THE MALE FERN DRYOPTERIS FILIX-MAS 

 means, can persist and multiply if not adversely affected by selection. Local popula- 

 tions may therefore virtually be single clones, and their appearance of extreme 

 uniformity already commented upon is almost certainly an expression of this. 





Fig. 42. Sexual and apogamous prothalli. a. Median longitudinal section through a prothallus of 

 diploid Dryopteris Borreri showing apical cell of prothallus and an early stage of an apogamous 

 embryo behind, x 100. b. The same through a sexual prothallus of Z). abbreviata Newm. showing 

 central cushion bearing archegonia and glandular hairs on the lower surface, x 100. c. The 

 same through a sexual prothallus of D. Filix-mas sens-strict. emend, showing glandular hairs and 

 young archegonia on the central cushion near the apex, x 100. d. Whole mount in glycerine 

 jelly of a prothallus of diploid D. Borreri Newm. at the stage sectioned in a. The apogamous embryo 

 appears like a dark spot, x 10. e. The same of a sexual prothallus of Z). Filix-mas with archegonia. 



The principal characteristics by which apogamy in these ferns can be diagnosed are 

 as follows : 



(i) The chromosome number of the prothalli is the same as that of the parent plant, 

 no matter what this may be. 



57 



