STERILE CELLS OCCASIONALLY FERTILE 



99 



has been fully made out, is that of Tmesipteris : the normal synangium 

 of this plant has when mature two loculi, divided by a septum some five or 

 six layers of cells in thickness. Certain synangia of small size are found 

 about the limits of the fertile zones : they appear non-septate, and it has 

 been shown that the cells of the septum in such cases develop as fertile 

 cells, undergoing the tetrad-division (Fig. 58). Such examples show that 

 occasionally a reversion may occur from cells normally sterile to the 

 function of spore-production. Putting together the two converse series of 



FIG. 57. 



Longitudinal sections of ovules showing multicellular archesporia. A, B = Astilbe 

 japomca. X 550. (After Webb.) C = Salix glaucopkylla. x6oo. (After Chamberlain.) 

 D = Rosa livida. X 224. (After Strasburger.) E=Alchciiiilla alpina. x 275. (After 

 Murbeck.) F=Callipeltis cucullaria. (After Lloyd.) G = Qucrcus vclutina. x 720. 

 (After Conrad.) From Coulter and Chamberlain, Morphology of Angiosperjns. 



facts of sterilisation which is relatively common, and of reversion to the- 

 fertile state which is comparatively rare two conclusions may be drawn : 

 first, that the facts indicate a preponderance of the former over the latter 

 in plants now living : in them sterilisation appears to be a more potent 

 factor now than reversion, and it has probably been the same in the past 

 also. Secondly, it may be stated generally for Archegoniate and Seed- 

 bearing Plants, that spore-production is not always strictly limited to, or 

 defined by pre-ordained formative cells or cell-groups. 



Voechting has formulated the proposition that " No living vegetative 

 cell of the plant-body, which is capable of growth, has a specific and 



