9 8 



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 



FIG. 55. 



Ca.sua.rina 



Median section of the nucellus ot an 

 ovule, with the group of sporogenous 

 cells shaded. X 285. (After Treub.) 



Rum phia.no., Mig. 

 dlus of a 



FIG. 56. 



Casuarina glauca, Sieb. Median section of 

 nucellus of an ovule showing the cells of the 

 sporogenous group differentiated : some are 

 becoming elongated in the direction of the 

 chalaza : one long cell has divided by six 

 swollen walls: another has developed as a 

 tracheid. X 285. (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 Bryuml 

 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 



1 Lanzius Beninga, Beitriige z. Kenntn. d. inn. Banes d. angew. Mooskapsels, 1847,. 

 Tab. 58, Figs. 9*, 9**; Kienitz Gerlofif, Bot. Zeit., 1878, p. 47, Taf. 2, Fig. 52. 



