Il4 THE POLYSPORANGIATE STATE 
the total output of spores from the numerous sporangia may be virtually 
unlimited: moreover, their production may be extended over many years 
on the same individual plant. Since, in the homosporous forms, each 
single spore is small, and therefore conveys with it only a small store 
of nutriment, the risks on germination are great; a reasonable chance of 
success is, however, secured by their large number. 
But with the heterosporous condition complications arose. Owing to 
the storage arrangements in the enlarged female spore, this more specialised 
state leads to economy in number of the spores necessary to secure survival 
and spread of area; for each female spore carries with it, in its higher 
store of nourishment, a higher probability of successful establishment of 
an embryo, and a sufficient degree of propagative probability can thus 
be attained with a moderate number of spores. Hence heterosporous 
types may be expected to present examples of reduction of number, not 
only of sporogenous cells, but also of sporangia. That is seen to be actually 
the case, and it might be illustrated by numerous examples. It will then 
be in homosporous types, which are certainly the more primitive, that 
we shall expect to meet with the best evidence as to the origin of the 
polysporangiate state, or with traces of increase in number of sporangia ; 
in fact, they will illustrate more faithfully than heterosporous forms the 
upgrade of complexity of their spore-producing parts. 
On grounds of nutrition of the spores, and of stability at the critical 
stage when the spore-mother-cells are floating in fluid, there is a clear 
advantage in the segregation of the spores into separate pockets—the 
sporangia—as against any method of indefinite enlargement of a single 
sac. It is probably such conditions as these which have also determined 
the limits of size of the individual sporangia of the Pteridophytes, and led 
to some degree of uniformity in their dimensions. But still considerable 
variations in size of the wsporangia are found to occur, even in close 
juxtaposition on the same plant: this is most conspicuous in the 
Eusporangiate forms. Sometimes the difference in size seems to be 
dependent on nutrition; for instance, it is usual to find about the’ upper 
and lower limits of the fertile strobilus of Lycopods sporangia of smaller 
size than those about the middle of the fertile region: the same is the 
case in the Psilotaceae and in £gwise‘wm. But in other cases this simple 
explanation will not suffice, for smaller sporangia may be found dis- 
tributed between the larger ones: this is especially so in the sori of 
the Marattiaceae, and a general survey shows that in many of the 
Eusporangiate forms the single sporangium is not quantitatively a definite 
unit. 
But though there may thus be wide variation of size of the individual 
sporangia in certain Pteridophytes, still in others their dimensions are 
often very uniform. In the Leptosporangiate Ferns, indeed, the number 
of spores in a single sporangium is often strictly constant. In that case 
change in the output of spores on the plant is effected by change in the 
