CASE OF PSILOTUM 89 
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 Psélotum, such as Lycopodium: here 
all the cells of a sporogenous group essentially similar to that of Ps¢lotum 
are equipotential; all of them normally undergo the tetrad-division 
Sirti 
Q 
Te ‘Go 
) 
{] 
Ui 
LY 
Raila? 
a yvetstne a 
SMES] 
/ é 
D £ 
Fic. 45. 
i 
a 
4 ba 
2 sat 
* Psilotum triguetrum, Sw. Various stages of development of the synangium and 
sporangium. In C the sporogenous group is shaded. J shows the differentiation of its 
cells, the fertile cells being shaded. £ shows the disorganisation of the remaining cells 
without forming spores. X 100. 
and develop spores. But in Psi/otum 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 £). The conclusion to be drawn is that in 
Lsilotum 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 séerzlésed, that is, diverted from. 
