702 CONCLUSION 
Land. The theory presupposes in the first instance post-sexual complica- 
tions involving reduction: by deferring that event through sterilisation 
of individual cells, a neutral cell-group is established: this shows con- 
tinued growth, and further progressive sterilisation as it is seen exemplified 
in the Bryophytes, and leading to their complete state with a vegetative 
system of considerable extent and a concrete fertile tissue. Such 
sterilisation of individual cells or cell-groups is also seen among 
Vascular Plants, and has been in them a source of vegetative increase. 
But in them, with their discrete sporangia another, and a more effective 
factor arises, viz., the abortion of whole sporangia and sporangiophores. 
This leads in a more rapid and wholesale fashion towards the same 
end, viz, the establishment of a vegetative system, by separation of 
the function of nutrition from that of propagation in a shoot primitively 
constructed to serve both purposes. Such an early state is seen in 
every plant which shows the ‘Selago” condition; it has been shown 
above that this exists in more or less obvious form among the representatives 
of all the main phyla of Vascular Plants: and that it figures among their 
early fossil forms. There is less certainty about the earlier steps of origin 
of the sporophyte in the poly-sporangiate type, and analogy with the 
Bryophytes has to serve in place of more direct observation. But the 
later steps, by abortion of spore-producing parts, are more secure, even 
though the observations are frequently of the negative fact established by 
comparison, viz., that certain parts are not present, having been com- 
pletely obliterated, so that not even a vestige remains to show what has 
happened. 
In the nature of things this theory of the origin of the sporophyte, 
and of its establishment as the leading factor in the Flora of the Land, 
is not susceptible of direct or full proof under present conditions. But 
it offers a coherent account of how the sporophyte may have arisen: it 
is based on a wide comparative study of known forms from the point 
of view of their individual development, their external morphology, their 
anatomy, spore-producing members, and embryology: it does not assume 
wide-spread reduction, nor does it postulate any imaginary types, but 
proceeds by comparison of those forms of which there is evidence 
actually existing either in the living or the fossil state. On _ these 
grounds the theory is put forward with some degree of confidence, though 
in the full knowledge that it has not been, and indeed that it cannot 
be, proved. 
