October 29, 19 14] 



NATURE 



245 



plants. Another common type in the Coal-period 

 forests was Lepidodendron, an arborescent member of 

 a class including the existing club mosses and similar 

 plants, which reached a height of at least loo ft., 

 and had the power of producing an ever-widening 

 cylinder of wood like that of a pine or an oak. The 

 stem rose from a dichotomously branched sub- 

 terranean organ that grew to a considerable length 

 in a horizontal position a short distance below the 

 surface of the ground, precisely like the underground 

 organs of plants in the partially water-logged soil of 

 a modern fen. The leaves were well provided with 

 stomatal pores situated in grooves on the under sur- 

 face, an arrangement suggestive of life under condi- 

 tions requiring economy in the expenditure of water. 

 Certain anatomical features in the stem suggest that 

 the tree grew in swampy soil or possibly with the 

 lower part of the trunk immersed in water, like the 

 plants in a tide-swept mangrove swamp. A different 

 t>'pe of Palaeozoic plant is illustrated by Lyginopteris, 

 a remarkable generalised genus with the habit of a 

 slender tree-fern, but agreeing with the higher plants 

 in the possession of true seeds. The stem bore aerial 

 roots characterised by a covering of thin-walled cells, 

 which may have enabled these organs to absorb water 

 from a moist atmosphere. Another extinct genus 

 from the Coal Measures, Sphenophyllum — so called 

 because of its wedge-shaped leaves — affords in its long 

 and slender stem and certain anatomical peculiarities 

 evidence of a climbing habit, and suggests a genial 

 climatic environment. The genus Cordaites, particu- 

 larly abundant in some of the French coal-fields, and 

 met with also in English localities, resembled in habit, 

 and to some extent anatomically, the Kauri Pine 

 (Agathis australis) of New Zealand. The wood is 

 uniform in structure and in specimens from the 

 northern hemisphere without regular rings of growth. 

 The long, flat leaves reveal a distribution of support- 

 ing tissue in the form of I-shaped girders as efficient 

 mechanically as the corresponding tissue in leaves of 

 existing plants adapted to resist the force of the wind. 

 Groups of small lateral roots of Cordaites have been 

 found in a petrified condition, containing in their 

 cortex the delicate tubular cells of a fungus, and it 

 is believed that this association of fungus and root 

 affords an example of symbiosis, the two organisms 

 living together to their mutual advantage. It is note- 

 worthy that certain recent trees, such as the Alder, 

 inhabiting swampy ground, are characterised by an 

 association of a fungus with short fleshy roots similar 

 to those of Cordaites, the fungus assisting the roots to 

 absorb water from a soil rich in humic acid. It may 

 be, therefore, that this discoverv- of symbiotic relation- 

 ship in the roots of a Coal-period tree supplies addi- 

 tional evidence in favour of a swampy habitat. 



Attention may now be directed to the vegetation 

 of a more recent geological period, namely, that 

 known as the Jurassic. Sedimentary rocks of this 

 age are often rich in fossil plants, but as yet the 

 present dominant class, the flowering plants, had not 

 begun to assert itself in the struggle for existence. 

 Jurassic floras are. known from the Arctic regions, 

 many parts of North America, Europe, and Asia, from 

 South America, and so far south as Graham Land, 

 also from India, China, Australia, and elsewhere. 

 There is conclusive evidence of the almost world-wide 

 distribution of certain plants during this period of the 

 earth's history, and despite our imperfect knowledge 

 of the floras, we are justified in stating that the 

 available data afford no satisfactory' evidence of any 

 well-marked differences in the nature of the vegeta- 

 tion in various latitudes such as might fairly be 

 expected had there been climatic zones comparable 

 with those of the present era. Identical or closely 

 allied species of Jurassic and Cretaceous age are 

 NO. 2348, VOL. 94] 



I recorded from Greenland, England, India, and 

 I Graham Land, and, so far as it is possible to base 

 any conclusions as to climate on a comparison of these 

 ; extinct plants with modern forms, they indicate an 

 environment characteristic of subtropical or tropical 

 regions. A remarkable example of wide distribution 

 I is afforded by some large fronds described from Lower 

 ! Cretaceous rocks in Greenland by the late Oswald 

 Heer, belonging to a class of seed-plants known as 

 I the Cycadales or Cycads, now represented by a few 

 I genera for the most part tropical in their distribu- 

 i tion ; the most northerly members of the class occur 

 ' in Florida, while the great majority grow in southern 

 countries. Fossil cycadean fronds very similar to 

 i those from the west of Greenland are common in the 

 I Jurassic plant-beds at Whitby and other localities on 

 the Yorkshire coast, also in strata of the same age in 

 I India, Siberia, California, and many other parts of 

 the world. A few years ago members of a Swedish 

 expedition discovered several Jurassic plants in 

 Graham Land (lat. 63° 15' S.), on the edge of the 

 Antarctic circle, among which were cycadean leaves 

 almost identical with the slightly more recent (Lower 

 Cretaceous) specimens from Greenland. Cycadean 

 plants of Jurassic and Lower Cretaceous age are repre- 

 sented in several parts of Britain, in most cases by 

 impressions of leaves, but sometimes, as in the Isle 

 of Portland, by large stems and occasionally by 

 petrified flowering shoots ; the latter have supplied 

 important information as to the affinitv of these 

 extinct members of the class, and incidentally their 

 structure points to circumstances necessitating efficient 

 protection against drought 



By treating with acid the carbonised film that can 

 often be detached from an impression of a fossil plant 

 preserved in shale, it is sometimes possible to obtain a 

 preparation of the mummified cuticle or superficial 

 covering of a leaf suitable for microscopical examina- 

 tion. Such specimens furnish information as to the 

 structure and number of the stomata, and the data 

 may be of value as criteria of climatic conditions. 

 The carbonised impressions of cycadean fronds from 

 the Jurassic plant-beds of Yorkshire, recentlv investi- 

 gated by Mr. Hamshaw Thomas, of Cambridge, 

 furnish particularly good preparations of the resistent 

 cuticle, the only part of the leaf that has escaped 

 destruction, and these supply a clue not only as to 

 systematic position but also with regard to the struc- 

 ture of the mechanism by which the plants regulated 

 their output of water and gaseous exchange with the 

 atmosphere. Although it is rash to institute a close 

 compa^rison between e.xtinct types and their nearest 

 living representatives as regards climatic conditions, 

 the data obtained bv a study of Lower Cretaceous 

 and Jurassic floras throughout the world point to a 

 greater uniformity in the vegetation, and indicate the 

 prevalence of a higher temperature in Arctic and 

 Antarctic regions than at the present day. It has 

 recently been stated by Dr. Gothan, of Berlin, that 

 the comparative study of petrified wood affords 

 evidence of seasonal changes in Arctic lands as opposed 

 to a greater uniformity of conditions — ^as indicated by 

 the absence of annual rings — in tropical regions ; but 

 the data are at present scarcely sufficient to justify 

 any definite pronouncement as to the occurrence of 

 climatic zones during the Jurassic period. 



Our knowledge of the more recent geological periods 

 is as yet too meagre to warrant any ver>- precise con- 

 clusions as to their climates. The older Tertian.- floras 

 are characterised by several flowering plants closely 

 I related to modern species in subtropicaf and tropical 

 countries ; fossil seeds from the London clay and from 

 beds of similar age in Belgium and practically identical 

 with those of Nipa, a palm which flourishes in the 

 swampy ground of tropical estuaries. As the buoyant 



