'44 



NATURE 



[October 29, 1914 



respects from the richer flora preserved in the coal 

 itself, and the differences are such as lend support 

 to the view that the forests which furnished the coal- 

 producing material grew in swampy ground, while 

 the roof-nodule plants grew on dry land. 



Before dealing with the anatomical structure of a 

 few Carboniferous plants further reference may be 

 made to the question of climate. It used to be 

 asserted that in the Coal period the climate was 

 uniform over almost the whole world. In recent years 

 it has been shown that whatever may have been the 

 temperature in the northern hemisphere, there is good 

 reason for believing that in India, South Africa, South 

 America, and Australia the climate was different. 

 During the latter part of the Carboniferous period the 

 vegetation of Europe, North America, and part of 

 China was fairly uniform in composition, and these 

 regions were also characterised by similar physical 

 conditions. The precise correlation of rocks in widely 

 separated localities is often difficult, but it is safe to 

 say that in India and the southern hemisphere strata 

 occur corresponding in geological position with Upper 

 Carboniferous and with Permian rocks in North 

 America, Europe, and China ; these southern rocks, 

 conveniently termed Permo-Carboniferous, contain 

 many plants some of which are closely allied to typical 

 northern species, while others are distinct, notably a 

 genus known as Glossopteris, a fern-like plant, though 

 probably a member of an extinct group intermediate 

 in some respects between ferns and seed-plants. From 

 the extraordinary abundance and wide geographical 

 range of Glossopteris in South America, South Africa, 

 India, and Australia, this southern flora is spoken of 

 as the Glossopteris flora ; it differs from the contem- 

 poraneous northern flora not only in the presence of 

 Glossopteris and several other types unknown in the 

 coal-fields of Europe, but in the absence of many of 

 the most abundant northern plants. With the Glosso- 

 pteris strata are associated extensive boulder beds, 

 clearly pointing to the existence of glaciers or water- 

 borne ice, and these beds sometimes rest on a platform 

 of solid rock, exhibiting in its rounded outlines and 

 smooth, grooved surfaces unmistakable evidence of 

 moving ice. The conclusions drawn from these and 

 other facts point clearly to the existence of two 

 botanical provinces in the Permo-Carboniferous era, 

 for the most part sharply contrasted, but In a few 

 places intermingling. In the southern hemisphere, 

 and stretching north of the equator into India, was a 

 vast continent, occup3-ing a large portion of what is 

 now the southern ocean, and of this lost continent 

 remnants are preserved in South Africa, South 

 America, and Australia. The abundance of boulder 

 beds and ice-scored rocks in the southern hemisphere 

 demonstrates the prevalence of conditions favourable 

 to the formation of glaciers, a marked contrast to the 

 physical environment of the swamp forests north of 

 the equator. 



It is unfortunate that the plants preserved in the 

 southern hemisphere strata are very seldom met with 

 in a petrified state ; they occur almost exclusively as 

 casts or impressions. A few specimens of petrified 

 stems recently received from South Africa and India, 

 with others previously described from Australia, afford 

 one piece of evidence bearing on the problem of 

 climatic conditions, namely, the presence of well- 

 defined rings of growth in the wood. Stems from 

 Carboniferous and Permian strata in the northern 

 hemisphere do not as a rule possess any regular rings 

 of growth ; their wood is composed of water-conduct- 

 ing tubes of uniform diameter denoting an absence 

 of seasonal changes. A tree growing in a district 

 where a period of inactivity or winter rest is suc- 

 ceeded by a vigorous awakening in the spring, regis- 

 ters the contrast by the production of large vessels 



NO. 2348, VOL. 94] 



in response to the greater demands on the water- 

 supply consequent on the sudden increase of activity in 

 the life of the plant ; the production of these water- 

 tubes of wider bore in juxtaposition to the narrower 

 vessels formed in the previous autumn at the close of 

 the growing period gives the appearance of an annual 

 ring. In tropical countries similar annual rings are 

 formed when alternating dry and wet seasons replace 

 spring and autumn, and it occasionally happens that 

 several rings are produced in one year. An extreme 

 case is afforded by a tree of Theobroma cacao, the 

 cocoa tree, planted in Ceylon in the summer of 1893, 

 and felled in January, 1901, in which during a period 

 just above seven years twenty-two rings were formed. 

 In this instance the tree shed its leaves three times a 

 year, and each break in the uniformity of its life- 

 processes was marked by the juxtaposition of wide 

 and narrow water-conducting tubes. In many tropical 

 trees and in stems of some plants growing in water 

 there are no annual rings because there are no 

 seasonal disturbances to interfere with the even tenor 

 of existence. As a general rule, however, the rings 

 are annual and afford a fairly accurate measure of 

 age. The occurrence of well-marked rings in the 

 South African and Indian stems of Palaeozoic plants 

 is therefore of some interest as an indication of regu- 

 larly recurring seasons in contrast to the more 

 uniform conditions characteristic of the more or less 

 contemporar}^ northern flora. " Petrified stems from 

 Jurassic, Cretaceous, and Tertiary rocks in the 

 northern hemisphere usually show well-defined rings 

 of growth. 



An interesting illustration of the employment of 

 the relative breadth of concentric rings of growth in 

 trees as a guide to climate is given in a recent paper 

 by Mr. E. Huntington in a Smithsonian Report for 

 1912. This author, from observations made in the 

 drier regions of Central Asia and other countries, was 

 led to the conclusion that during the last 3000 years 

 there were periods characterised by a greater amount 



' of moisture, the climatic changes being of a fluctuat- 

 ing kind. He afterwards extended his observations 

 to California, Arizona, and New Mexico, where ruins 

 of prehistoric settlements afforded evidence of less arid 

 conditions than now prevail. Evidence derived from 

 different sources points to the occurrence of three 

 main periods of relative prosperity in both the eastern 

 and western hemispheres. To test this hypothesis an 

 examination was made of the growth-curve of certain 

 forest trees in relation to the rainfall during the last 

 fortv vears ; this showed a close agreement and 

 justified the use of the measurement of rings in trees, 

 reaching in some cases an age of more than 3000 

 years, as an index of external influences. A curve 

 of growth based on the relative breadth of the rings 

 in several stumps of old Sequoia trees showed marked 

 pulsations which on the whole coincide with climatic 

 changes as deduced from other data 



We may next briefly consider the nature of the 

 evidence afforded by anatomical features exhibited by 

 petrified plants from English coal seams. One of the 

 commonest genera in the forests of the Coal period 

 was that known as Calamites, similar in habit to 

 modern Horsetails, but attaining the proportions of a 

 tree with a thick woody stem. The leaves of slender 

 foliage shoots occasionally preserved in the calcareous 

 nodules from the coal are characterised by palisade 

 cells disposed radially and at right angles to the sur- 

 face, an arrangement correlated with fairly bright illu- 

 mination rather than dull diffused light. The structural 

 features on the whole suggest a plant living under 

 conditions where the output of water from the leaves 

 was kept within prescribed limits. The roots of 

 Calamites contain large air-spaces in the cortex 



i similar to those in the stems and roots of recent water- 



