94 STERILISATION 
of the arrested cells is sometimes simply nutritive, sometimes they form 
permanent tissue-masses. In Lycopodium and Phylloglossum, and in the 
homosporous Ferns, after the sporogenous tissue is first defined, all its 
cells normally undergo the tetrad-division, and develop spores: occasional 
cells may become disorganised without full development, though, as a 
tule, all the potentialities are realised. But among the Marattiaceae, 
where, as a rule, all the cells of the sporogenous groups are fertile, a number 
of exceptional cases have been noted: the most remarkable is that of 
Danaea, in which it has been shown how certain cells of the larger sporo- 
genous groups remain sterile, and may be developed as tapetum, or even 
as component cells of a partial septum (Fig. 49). Somewhat similar 
conditions have been seen in Kaulfussia, Marattia, and Angiopterts. 
Fic. 50. 
A, apex of sporangium of guisetum limosum, L., showing the sporogenous cells, 
surrounded by the tapetum (shaded), and sporangial wall. 28, shows part of an older 
sporangium with its tapetum (¢) still clearly defined, though the individuality of the cells 
is lost: within this the sporogenous tissue, of which certain cells (a) are abortive. 
X 200. 
In other cases it has been found that only a portion of the cells of the 
sporogenous group are fertile, as already described for Psclotum (Fig. 45): 
this has been shown to be the case also in Zmeszpteris, and in Egutsetum 
(Fig. 50), and it has been described also as an occasional feature in the 
Ophioglossaceae. In all of these a varying proportion of the sporogenous 
cells are sterile. and become disorganised without forming tetrads. As. 
the proportion of the sterile to the fertile cells is not fixed, an elastic 
arrangement exists which leads to the largest number of spores being 
brought to maturity that the plant at the time can support. The limits 
of the sporogenous tissue in early stages are difficult to define in these 
large sporangia, and they show considerable irregularities: this is especially 
So in Psilotum, Tmesipteris, and Ophioglossum, and it appears to be partly 
due to the ill-defined and broad tapetum which is formed peripherally, 
