. SIZE AND POSITION OF SPORANGIA IT5 
number of the sporangia, not by variation of their dimensions: thus the 
number of the sporangia may come to be an approximate measure of 
the spore-output, as it is in fact in the Leptosporangiate Ferns. 
The several types of Pteridophytes differ greatly in the closeness of 
the relation of their sporangia to the axis of the shoot; it will be pointed 
out in detail below how the five main series of them—the Lycopodiales, 
the Equisetales, the Sphengphyllales, the Ophioglossales, and the Filicales 
—exhibit successive degrees of enlargement of the appendicular organs, 
and of the consequent removal of the sporangia from the central axis. 
The strobiloid character, with small appendages, and one sporangium at 
the base of each, or even seated on the axis itself, is characteristic of 
the Lycopods; but this regularity is not characteristic of the larger-leaved 
types: thus the definiteness in number and in position of the sporangia 
relatively to the other parts, which is as a rule absolutely exact in the 
Lycopods, is less strictly observed in the Equisetales and Sphenophyllales, 
and it is almost entirely absent in the Ophioglossales and Filicales, in 
which the sporangia are borne upon the large leaves, far removed from 
the central axis: their number and their arrangement there tends to be 
indefinite. These facts may be summarised into the statement that in 
the Pteridophytes those forms which bear their sporangia in closest relation 
to the axis show the most strict definiteness in their number and position : 
where the sporangia are removed from the central axis, being borne upon 
larger appendicular organs, they habitually show less definiteness in number 
and in position. 
The indefiniteness of number of the sporangia thus seen in the 
Ophioglossales and Filicales is an illustration of the variability of multiple 
structures, alluded to by Darwin as follows: “It seems to be a rule, as 
remarked by Is. Geoffroy St. Hilaire, both with varieties and species, 
that when any part or organ is repeated many times in the same individual 
(as the vertebrae in snakes and the stamens in polyandrous flowers) the 
number is variable: whereas the same part or organ, when it occurs in 
lesser numbers, is constant.” That constancy is seen in the Lycopods 
in high degree: it is departed from to some extent in the Sphenophyllales 
and Equisetales, and it becomes unrecognisable in the Ophioglossales 
and Filicales, in which the number of sporangia on each appendage is 
large. 
It has been remarked above that it is still an unsolved problem what 
those intimate influences are which determine the development of any 
specific cell of the plant-body as a spore-mother-cell on the one hand, or 
as a vegetative cell on the other. This determination lies at the root 
not only of the limitation of sporogenous tissues, but also of the initiation 
and consequent number of sporangia. The determining factors are probably 
numerous: suitable nutrition is certainly one. Speaking generally, better 
nutrition is clearly connected with more ample spore-formation ; but it is 
also well known that a plethoric state may lead to sterility in certain 
