350 GENERAL BOTANY 



it still bears definite archegonia with neck cells and a ventral-canal 

 cell nucleus inherited from its fernlike ancestors of the coal period. 



In the angiosperm no cellular gametophyte is formed before fer- 

 tilization, and the gametophyte is reduced to the egg apparatus, 

 polar nuclei, and antipodals. Pollination and fertilization methods 

 are also greatly modified, in both the higher gymnosperms and 

 angiosperms, by the permanent inclosure of the gametophyte in the 

 megasporangium. In Selaginella the motile sperms reach the eggs 

 in the exposed archegonia by swimming in films of water. In cycads 

 the sperms are still motile, with cilia, but they are liberated in a 

 special chamber, called the archegonial chamber, into which the 

 mouths of the archegonia open. In the higher gymnosperms and 

 angiosperms, however, fertilization depends upon the growth of the 

 pollen tube through the micropyle down to the egg ; by this growth 

 a canal is formed, down which the nonmotile male gametes reach 

 and fertilize the eggs. This is true siphonogamy, or the fertilization 

 of the egg through the intermediary of a pollen tube. In the angio- 

 sperms the development of a pistil, with stigma and style, renders 

 pollination and fertilization even more difficult and is correlated 

 with the elaborate devices for securing pollination observed in many 

 angiosperm flowers. 



The seed is a complicated structure composed of the megasporan- 

 gium of the mother plant and the embryo. In the angiosperms the 

 true gametophyte of the gymnosperms is replaced in many species 

 by the endosperm, which, as we have learned, is a nutritive tissue 

 formed as a result of fertilization. In 'other respects the seeds of 

 angiosperms and gymnosperms are quite similar in structure. The 

 most striking advances and changes leading to the evolution of 

 seeds are, therefore, the reduction of both male^and female gameto- 

 phytes, the reduction in the number of megaspores produced by the 

 megasporangia, the retention of the megaspores and female gameto- 

 phy tes permanently within the sporangia, and the changes in methods 

 of pollination and fertilization correlated with this retention. 



