LINNEAN SOCIETY OF LONDON. 23 



contradictiou to the principles of the engineei* '' merely showed 

 that the critic had failed to distinguish between the supporting 

 and conducting tissues of the plaut. It appears to have been 

 characteristic of PaliBozoic plants that their mechanical tissues 

 were, to a great extent, independent of the wood and concentrated 

 in the outer cortex — the most advantageous position on engineering 

 principles. For example, the extremely prevalent " Dictyoxylon " 

 type of cortex, in which bands of strong, fibrous tissue, united to 

 form a network, alternate with the living parenchyma enclosed in 

 their meshes, was an admirable mechanical construction for stems 

 Avhich did not attain any great thickness by secondary growth. 

 Where such growth was so extensive as to put the primary sup- 

 porting system out of action, we iind, as in species of Siglllaria and 

 Lepidodendron, a secondary Dictyoxylon framework set up in the 

 periderm, and no doubt renewed as further growth went on. The 

 periderm, so typical a feature of the tree-Lycopods of the Palaeozoic, 

 was not a mere bark, but constituted the chief mechanical tissue of 

 the older trunks. The wood, only moderately developed, was, as a 

 rule, too centrally placed to afford an efficient resistance to bending 

 strains, and was a comparatively soft, thin-walled tissue, evidently 

 adapted solely or chiefly for conducting purposes. 



In the Calamites, we find, in young stems, the same alternation 

 of fibrous and parenchymatous bands in the cortex, which is so 

 familiar to physiological anatomists in the stems of our living 

 Horsetails. In the more mature Calamitean stems we meet with an 

 immense development of periderm, which may have had a mechanical 

 function like that of the Lepidodendrea), though in Calamites the 

 wood often had a denser structure, and may have contributed more 

 materially to suppoi't. 



The great Tree-ferns of the later Carboniferous (if Ferns they 

 were) evidently depended for their mechanical strength on a 

 stereome or siipporting tissue quite distinct from the vascular 

 system, and for the most part peripherally disposed, as it should 

 be. Their power of i"esistance to bending strains was no doubt 

 greatly increased by the dense external envelope of strongly con- 

 structed adventitious roots, imbedded in the cortex, a mode of 

 support which we meet with in some Monocotyledons such as 

 Khigkt (Liliaceae) and species of P«?/a (Bromeliacete) at the present 

 day. 



The remarkable Palaeozoic genus SplienophyUum shows an only 

 moderately strong construction, and it may be that here the central 

 woody axis was of greater relative value as a support, but from the 

 habit we may be sure that the species were not ordinary upright 

 terrestrial plants, and that the conditions of stability were dilferent 

 from those in the other cases cited. The old view was that Spheno- 

 pihyllam was an aquatic genus; thei'e are, however, many argu- 

 ments against this, and of late years Prof. Seward's suggestion 

 that the species may have been scrambling climbers, supporting 

 their weak stems by the aid of their more robust neighbours, has 

 found favour and would explain a construction possibly adapted 

 to tensile strains. 



