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. 



