504 MOSSES AND FERNS chap. 



ginella continues up to the time of the final division of the spore 

 mother cells, each of which divides into four tetrahedral spores. 

 These in the microsporangium all develop, but in the macro- 

 sporangium only one of the tetrads reaches maturity. The 

 tetrad of macrospores fills the sporangium completely, and 

 with their growth the sporangium itself becomes four-lobed, 

 and very much larger than the microsporangia. The cells of 

 the wall remain green and fresh up to the time that the 

 macrospores are ripe, and sections show that the tapetal cells 

 are in close contact with the wall of the spores. The episporic 

 ridges are very evident before the spore has reached half its 

 final diameter, and sections of the spore wall at this time (Fig. 

 266, C) show the spine-like section of the surface ridges. The 

 wall rapidly increases in thickness as the spores grow, and this 

 increase is evidently due almost entirely to the activity of the 

 tapetal cells, as the spore at this stage contains very little 

 protoplasm. The first nuclear division in the macrospore 

 takes place when the spore is about half-grown, and by the 

 time it has reached its full size the cell divisions in the apical 

 region are complete and the archegonia have begun to form. 



The ripe sporangium, as in Lycopodiuni, opens by a 

 vertical slit. 



TJie Affinities of the LycopodinecE 



Among the living Lycopodineae there are tvv^o well-marked 

 series, one including the Lycopodiaceae and Selaginelleae, the 

 other the Psilotaceae. In the first, beginning with PJiyllo- 

 glossum, the series is continued through the different forms 

 of Lycopodium to the Selaginelleae. The relation of the 

 Psilotaceae to this series is doubtful, and must remain so until 

 the sexual generation of the former is known. The probable 

 saprophytic or parasitic life of these plants makes it impossible 

 to determine just how far their simple structure is a primitive 

 character rather than a case of degradation. 



Of the first series, it seems probable that of the forms 

 whose life history is known, the type of L. cernuum represents 

 the most primitive form of the gametophyte. It is reasonable 

 to suppose that in all these forms the prothallium was green, 

 and that the saprophytic prothallia, like those of L. pJdegmaria 

 and L. annotinuin, are of secondary origin. The prothallium, 

 of the type of L. cernuum, may be directly connected with 



