154 THE PLANT CELL. 



(macrospore) are to be regarded as a remnant of a former 

 female prothallium, which is represented in the Gymnosperms 

 by the prothallium formed earl}- in the embryo-sac, and which 

 is existent in the macrospore of the Heterosporous Pteridophyt.n. 

 The synergidse, in like manner, have been looked upon as 

 remnants of archegonia, or oogonia, occurring in Gymnosperms, 

 or Pteridophyta, as special organs growing upon the female 

 prothallium or gametophyte (sexual generation). The prothallus 

 of Homosporous Pteridophyta is in reality a double structure, 

 homologically, although the spore from which it is produced 

 shows no signs of a mixed or hermaphrodite nature. It is not 

 homologous to the prothallium of Gymnosperms alone, but to 

 the combined prothallial cell of the microspore, and the pro- 

 thallium of the embrj'o-sac or macrospore. The antherozooids 

 of the Pteridophyta and lower types are homologous, not with 

 the whole microspore of Augiosperms and Gymnosperms, but 

 with the generative cells only in that structure. 



In the Heterosporous Pteridophyta, the male prothallium 

 formed in the microspore is homologous with the prothallial cell 

 formed early during the maturation of the microspore in some 

 Angiosperms {Sparganium) and Gymnosperms, and the female 

 prothallium formed in the macrospore is homologous with the 

 prothallium formed early in the embryo-sac of Gymnosperms, 

 and, as was above stated, probably with the antipodal ells 

 produced during the maturation of the macrospore in the 

 Angiosperms. 



In the Cycadeae (Gymnosperms) antherozooids or spermato- 

 zooids are met with which are produced in the microspore, and 

 the prothallial cells cut off early in this microspore somewhat 

 recall the male prothallium formed in the microspore of Hetero- 

 sporous Pteridophyta. The C3-cads probably form the nearest 

 existing link between the Angiosperms and the Pteridophyta, 

 more especially with the Heterosporous members of that group. 



The phenomenon of double fertiUsation in the Angiosperms 

 has no parallel in the lower groups. By this method a secondary 

 prothallium is produced in the macrospore (embryo sac), by 

 means of which the embrj'o is nourished during the period 

 which precedes germination, and for a short time after it. In 

 the lower gioups, however, the embryo-sporophyte depends 

 partly for its first nutriment upon the cells of the prothallus 



