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. 



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- 



