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 ithe 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 XI.). 
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 femain 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. 
