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 = Astilbf 

 japonica. X 550. (After Webb.) C Salix glaucophylla. X6oo. (After Chamberlain.) 

 D=Rosa livida. X224- (After Strasburger.) EAlchemilla alpina. X275. (After 

 Murbeck.) F^Callipeltis cucitllaria. (After Lloyd.) G = Quercus velutina. Xj2o. 

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



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 



