SIZE AND POSITION OF SPORANGIA 115 



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 Sphenophyllales, 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 definite'ness 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 



