98 STERILISATION 
in homosporous and in heterosporous forms; the sterile cells may be 
functional sometimes only as transitory, nourishing cells; or they may 
persist as permanent tissue, forming in some cases partial, in others even 
complete septa. 
The converse case, viz. the conversion of cells normally sterile into: 
fertile cells, is a much less common phenomenon, though instances of it 
have been observed. This change is not to be confounded with the 
formation of whole organs of propagation, such as sporangia, in places. 
where they do not normally exist: what is here meant is the change in 
Fic. 55. Fic. 56. 
Casuarina Rumphiana, Mig. Casuarina glauca, Sieb. Median section of 
Median section of the nucellus of an nucellus of an ovule showing the cells of the 
ovule, with the group of sporogenous sporogenous group differentiated: some are 
cells shaded. 285. (After Treub.) becoming elongated in the direction of the 
chalaza: one long cell has divided by six 
swollen walls: another has developed as a 
tracheid. X285. (After Treub.) 
individual cells, which are normally vegetative, to the sporogenous condition. 
A case of this has been recorded by Lanzius Beninga in a specimen of 
Syntrichia subulata: certain cells of the normally sterile columella were 
found to be undergoing tetrad-division prior to forming spores: a similar 
condition has also been noted by Kienitz Gerloff in a species of Bryum. 
It has also been seen in rare cases in the Pteridophytes, that cells outside 
the limits of the normal sporogenous group, but contiguous with it, may 
show the characters of fertile cells. But the most distinctive case, which 
lLanzius Beninga, Bertraige z. Kennin, d. inn. Baues ad. angew. Mooskapsels, 1847,. 
Tab. 58, Figs. 9*, 9**; Kienitz Gerloff, Bot. Zett., 1878, p. 47, Taf. 2, Fig. 52. 
