350 



BOTANY OF THE LIVING PLANT 



In the relatively simple case of Nephrodium pseudo-mas, var. polydactyla 

 a young sporophyte is produced as a direct outgrowth from the prothallus. By 

 a careful examination of the bud-forming tissue it has been found that the bud 

 is preceded by a sort of irregular fertilisation. The nucleus passes from one 

 cell through a pore in the cell-wall into the next cell. There it fuses with the 

 nucleus of the invaded cell (Fig. 294). Doubtless there is here a doubling of 

 the chromosomes, as in normal fertilisation ; and such a cell, like a false 

 zygote, may serve to initiate the sporophytic bud. The process has been 

 styled pseudomixis to suggest a comparison with sexuality, while marking its 

 distinctness from it. 



In other cases careful investigation has shown that a gametophyte may 

 be diploid. Transition from one generation to the other may then be repeated, 



FIG. 293. 



Pteris cretica ; prothallus seen from below, bearing an apogamous bud derived not by 

 fertilisation but by direct growth from the cushion. (After De Bary.) 



while uniformity of chromosome-number is maintained throughout. This 

 is seen in Athyrium filix-foemina, var. clarissima, where the number is 90, 

 approximately that for the normal sporophyte of that species. The same is 

 the case for certain plants of Marsilia Drummondii, which are diploid through- 

 out, with 32 as the number. It is probable that the converse is the case for 

 Lastraea pseudo-mas, var. cristata (Fig. 295}, for the chromosome-number 

 throughout was found to be variable, from 60 to 78, whue in that species the 

 normal number for the sporophyte is 144. Not only do such cases show that 

 the usual chromosome-cycle may be departed from, but also that the external 

 characters are not directly dependent upon the chromosome-number. 



The cycle of life of a Fern shows more clearly than that of any of 

 the Vascular Plants hitherto described the antithesis of the two 



