

 larger part of the interior 

 of the nucellus in this large 

 ovule. The nuclei at first lie 

 freely in the protoplasm, but, 

 later, walls are formed and 

 the embryo sac becomes filled with a delicate tissue, called the 

 endosperm (Fig. 295, D), which corresponds to the vegetative part 

 of a prothallium in a fern. Several archegonia are developed at 





FIG. 295. The sperms and ovule of a 

 cycad (Zamia) 



A, lower surface of a stamen, with numer- 

 ous pollen sacs in two groups. B, the two 

 large top-shaped motile sperms at the end 

 of the pollen tube ready to he discharged 

 above the archegonia. C, a sperm viewed 

 from the end, showing the spiral band 

 which bears the cilia. D, diagram of a 

 section of an ovule after pollination : m, 

 micropyle; i, integument ; p, pollen cham- 

 ber; n, nucellus containing developing 

 pollen tubes; a, archegonia, with large 

 eggs imbedded in the endosperm (female 

 gametophyte) . B, C, after Webber 



