ii6 



NA TURE 



[July 22, 1909 



in the plants of our own part of Europe since Glacial or 

 pre-Glacial times. 



The conclusion follows that at any given time, apart 

 from the relatively short critical periods when changed 

 conditions had to be met, we must expect to find organisms 

 in a state of complete adaptation to their surroundings. 

 When physical, and especially mechanical, conditions are 

 in question, such as have practically remained constant 

 through all geological time, we may recl<on on finding 

 the corresponding adaptive structures essentially the same 

 at the earliest periods as they are now. 



Hence the attempt to support the Darwinian theory by 

 the detection of imperfect mechanical adaptations in 

 Palaeozoic plants is wholly futile, as was well shown by 

 the late Prof. Westermaier. This author's own point of 

 view was not that of a Darwinian, but, nevertheless, his 

 conviction that efficient adaptation has always been 

 characteristic of living organisms is a perfectly sound one, 

 tlioroughly in harmony both with the principles of Darwin 

 and Wallace, and witli the observed facts, as far back, at 

 any rate, as the palaeontological record extends. In par- 

 ticular, Westermaier 's contention that the construction of 

 the Carboniferous plants followed the laws of mechanical 

 stability and economy of material, just as is the case in 

 plants of our own day, is completely confirmed by accurate 

 observations on their structure, while an opponent's sup- 

 posed detection of Palaeozoic constructions " in direct con- 

 tradiction to the principles of the engineer " merely show-ed 

 that the critic had failed to distinguish between the sup- 

 porting and conducting tissues of the plant. It appears 

 to have been characteristic of PaUeozoic 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 which did not attain any great 

 thickness by secondary growth. 



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. 



The great tree-ferns of the later Carboniferous (if ferns 

 they were) evidently depended for their mechanical strength 

 on a stereome or supporting tissue quite distinct from the 

 vascular system, and for the most part peripherally dis- 

 posed, as it should be. Their power of resistance to 

 bending strains was no doubt greatly increased bv the 

 dense external envelope of strongly constructed adventitious 

 roots, imbedded in the cortex, a mode of support which 

 we meet with in some monocotyledons such as Kingia 

 (Liliaceae) and species of Puya (Bromeliacea;) at the 

 present day. 



When we come to the most highly organised of the 

 Palaeozoic plants, the Cordaitales, constituting the 

 characteristic gymnospcrms of that epoch, we find that 

 the young stems had the same " Dictyoxylon " construc- 

 tion of the cortex as was so common among the con- 

 temporary fern-like seed-plants. The cordaitean wood, 

 however, often assumed a dense structure, and in many 

 cases (as also sometimes occurred among the pterido- 

 sperms) there were tangential bands of narrow fibre-like 

 wood-elements, suggesting, though 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, ultimately 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 principles. The old plants were, on the 

 whole, more correct in their methods ; their successors 

 have more often had to adopt a compromise, which 

 sacrifices a certain degree of mechanical efficiency in order 

 to facilitate construction. 



In the leaves of the Cordaitere we meet with remarkably 

 perfect types of mechanical construction showing various 

 applications of the I-girdle principle, with utilisation of 

 the " web " for the protection of the conducting vascular 

 strands. The construction i? on the same lines as that 

 NO. 2073, VOL. 81] 



of many of the monocotyledonous leaves investigated by 

 Schwendener in his classical work. It will be remembered 

 that the cordaitean leaves 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. 



Without 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 living flora. 



These few illustrations may suffice to show that, from 

 an engineering point of view, the plants of the Palseozoic 

 were just as well constructed to resist the strains to which 

 their organs were exposed as are their recent successors. 



I have elsewhere dwelt on the gradual change in the 

 construction of the wood, correlated with the on-coming 

 of secondary growth, and have traced the slow extinction 

 of the old, " cryplogamic," centripetally developed -wood, 

 as the newer, centrifugal wood, derived from a cambium, 

 more and more effectually 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." that the central wood 

 had changed its character, and shows by its structure that 

 it had become adapted to the storage rather than to the 

 transmission of the water-supply, its earlier function now 

 being more conveniently left to the external parts of the 

 wood. Such utilisation of vestigial structure appears to 

 be a good mark of a high standard of adaptation. 



Another interesting case of adaptive specialisation in an 

 organ which may be regarded as of an old-fashioned type 

 is to be found in the rootlets of Stigmaria. The nature 

 of these appendages has been much disputed ; last year 

 we had an interesting discussion on the subject, opened 

 by Prof. Weiss. 1 have used the word " old-fashioned " 

 because there is some reason to suppose that these organs 

 wore not yet quite sharply differentiated as roots ; at any 

 rate, there are certain points in which they rather resemble 

 modified leaves, though in my opinion the root-characters 

 predominate. Though they may thus be " primitive," 

 from the point of view of our current morphological 

 categories, these organs, as Prof. Weiss has discovered, 

 show a remarkable adaptive mechanism in the presence 

 of strands of water-conducting elements running out from 

 the central vascular bundle, and terminating in plates of 

 trachea; placed in the outer cortex. The whole constitutes 

 an absorptive apparatus more elaborate than anything 

 found in recent roots, if we except a few highly specialised 

 haustorial roots of parasites. This example seems to me 

 instructive, for it shows how a very high degree of adapta- 

 tion may co-exist with characters which suggest a some- 

 what archaic type of organ. 



As an example of adaptation to more special conditions, 

 I may instance the xerophytic characters shown by various 

 Carboniferous plants, especially in the structure of their 

 leaves. 



Though there is no question of absolute perfection in 

 nature, it appears that, under given conditions, adaptation 

 is and was suflRciently perfect to make it very difficult 

 to put one's finger on any defect. When we think we can 

 do so, it generally turns out that the defect is in the mind 

 of the critic ratlier than in the organism criticised. We 

 will take a particular case, where the history seems to 

 give some justification for our fault-finding. 



The late Palaeozoic family Medulloseae were in some 

 respects the most remarkable plants, from an anatomical 

 point of view, that we know of. Most of them were 

 plants of great size, with rather sturdy stems bearing 

 immense fern-like fronds ; the habit altogether must have 

 been something like that of a tree-fern, but their repro- 

 duction was by large seeds, borne on the fronds. To 



1 Scott, "The Old Wood and the New " (iW-zc Fliytolog;ist, vol. i., 1902). 

 - Megaloxylon, Zatesskya, Lcpidodcndron sciaginoides. 



