510 Scientific Proceedings, Royal Dublin Society. 



intemode. It appears from the Kiltorcaii material as if there must also be 

 present internal sclerotic bands or stereom-]amella3, possibly in connection with 

 tlie vascular bundle and parichnos-strands, and possibly capable of being 

 added to as the plant ages, because the ribbing is often more pronounced in 

 older stems in which the leaf-scars are still clearly recognizable. I have seen 

 it very well developed in stems 12 inches in diameter, and 7 or 8 feet long. 

 In the specimen figured the ribs are 3'3 mm. in width. The apparent absence 

 of a pronounced cylinder of secondary wood may be correlated with the 

 marked deciduous character of the foliage. Though conducting tissue may 

 not be much required, strengthening bands of sclerotic tissue would be for a 

 stem 20-25 feet long, 10-12 inches wide, and much branched. Tiie ribs in 

 B. hiltorkense might arise, like the zone of prosenchymatous tissue in 

 Lepidodendron, as the plant aged, from formative tissue in the cortex. 

 Potonie regards the ealamitoid condition as indicative of a special state of 

 preservation of the Knorria-stage. I go further than this, and regard the 

 ribbed and zoned condition as a natural feature of tlie Botlirodendron stem. 



It is possible that the fluting of the stem in Bothrodendron, with its early 

 deciduous leaves, is a physiological adaptation comparable to that found in a 

 recent Equisetum, with its inconspicuous leaf-sheaths, wliere the stem by its 

 adaptation of structure functionally replaces, to a large extent, the leaves. 

 One can picture a Bothrodendron in its swampy habitat, with its terminal 

 tufts of long subulate leaves, and grooved, dull-green, pseudo-joiuted stems 

 suggestive of our much smaller Equisetiini sylvaiicum in its boggy mountain 

 glen. 



The marked intervals between the leaf-scars in the older stem is, it is 

 generally agreed, explicable by the extension of the stem-surface in the 

 enlarging shoot. The younger stems show the leaf-scars in close contact 

 before expansion begins ; and this closeness of the scars is illustrated in the 

 modern Lycopodium throughout its thin stem. Thus we seem to have a 

 modern representative (Lycopodium) of an ancient group, throughout its life 

 in the condition passed through in its early stages by one of the most ancient 

 members of the group (Bothrodendron). Just as in metabolism there is 

 marked reversibility of physiological processes, so here we apparently have 

 reversibility of evolution. The most recent member of the group represents 

 in its permanency a stage passed through in its ontogeny by one of the 

 earliest members ; thus the phylogeny of the Lepidophytes is not recapitulated 

 in the ontogeny of its latest representative. The group has now retrograded 

 to its earliest type ; the intervening stages have been tried, found wanting, 

 and now lie buried in the rocks. Bothrodendron itself dies out in the Upper 

 Carboniferous. Thus in an undoubted Lepidophyte we have those clearly 



