CASE OF PSILOTUM 



group have a common origin, and as sister cells they develop alike at 

 first : they may therefore be held to be equipotential cells (Fig. 45 c). 

 The conclusion that they are so is supported by comparison with plants 

 having some degree of affinity with Psilotum, such as Lycopodium : here 

 all the cells of a sporogenous group essentially similar to that of Psilotum 

 are equipotential ; all of them normally undergo the tetrad-division 



D 



FIG. 45. 



Psilotum triqnetruin, Sw. Various stages of development of the synaiigium and 

 sporangium. In C the sporogenous group is shaded. D shows the differentiation of its 

 cells, the fertile cells being shaded. E shows the disorganisation of the remaining cells 

 without forming spores, x 100. 



and develop spores. But in Psilotum the originally uniform group 

 differentiates at a relatively late period into fertile and sterile cells, the 

 former having dense protoplasm, and undergoing the tetrad-division ; the 

 latter having less dense protoplasm (Fig. 45 D) : these become dis- 

 organised without tetrad-division, and their substance goes to nourish 

 the young spores (Fig. 45 E). The conclusion to be drawn is that in 

 Psilotum all the cells of the sporogenous mass are potentially sporogenous, 

 as they are in Lycopodium, and probably were actually fertile in ancestral 

 forms : that some of them have been sterilised, that is, diverted from 



