GROUP III. PTERIDOPHYTA : FILICINVE. 



357 



GAMETOPHYTE. As Isoetes is heterosporous, the gametophyte is repre- 

 sented by distinct male and female individuals, which remain connected 

 with the spores producing them: they resemble those of Selaginel la ami 

 of some Gymnosperms (q. r.). 



The male individual is developed from a microspore. The microspore 

 which has the form of the quadrant of a sphere and is consequently of 

 the bilateral or radial type undergoes,, on germination, division by a. 

 transverse wall, 

 formed near one 

 of its somewhat 

 pointed ends, into 

 two cells, a large 

 and a small : the 

 latter is the vegetav- 

 tive cell, and un- 

 dergoes no further 

 change ; the former 

 is the mother-cell 

 of the male organ or 

 antheridium. The 

 prothallium here is 

 thus very much re- 

 duced, consisting of 

 a single antheri- 

 dium and of a 

 single purely vege- 

 tative cell. The 

 antheridium, de- 

 veloped by the 

 growth and divi- 

 sion of the mother- 

 cell, consists of four 

 peripheral cells 

 forming the wall, 

 and of four central 

 cells, each of which 

 gives rise to a 

 single spirally 

 coiled multiciliate 

 spermatozoid. 



The female indi- 

 vidual is developed 

 from a macrospore. 

 The macrospores 

 are much larger than the microspores, and are nearly globular in form ^ 

 though they belong to the tetrahedral type, as can be seen by the three 

 ridges on the spore where it was in contact with the other three developed 

 from the same mother-cell. On germination, the nucleus of the macro- 



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Fie. 213. Itoetes lacustris (after Luerssen). A Plant, half 

 naLsize: r dichotomously branched roots. B Inner (ventral) 

 surface of base of a sporophyll : I ligule ; / fovea. C Longi- 

 tudinal section of base of a sporophyll : p the sporangium 

 in the fovea ; tr the trabeculae ; e the velum ; I the ligule. D 

 Transverse section of the base of a sporophyll : letters as in C. 



