PHYSIOLOGY OF REPRODUCTION IN PLANTS. 243 



organic continuity is established which justifies our regarding 

 the two alternate "generations " as we do. It is clear, how- 

 ever, that some assumptions lie behind this, and that tacit 

 necessity for fitting- in the life-history to the ascertained 

 alternation in Ferns plays some part ; for, in the first place, 

 mere breach of continuity would apply to any naturally 

 separated tubercule or bud or even a piece of protonema, 

 and, in the second, we know that the protonema need not 

 arise from a spore, but may be developed from any part of 

 the gametophyte or sporophyte — e.o-., a rhizoid, a piece of 

 stem, or of seta, etc. 



Obviously one underlying assumption is that the normal 

 life-history does not admit of the breaches of vegetative 

 continuity, or of the "misplaced" origins of protonema 

 referred to above. 



But normal means 7(sual in these cases. The same ap- 

 plies to the Ferns, where apospory and apogamy have now, 

 moreover, been shown to be so common that one is almost 

 driven to ask how soon it will be necessary to take a sort 

 of census as to the proportion of cases where "normal" 

 alternations of generations occur, and those where we find 

 the new Fern-plant — the Sporophyte — springing direct from 

 the previous sporophyte without the intervention of the 

 gametophyte- generation ; or sporangia springing direct 

 from the prothallus, and so on. 



The absolute number of these "abnormal" cases is at 

 any rate large, and when we come to look over the whole 

 domain of the Cryptogams, our doubts as to any fixed 

 necessity for the alternation of generations increase in pro- 

 portion as the question, How far is it all a matter of 

 conditions of nutrition, moisture, illumination, temperature, 

 etc. '^ assumes definite shape. For however much we may 

 assent to the typical character of alternations of generation 

 in Bryophyta and Ptei'idophyta, we must concede that such 

 alternation is not essential, and the gradual reduction of 

 one generation in the higher plants till it is merely repre- 

 sented by the most obscure traces, detected only with 

 difficulty aud by special methods, is a further proof of 

 this. 



