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 



f 



FIG. 55. 



Casuarinn Rumphiana, Mig. 

 Median section of the nucellus of an 

 ovule, with the group of sporogenous 

 cells shaded. Xi>8 5 . (After Treub.) 



FIG. 56. 



Casuarina glanca, 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 MX 

 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 

 Syntrich'm siihulata : 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 GerlofT in a species of Biyum^ 

 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, Iti-itriige z. Ketiutn. d. inn. Uaues d. angt'w. Mooskafsels, 1847,. 

 Tal>. 58, Kigs. 9*, 9**; Kienit/. U-il.ilV, AW. Zeit., 1878, p. 47, Taf. 2, Fig. 52. 



