164 EVOLUTION OF PLANTS 



and thus is capable of a certain degree of independent 

 existence, a condition not known in any other Spermato- 

 phytes. The several archegonia produced upon the 

 gametophyte do not differ in any essential particular 

 from those of the true Archegoniates. 



o 



The microsporangia occur in great numbers upon the 

 backs of sporophylls which are arranged spirally about 

 a thick axis and form a cone or strobilus. The micro- 

 sporangia are very much like those of the ferns, and 

 are usually grouped in clusters or sori (Fig. 40, C, D). 

 The microspore on germinating produces a rudimentary 

 plant with a simple antheridium containing two sperm- 

 cells. From these are produced the spermatozoids, 

 much like those of the ferns, but, especially in Zamia, 

 enormously larger than any other known spermato- 

 zoids. These are formed shortly before fertilization 

 takes place. 



The pollen falls upon the top of the ovule (macro- 

 sporangium), where there is an opening in the integu- 

 ment with which it is surrounded, and this opening at 

 the time of pollination is filled with a fluid which on 

 evaporating deposits the pollen-spores upon the top of 

 the sporangium itself, where they germinate by sending 

 out the pollen-tube, which forces its way through the 

 upper part of the ovule to a cavity just above the arche- 

 gonium (Fig. 40, H). Simultaneously with the ripening 

 of the latter, the two spermatozoids within the pollen- 

 tube are discharged into the cavity, which is filled with 

 a watery fluid derived from the distended pollen-tubes, 

 and in this they swim to the archegonium by means of 

 the numerous cilia with which they are furnished. Fer- 

 tilization is thus effected precisely as in the Arche- 



