442 GYMNOSPERMS 



The zygote is the first cell of the 2x generation, whether it divides 

 many times, buihhng up a more or less extensive 2X body, or does 

 not divide at all. The zygote, even in Spirogyra, might be called 

 a sporophyte; for it produces four nuclei, three of which degenerate, 

 while the other remains as the only nucleus of the cell which grows 

 into a new filament. In Mesocarpus the four nuclei organize pro- 

 toplasm about themselves and become recognizable spores, so that 

 the older taxonomists called the whole structure a sporocarp. 



In Riccia, and in most of the liverworts, the sporophyte is para- 

 sitic upon the gametophyte. In the moss, while still parasitic, it is 

 green and partly independent. In the ferns, and in all plants above 

 that level, the sporophyte, although at first parasitic, fmally be- 

 comes entirely independent of the gametophyte. 



In the gymnosperms, and in all plants which have reached the 

 seed level, the sporophyte is parasitic until the seed germinates. 



In some of the extinct seed plants it is possible that the male 

 gametophyte may have protruded somewhat from the spore, and 

 may have developed some chlorophyll, but in all known gymno- 

 sperms, both living and extinct, the male gametophyte is contained 

 within the spore until a pollen tube is formed by those gymno- 

 sperms which have pollen tubes. The vegetative part of the male 

 gametophyte of gymnosperms is most extensive in the Araucaria- 

 ceae, where it may consist of as many as forty cells. In most of the 

 Podocarpaceae there is a vigorous development of prothallial cells, 

 but the number is far less than the Araucariaceae. In the Abietaceae 

 there are constantly prothallial cells, but not so many, and in Finns, 

 the most primitive genus, there are only two small evanescent cells. 

 In the other three families, Taxodiaceae, Cupressaceae, and Taxa- 

 ceae, prothallial cells are practically entirely lacking, so that the 

 microspore, as in the angiosperms, is reduced to an antheridial 

 initial. 



In Ginkgo there is one evanescent prothallial cell and one more or 

 less permanent prothallial cell. In the Cycads there is one rather 

 permanent prothallial cell. Nothing is known definitely about the 

 prothallial cell situation in any extinct forms; but it is very probable 

 that there were numerous prothallial cells in the Cordaitales, and 

 probably some in the Cycadofilicales and Bennettitales. 



