388 GENERAL MORPHOLOGY OF PLANTS 



re- 



is not necessarily cross fertilization so far as the plants (sporo- 

 phytes) are concerned. 



Sixth. In the retention of the macrospore in the ovule (macro- 

 sporangium) , and the development of an embryo plant, while still 

 attached to and nourished by the old sporophyte, the seed is pro- 

 duced with its protecting coats and stored food, which more cer- 

 tainly provides for the distribution and perpetuation of the species 

 under a great variety of adverse conditions. 



Seventh. The increasing importance and greater complexity of 

 the sporophyte in the liverworts, mosses, fern plants and Gymno- 

 sperms have been accompanied by a gradual diminution and 

 degeneration of the gametophyte, and in the Angiosperms this 

 degeneration of the gametophyte phase has gone still farther, 

 which is another evidence of the higher evolution of the Angio- 

 sperms. The male gametophyte * consists of two cells, a tube 

 cell with a cell wall enclosing a central cell or generative cell 

 (pollen grain or microspore) which lacks a cell wall and floats in 

 the protoplasm of the cell, and forms two sperm cells on division, 

 the tube cell growing as a parasite through the stigma and stye to 

 the ovule. In some of the Gymnosperms the male gametophyte 

 is just as simple (the yew, Taxus, for example). The female 

 gametophyte shows a greater degeneration even than in the 

 Gymnosperms, being in the Angiosperms reduced to a few nuclei 

 in cytoplasm not separated by cell walls (the embryo sac) , and the 

 archegonium is reduced to the egg cell. Sometimes the female 

 gametophyte is developed from what may ba considered a true 

 spore, and in other plants from the mother cell direct, thus still 

 farther cutting short the development of the gametophyte gen- 

 eration. 



Eighth. The development of the flower, which marks the 



* The male gametophyte probably represents solely a reduced anthe- 

 ridium, or sexual organ. The tube cell may be the wall of the antheridium 

 here reduced to a single cell, for in Selaginella and Isoetes the antheridium 

 is formed from the larger part of the microspore, though wall cells com- 

 pletely surround the central cell which forms the sperms. However, in some 

 of the heterosporous ferns (the water fern Salmnia) the antheridium does not 

 have a complete wall of cells surrounding the central cell. 



