LINKEAK SOCIETY OF LOXDOX. 2$ 



Westermaier, in the coutroversy already mentioned, perhaps 

 went too far when he maintained that histological differentiation 

 was as far advanced in Pala-ozoic plants as in those of our own 

 day. We have no evidence that the complex structure of the 

 wood characteristic of our Dicotyledonous trees had any close 

 parallel in the Carboniferous Flora. The mechanism was con- 

 structed on other lines, but in its own way was elaborate enough. 

 For example, the extraordinary lattice-work structure of the 

 scalariform vessels, recently discovered by Mr. Gwynne-Yaughan, 

 was first suggested to him by observations on the wood of fossil 

 Fei-ns, belonging to the Osmundaceae. and goes back to Palaeozoic 

 members of that family. The complex mechanism of the wood- 

 elements with bordered pits was peculiarly characteristic of 

 extensive groups of plants in the Palaeozoic Flora. The horizontal 

 tracheides in the medullary rays, serving no doubt for the 

 transference of water in a radial direction, now peculiar to the 

 wood of the more liighly differentiated Coniferse, was anticipated 

 by the Palaeozoics Lycopods, as was also the remarkable '"trans- 

 fusion-tissue " of the leaf, a system of water-conducting elements 

 servins: to reinforce the vascular bundle in the irrigation of the 

 tissues of the leaf, and thus replacing the more complex venation 

 of other types. In the cases last mentioned — the ray-tracheides 

 and the transfusion-tissue — the peculiar differentiations in question 

 were, in my opinion, of quite independent origin in the two 

 groups of plants which have possessed them. 



1 have elsewhere dwelt on the gradual change in the con- 

 struction of the wood, correlated with the on-coming of secondary 

 growth, and have traced the slow extinction of the old, 

 " Cryptogamic," centipetally-developed wood, as the newer, 

 centrifugal wood, derived from a cambium, more and more 

 eft'ectually took its place *. In the former we have to do with a 

 structure becoming vestigial, but it is interesting to note how 

 the doomed tissue was not always left in its old age to be a mere 

 pensioner on its more active neighbours, but was often employed, 

 while it survived, on such work as it was still able to do. We 

 find, in quite a number of cases f, that the central wood had changed 

 its character, and shows by its structure that it had become 

 adapted to the storage, rather than the transmission of the water- 

 supply, its earlier function no'w being more conveniently left to 

 the external parts of the wood. Such utilization of a vestigial 

 structure appears to be a good mark of a high standard of 

 adaptation. 



Another interesting case of adaptive specialization in an organ 

 which ma}^ be regarded as of an old-fashioned type is to be found 

 in the rootlets of Sfir/maria. The nature of these appendages has 

 been much disputed — last year we had an interesting discussion 

 on the subject, opened by Prof. "Weiss. I have used the word 

 " old-fashioned "' because there is some reason to suppose that 



* Scott, D. H., " The Old Wood and the New." New Pliytologist, vol. i. 1902. 

 t Megaloxyloii, Zolesskyu, Lepidodvndrcm selafjinoides. 



