246 SUMMARY OF THE WORKING HYPOTHESIS 



number the better the chance of survival ; in this may be found the 

 rationale of the enormous numbers of spores habitually produced by 

 the homosporous Archegoniatae. To protect them while young, and to 

 nourish them during their development presupposes some vegetative 

 system, which will require to be more elaborate the larger Zthe number 

 of spores. The protection is in part supplied by the parent gametophyte, 

 though in all but the simplest it falls on the sporophyte. The nutrition 

 may also in some cases be supplied by the gametophyte, as it is in the 

 simpler Liverworts and Mosses ; but in the more advanced forms, after 

 the first embryonic stages are passed this duty falls on the sporophyte 

 itself, as in the Vascular Plants. The comparative study of the sporophyte 

 in its various living forms suggests certain factors of advance, which led 

 to its becoming efficient for carrying out these functions of protection 

 and self-nutrition, and thus conduced to its final independence ; the most 

 important of these are : (i) sterilisation of cells potentially sporogenous, so 

 as to supply a vegetative system (Chapter VIII.) ; (ii) the segregation of 

 the sporogenous tissue into distinct pockets, or sporangia, thereby facilitating 

 nutrition and dispersal (Chapters VIII. and IX.); and (iii) the origin of 

 appendicular organs, which serve a variety of purposes beyond the usual 

 direct ones of supporting the sporangia, and of nutrition (Chapter XL). 



Sterilisation of cells potentially sporogenous is a feature which is very 

 widespread among living sporophytes : evidence of its occurrence may be 

 drawn from all the main groups composing the characteristic Flora of 

 the Land (Chapter VII.). The argument to be based on this fact is as 

 follows : it is seen in plants of the present day that in definite cell-groups 

 of the sporophyte, which may be recognised as sporogenous, sometimes 

 the whole body of the cells undergo the tetrad-division, and form spores ; 

 in other cases, while certain cells of such groups are fertile, other cells 

 of like origin with them remain sterile : these may, however, subserve 

 various purposes in less direct relation to the production of the spores : 

 in certain cases the sterile cells may even develop as permanent tissue. 

 The conclusion from this is first ontogenetic : viz., that the sterile cells, 

 being sister cells with those which are fertile, are potentially sporogenous 

 cells which have been diverted from their original purpose, and that their 

 potential spore-producing capacity has been sacrificed to ensure the 

 success of those which remain fertile. The second conclusion is phylo- 

 genetic, and it follows from the fact that examples of such sterilisation 

 may be drawn from all the main groups of Plants which form the 

 characteristic Flora of the Land : it is that such transformation of cells 

 from the fertile to the sterile condition as is seen so commonly at the 

 present day, was also of common occurrence in the course of evolution 

 of the sporophyte. It would be going too far to say that there is in 

 this any demonstration of the source from which all vegetative tissues 

 of the sporophyte have been traced ; but at least this is a justifiable 

 working hypothesis. 



