326 LYCOPODIALES 



the rest in the distal insertion of the sporangium upon the sporophyll, 

 corresponding in this respect to Spencerites. 



Taking a general view of the fructifications of the Lycopodiales, the 

 most salient feature is the constancy of the numerical relation of sporan- 

 gium to sporophyll. In the whole phylum of the Lycopodiales each 

 sporangium is subtended by its sporophyll, while the median planes of 

 both those parts coincide. In most cases the sporangium is in close 

 proximity to the axis, or it may even be inserted upon it : occasionally 

 its position is further removed from the axis and inserted towards the 

 distal end of the sporophyll : these differences are of secondary importance 

 so long as the median position is regularly preserved. It is to be noted 

 that such extreme conservatism in number and in place of the sporangia 

 is peculiar to this phylum of Vascular Plants, in which also the closest 

 relation exists between the sporangia and the axis : in all other types 

 the sporangia show not only a less close relation to the axis, but also 

 less definiteness in number and in position : there is often, indeed, some 

 rough proportion between the size of the appendages and the number of 

 the sporangia which they bean 



The type of the sporangium itself is constant, though liable to differences 

 in proportion : it is always more or less fan-shaped in tangential section, 

 but the angle of spread of the fan is liable to considerable variation. It 

 is, however, in the extension radially outwards from the axis that the 

 greatest differences of proportion are seen, and it has been shown above 

 that in the living species of Lycopodium the differences may be correlated 

 with the degrees of differentiation of the strobilus from the vegetative 

 region; the narrow compressed form of sporangium with relatively thin 

 stalk is found in the less differentiated, the sporangium more radially 

 extended with short thick stalk in those with more clearly differentiated 

 strobili. The extremes of radial extension are seen in the dendroid fossils, 

 as well as in Isoetes. It would seem probable, as suggested by the com- 

 parative study of the living species of Lycopodium, that the larger sporangia 

 are derivative types, and that the enlargement was consequent upon 

 increased facilities of nutrition : such increased facilities are afforded by 

 the large size of the assimilating leaves in Isoetes; but in the more 

 differentiated species of Lycopodium, and in still higher degree in the 

 dendroid fossils, by the extensive vegetative system which precedes the 

 production of cones. The abortion of sporangia, and consequent reduc- 

 tion of their number in proportion to the foliage leaves, would tend in 

 the same direction. Such circumstances would encourage enlargement 

 of the spore-output, which is most readily and directly secured by increase 

 in size of the individual sporangium in so hide-bound a type as that of 

 the Lycopodiales. The extreme enlargement led to mechanical and 

 nutritive difficulties, which were met, perhaps independently, in Isoetes 

 and in some Lepidodendrons by the formation of trabeculae : these origi- 

 nated in Isoetes by partial sterilisation of sporogenous tissue. But though 



