532 MOSSES AND FERNS CHAP. 



Miss Lyon (2) thinks that in both S. apus and 5\ rupestris 

 the whole sporangium may be traced back to a single super- 

 ficial cell, which she calls the archesporium. 



Bower (15) considers it probable that in S. spinosa and 5\ 

 Martensii the sporogenous tissue cannot be traced back always 

 to a single cell (in radial section), and has also shown that 

 when tangential sections are examined, as in Lycopodium, the 

 archesporium always is a row of cells. 



In all species of Selaginella yet examined, the sporangium 

 is not of foliar origin, but originates from the axis above the 

 insertion of the leaf by which it is subtended. 



As in Lycopodium the tapetal cells do not become disorgan- 

 ised, but remain intact as the inner layer of cells of the three- 

 layered sporangium wall. They form an epithelium-like layer 

 of papillate cells, distinguished by their dense granular con- 

 tents, and it is evident that they are actively concerned in the 

 elaboration of nutriment for the growth of the young spores 

 (Fig. 308). 



As in the other heterosporous Pteridophytes, the two sorts 

 of sporangia are -alike in their earlier stages, and this in Sela- 

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

 mother cells. In the microsporangium, all of the sporogenous 

 cells undergo the usual tetrad division; but in the macrospo- 

 rangium only a single one normally divides. Occasionally 

 one of the divisions is suppressed so that but two macrospores 

 result. In the microsporangium all of the spores mature, and 

 the spores remain small. The single tetrad of macrospores in- 

 creases enormously in bulk, and finally completely fills the mac- 

 rosporangium, which is itself much larger than the microspo- 

 rangia, and by the crowding of the enclosed spore-tetrad, as- 

 sumes a four-lobed form. 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 be- 

 fore the spore has reached half its final diameter, and sections 

 of the spore wall at this time (Fig. 308, 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 



