(Chee TER UX, 
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 most 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 
