CHAPTER IX. 



SOME GENERAL ASPECTS OF THE POLYSPORANGIATE 



STATE. 



IT has been pointed out in Chapter VI., which dealt with the Biological 

 Aspect of Alternation, that in the case of plants of aquatic origin migrating 

 to the land an increasing production of non-sexual germs, or spores, would 

 become important. Since under those circumstances dependence could no 

 longer be placed on frequent recurrence of fertilisation, the production of 

 numerous spores as a consequence of a single fertilising act will be essential, 

 if the race is to survive and be in a position to compete and to extend 

 its area. Other things being equal, the larger the spore-output the better. 

 This should be constantly before the mind in the comparative study of 

 the more primitive types of sporophyte, and the same principle should 

 be applied to the more complex forms also, though in them the evidence 

 is necessarily less obvious. 



The antithesis between the Bryophyta and the Pteridophyta, as regards 

 the method of spore-production, is chiefly marked by the former having 

 one concrete sporogenous tissue, the latter numerous discrete sporogenous 

 groups which form the centres of more or less distinct sporangia. The 

 Bryophyte type is essentially a limited one, for indefinite enlargement 

 of the concrete sporogenous tissue introduces mechanical and nutritive 

 difficulties : these are mos't urgent at the critical period of separation of 

 the spore-mother-cells, when they are floating freely in the fluid contents 

 of the spore-sac. In actual life the Bryophyte type is almost always 

 annual, and does not extend beyond limited proportions; nor is there 

 evidence that it ever attained a larger size in earlier periods. This is 

 exactly what biological considerations would have led us to anticipate. 



But in the homosporous Pteridophytes, given an enlarging vegetative 

 system, which in them is usually perennial, there seems no limit to the 

 number of sporangia which may be borne on the individual plant ; and 

 as each sporangium is of moderate size, the mechanical and nutritive 

 requirements at the critical period of tetrad-division are suitably met, while 



H 



