SPENCERITES 193 



reached at least as high a differentiation as the most 

 highly organised Lycopods of the present day. They 

 differ most strikingly from recent strobiliferous forms in 

 the mode of insertion of the sporangia, which in existing 

 Lycopodiaceous cones are either axillary {Selaginelld), 

 or are seated on the sporophyll, close to the axis l -(Ly co- 

 podium and Phylloglossuni). In Lepidostrobus, as in the 

 recent, non-strobiliferous Isoetes, the enormous develop- 

 ment of the sporangia is due chiefly to radial elongation, 

 and this brings with it a corresponding extension of the 

 base of the sporangium, which is attached to' the long 

 pedicel of the sporophyll from end to end. The displace- 

 ment of the ligule, as described above, affords a good 

 measure of the radial extension of the whole organ. 

 The presence in some cases of trabeculae was no doubt 

 an adaptation to the great bulk of the sporangium. 



4. The Seed-like Lycopodiaceous Fructifications. — 

 We can well understand that an enormous output of 

 spores must have been necessary in the Lepidostrobi, 

 in so far as they were heterosporous. For fertilisation 

 and the development of an embryo to take place, it was 

 essential for microspores and megaspores to germinate 

 together, and where these cones were borne on trees, and 

 often at a great height above the ground, the chances 

 must have been enormously against such an association. 

 The successful accomplishment of the reproductive act 

 could only be ensured by the production of a prodigious 

 number of spores, and especially of microspores. It 



1 Psilotum and Tmesipteris are left out of consideration, as it is quite 

 doubtful whether the synangium of these genera is comparable to a single 

 sporangium of the typical Lycopods. See Chapter XIV. 



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