FILICALES 



509 



* & 



./- 



frcn 



is preceded by a sort of irregular fertilisation. The nu< letlfl | 



cell through a pore in the cell-wall into the next cell. I here it hues with the 



nucleus of the invaded cell (Fig. 403). Doubtless there is here a donblin| 



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

 zygote, may serve to initiate the sporophytic bud. The proo 

 styled pseudomixis to suggest a comparison with sexuality, while marking it^ 

 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 re] 

 while uniformity of chromosome-number is maintained throughout. Thi 

 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 

 throughout, with 32 as the number. It is 

 probable that the converse is the case for 

 Dryopteris pseudo-mas, var. cristata (Fig. 404), 

 for the chromosome-number throughout was 

 found to be variable, from 60 to 78, while in 

 that species the normal number for the sporo- 

 phyte 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 generations which constitute it. 

 Each can grow, nourish itself, and even 

 multiply independently of the other. It 

 is true that the young sporophyte is nursed 

 temporarily in the parent prothallus. But 



this is only a transient event and is soon over. A similar nursing 

 period, with much greater adaptive detail that lends added efficiency, 

 is seen in the Seed-Plants. The main difference between the Seed- 

 Plant and the Fern lies in their spores. The former are lit (cros porous, 

 the latter are, with few exceptions, homosporous. The advantage of the 

 large female spore is that it contains already a supply of nourishment 

 for the young germ after fertilisation, so that a vegel rive prothallus 

 is not necessary. Especially is this so in Seed-Plants where the spore 

 is retained in the tissue of the parent, and can draw nourishment 

 continuously from it. On the other hand, in the primitive homo- 

 sporous state, as it is seen in the Bryophytes and Pteridophytes, the 



Fig. 404. 

 Dryopteris pseudo-mas, v. cristata 

 (Cropper). Drawing by Dr. Lang 

 showing apogamous transition from 

 prothallus to sporophyte, and sub- 

 sequent aposporous transiting from 

 sporophyte to prothallus at the apex 

 and margins of the leaf. 



