242 



NATURE 



[October 29, 19 14 



CLIMATE AS TESTED BY FOSSIL PLANTS.'^ 



PROBLEMS connected with the climates of past 

 ages have long exercised the minds of scientific 

 writers, both from the astronomical point of view and 

 from the point of view of the gradual development and 

 distribution of plants and animals, since the earliest 

 periods that have left recognisable records in the 

 earth's crust. My task this evening is to deal with 

 the nature and value of the evidence afforded by plants 

 as to the climates at different periods of the world's 

 history. Even before the study of fossil plants 

 attained the dignity of a science (1706) the opinion 

 was expressed that certain leaves preserved as im- 

 pressions in Palaeozoic strata in Germany bore a closer 

 resemblance to existing tropical genera than to any 

 European forms ; and as investigation of the botanical 

 records of the rocks progressed it became increasingly 

 evident that fossil plants often exhibit a close agree- 

 ment with species characteristic of regions warmer 

 than those where the fossils are found. Plants, it has 

 been said, "are the thermometers of the ages, by 

 which climatic extremes and climate in general 

 through long periods are best measured." It seems a 

 simple matter to draw conclusions as to the climates 

 of former ages from the nature of the vegetation 

 embedded in the rocks ; but the more we consider 

 the facts the more fully we recognise the difficulties 

 of interpretation. 



At the outset of our inquiry we must endeavour to 

 obtain some general conception of the relation of 

 ■existing plants to the diverse influences to which they 

 are exposed in ord^r to appreciate their plasticity or 

 power of modifying form and structure in response 

 to the demands of the environment. Plants have 

 generally been preferred to animals as indices of 

 climatic change on account of their inability to escape 

 from uncongenial or injurious conditions unless the 

 change in climate is sufficiently gradual to allow time 

 for migration by the precarious method of travelling 

 afforded by the adaptation of fruits and seeds to dis- 

 persal by wind, water, or animal agency. Plants 

 must either become acclimatised or suffer extinction, 

 while animals, unless faced by impassable barriers, 

 can change their home. It is therefore of importance 

 to obtain some general idea of the power of plants 

 to accommodate themselves to changed conditions, 

 and to inquire how far the factors influencing plant- 

 life are able to cause alteration in form or structure 

 and so maintain equilibrium between the organism 

 and its environment. It is well known that closely 

 allied plants can exist under very diff'erent conditions, 

 but we are unable to give any satisfactory explanation 

 of this power of adjustment inherent in the constitu- 

 tion of a species. As Prof. Judd reminds me, tropical 

 species in St. Petersburg live through the winter in 

 semi-darkness in glass houses with the roof darkened 

 by a thick covering of matting and snow. The same 

 ■species is occasionally met with in both temperate and 

 tropical regions : the bracken fern, which monopolises 

 wide stretches of British moorland, grows in tropical 

 Africa, in the Alps and Himala}-as, China, the Malay 

 Peninsula, Tasmania, and many other parts of the 

 Avorld where it is exposed to a wide range of climatic 

 conditions. In contrast to this and other cosmopolitan 

 tvoes there are many instances of ferns and flowering 

 plants characterised by a narrowly circumscribed geo- 

 graphical range : some of the genera now confined to 

 a comparatively small area in the tropics are sur- 

 vivals from a remote past when they, or closely allied 

 forms, were widely spread in northern countries. If 

 we take the climatic environment of the surviving 



1 Lecture delivered before the Roval Meteoro'ogical Society on March i8 

 ■fay Prof. A. C. Seward, F.R.S. Reprinted from the Quarterly Journal of 

 »he Society. 



NO. 2348, VOL. 94] 



species as an index of the conditions under which 

 their predecessors flourished, their presence in 

 European sedimentary rocks points to tropical or sub- 

 tropical conditions in the Mesozoic era in latitudes 

 now characterised by a temperate or even an Arctic 

 climate. It is, however, very probable that the last 

 strongholds of these ancient and possibly enfeebled 

 types are characterised by climatic conditions less 

 rigorous than those under which their more robust 

 ancestors were able to exist. The '"big trees" of 

 California, the genus Araucaria, and the Malayan 

 ferns Dipteris and Matonia afford striking examples of 

 genera now restricted to a small area but formerly 

 very widely distributed. The two surviving species 

 of Sequoia (the redwood and the mammoth tree), now 

 confined to a narrow strip of land bordering the 

 Pacific coast, are the last members of a family that 

 has left many traces of its existence in Tertiary- 

 Europe and in other parts of the world. Similarly 

 Araucaria, one of the most venerable of our Conifers, 

 is now confined to Chile and Brazil in the west, and 

 to the eastern part of Australia, New Caledonia, and 

 other Australasian islands, whereas in the Jurassic 

 period species closely allied to the monkey puzzle 

 (Araucaria itnbricata) and the Norfolk Island "Pine" 

 (Araucaria excelsa) flourished in North America, 

 Europe, and other regions north of the equator. A 

 similar history of retreat from northern latitudes to a 

 much more limited tropical home has been deciphered 

 from the remains of Mesozoic ferns that have their 

 modern counterparts in the Malayan genera Matonia 

 and Dipteris. 



Though we cannot make any definite statement as 

 to the mode of action of heat on the living protoplasm 

 of a plant, it is possible to formulate some general 

 rules governing response to external factors as illus- 

 trated by differences in habit and anatomical char- 

 acters. The striking contrast in the environment of 

 land and water plants means a considerable difference 

 in the conditions affecting the working of the plant- 

 machine. A species surrounded by water has no need 

 to take measures for the reduction of evaporation or, 

 more accurately expressed, transpiration : the super- 

 ficial cells require no waterproof covering to prevent 

 loss of moisture from the internal tissues ; the water- 

 conducting tissue (or wood) is much less developed 

 in a plant that is not dependent on its roots alone for 

 a supply of raw material. If we cut across the stem 

 of an aquatic flowering plant, e.g. the mare's-tail or 

 the water milfoil, we find that the feebly-developed 

 conducting strands are nearer the centre than is the 

 conducting tissue of a land plant. The tensile strain 

 to which the stem is exposed renders desirable a con- 

 centration of the strongest tissue, in this case the 

 wood, towards the axial region as compared with the 

 more peripheral disposition of the corresponding tissue 

 in a stem exposed to the bending force of the wind. 

 The stem of the water-plant contains large air-spaces 

 which ensure the provision of an internal atmosphere 

 and an adequate supply of oxygen to the living cells ; 

 the support afforded by the surrounding water renders 

 superfluous any special strengthening tissue such as 

 characterises the stems of land-plants in which it is 

 so arranged as to secure maximum efficiency with the 

 least expenditure of material. 



Plants growing in dry climates where water is 

 available only at certain seasons, often separated by 

 long intervals of drought, are characterised by struc- 

 tural features correlated with the economising or stor- 

 ing of water. A relatively thick cuticle — an imper- 

 vious film on the surface of the aerial organs — reduces 

 the loss of water in the form of aqueous vapour from 

 the system of intercellular spaces permeating the 

 internal tissues. The minute pores or stoniata regu- 

 lating gaseous exchange between the plant and the 



