24 PEOCEEDINGS OF THE 



When we come to the most highly organised of the Palaeozoic- 

 plants, the Cordaitales, constituting the characteristic Gymno- 

 spenns of that epoch, we find that the young stems had the 

 same " Dictyoxylou " construction of the cortex as was so comniou 

 among the contemporary fern-like Seed-plants. The Cordaitean 

 wood, however, often assumed a dense structui'e, and in many 

 cases (as also sometimes occurred among the Pteridosperms) there- 

 were tangential bands of narrow fibre-like wood-elements, sug- 

 gesting, tliough not identical with, the autumn wood of recent 

 Coniferous trees, and no doubt subserving a special mechanical 

 function. 



The exigencies of secondary growth, when occurring on a great 

 scale, idtimately demand that the mechanical tissues should be- 

 seated in the wood, on the inner side of the growing zone, though 

 this is not the best position on engineering pi'inciples. The old 

 plants were on the '\\hole more correct in their methods ; their 

 successors have more often had to adopt a compromise, which 

 sacrifices a certain degree of mechanical efficienc}' in order to 

 facilitate construction. 



In the leaves of the Cordaitese we meet with remarkably perfect 

 types of mechanical construction showing various applications of 

 the I-girder principle, "with utilization of the " web '* for the 

 protection of the conducting vascular strands. The construction is 

 on the same lines as that of many of the Monocotyledonous leaves 

 investigated by Schwendener in his classical work. It will be 

 remembered that the Cordaitean leaAes were originally classed as 

 those of Monocotyledons, which they closely resemble in form 

 and mechanical requirements. Here there is no secondary growth 

 to disturb the lines of a rational construction ; the leaves were of 

 great length and borne on lofty stems, requiring a strong 

 mechanical system for their support, and hence we find that they 

 present admirable illustrations of engineering principles. 



AVithout pursuing the subject further it may be added that 

 other Palaeozoic leaves show essentially the same types of 

 mechanical construction as are found in leaves of corresponding- 

 shape and dimensions in the \i\ ing Flora. 



These few illustrations may suffice to show that from an 

 engineering point of view, the plants of the Palaeozoic were just 

 as well constructed to resist the strains to which their organs 

 were exposed, as are their recent successors. Mechanical con- 

 struction provides a favourable means of testing the standard of 

 adaption in early fossil plants, for we may assume that in this 

 respect the conditions were essentially the same then as now. In 

 other cases, it is often difficult to estimate the perfection of the 

 mechanism, because we have no sufficiently exact data as to the 

 end which it served ; in many cases our knowledge of the working 

 of the machine even in a recent plant is still very imperfect. 

 This is especially true of the water-conducting apparatus in \ ascular 

 plants, the mode of action of which is still the subject of dispute 

 among physiologists. A few points bearing on the structure of 

 the wood in fossil plants may, however, be mentioned. 



