CONCLUSIONS. 43 



more central, non-exogenous cylinder, through the branches of which it passed to 

 the leaves. Assuming that my morphological data are accurate, about which I 

 have no doubt, the above appears to me to be the only physiological explanation 

 that can be given of the primary functions of the exogenous zone in these giant 

 Cryptogams. In the living Lycopodiacese, with the exception of Isoetes, nothing 

 like this arrangement exists. The rootlet bundles are all derived directly from the 

 same central source as the leaf-bundles, the two sets of bundles differing only in 

 the upward direction followed by the former and the downward one pursued by the 

 latter ; hence no lateral transference from one vascular system to another is neces- 

 sary. Another structure introduced into the extinct plants is the bark cambium 

 (Plate VIII, figs. 22 and 23), which has no definite existence in any recent form 

 except Isoetes. In this plant the cortex is more dependent upon a cambial layer and 

 is developed by that cambium in a more conspicuous manner than in any other 

 known plant ; but in this instance the same cambium adds centrifugally to the loose 

 axial mass of Tracheids on its inner side, and, centripetally, to the cortex on the 

 outer one. "We thus see that the ancient Lycopods had two cambium zones, the 

 functions of the outermost being limited to bark-growth, and the inner one appa- 

 rently to extension of the exogenous layers, the latter alone being concerned in the 

 orientation of the rootlet bundles. Isoetes, on the other hand, has but one cambium 

 zone which is equally concerned in the orientation of the leaf-bundles, the rootlet 

 bundles, and the development of the bark. Degradation from a higher to a lower 

 type of organisation has been followed by a generalisation of function, instead of 

 the opposite process of multiplication of organs and specialisation of functions 

 which attends progressive evolution. The supremacy of the Carboniferous Lyco- 

 podiaceas over their modern representatives therefore is not limited to their greater 

 magnitude, but includes a more complex organisation. 



There appears no doubt whatever that the Stigmaria is found in the Devonian, 

 Carboniferous, and Permian Rocks. In the two former it is in some cases asso- 

 ciated with both Lepidodendron and Sigillaria. But in the Arran Deposit at 

 Laggan Bay, where it was not rare, though we have Lepidodendroid branches in 

 profusion, we find no trace of Sigillaria. This association is yet more remarkable 

 at the plant-bearing deposit at Burntisland. The rock at that locality is largely 

 composed of Lepidodendroid twigs, but has not yet furnished, so far as I know, the 

 smallest trace of a Sigillaria. The late Professor Heer tells us that the Spitzbergen 

 deposits supplied Lepidodendron Veltheimianum, along with fine and large Stigmarias, 

 but no Sigillarias. The same author also obtained Stigmaria along with several 

 species of Lepidodendron at Bear Island, but again no Sigillarias ; and M. Lesquereux 

 cites Schimper's authority for the fact that a deposit in the Vosges is " filled with 

 a prodigious quantity of fragments of Stigmaria without trace of any Sigillaria," 

 but adds significantly, "that these strata contain abundant remains or trunks of 



