October 29, 1914] 



NATURE 



243 



external air, and by which aqueous vapour escapes, 

 are often sunk below the level of the epidermis, or 

 the leaves are variously modified in response to the 

 need of reducing evaporation. The formation of 

 water-storing tissue may result in the development of 

 a succulent habit ; hard spines replace the flat green 

 leaves, thus by a reduction of exposed thin surfaces 

 effecting a considerable diminution in the amount of 

 water evaporated. In some cases the leaves persist 

 as fleshy storage-reser\oirs. Occasionally special cells 

 occur in the leaves of dr\-climate plants, in plants the 

 water-supply of which is precarious, which act as 

 small reservoirs to be drawn upon in times of stress. 

 The intercellular spaces that form large cavities in the 

 stems and leaf-stalks of aquatic plants are reduced 

 to the minimum consistent with adequate aeration. 

 Such anatomical features are by no means the mono- 

 poly of members of desert floras ; they are character- 

 istic of plants growing under conditions in which for 

 various reasons economy in the use of water must be 

 exercised. In salt marshes the water, though 

 abundant enough, contains a relatively high per- 

 centage of salt, and this checks absorption by the 

 roots ; to avoid the danger of a greater loss of water 

 from the leaves than can be made good by the roots, 

 the plant assumes a habit and anatomical features 

 similar to those characteristic of desert forms. 

 Habitats where water may be plentiful, but to which 

 plants react by acquiring the appearance and structure 

 of species growing in dry regions, have been termed 

 physiologically Ary. A salt-marsh plant lives in a 

 physiologically dry habitat ; desert plants live in a 

 physically dry locality, and though the environments 

 differ they induce a similar reaction on the part of 

 the two sets of plants. Peat-bogs afford another 

 example of a physiologically dry habitat : the abund- 

 ance of humic acids in the soil retards water absorp- 

 tion and so reacts on form and structure. In the 

 swampy soil of a fen oxygen is scarce, and the hori- 

 zontal underground stems of fen-plants tend to avoid 

 the deeper water-logged soil by growing unusually 

 near the surface. 



The action of heat and cold is less easy to analyse ; 

 the thick covering of bark on the stem of a woody 

 olant is primarily a protection against drought and 

 not against cold. It is rather in the habit of plants 

 than in any specific anatomical features that exposure 

 to cold is reflected. In Russian Lapland it has been 

 shown that it is not the low temperature but the effect 

 of the dry winds that sets a limit to the northward 

 extension of the forests ; the young shoots, exposed 

 to the desiccating influence of the air, lose more water 

 than the roots can replace by their enfeebled power 

 of absorption from the cold ground. 



The contrast in habit and anatomical structure 

 between individuals of the same species grown at high 

 elevations in the Alps and in the lower meadows is 

 due in part to the action of light and to other factors 

 affecting water-supply. Plants grown before a con- 

 tinuous light under conditions similar to those in high 

 latitudes, afford evidence in their power of response 

 to a changed environment of the remarkable plasticity 

 of vegetative organs ; the stems are shorter, the amount 

 of wood and fibrous tissue is reduced, and the cell- 

 walls are thinner than in plants grown under normal 

 conditions. The difference in the intensity of light 

 on the sunny as compared with the shaded side of a 

 tree leaves an impress on the structure of the leaves ; 

 a sun-leaf is thicker and is richer in the so-called 

 palisade cells, that is, the cells containing chlorophyll 

 elongated at right angles to the surface of the leaf, 

 than the leaves which grow in diffused light. 



The occurrence of the same or closely related species 

 of existing plants under very different climatic condi- 

 tions is a serious obstacle in the way of employing 

 NO. 2348, VOL. 94] 



fossil plants as indices of climate. While it is possible 

 to draw certain general conclusions from the facies 

 of a flora as represented by fossils that can be iden- 

 tified with recent forms, the difficulties are enormously 

 increased when the fossils are extinct types and too 

 distantly connected with living species to afford any 

 safe guide as to the conditions under which they were 

 able to live. It is, for example, certain that in the 

 Tertiary and Cretaceous epochs the vegetation of 

 Arctic regions was such as could not have withstood 

 the low temperature that now characterises Greenland, 

 Spitsbergen, and other ice-covered countries. It is 

 probably safe to assert that in latitude 70° N. in the 

 Cretaceous and the succeeding Tertiary period the 

 temperature was at least as high as that in southern 

 Europe at the present day. This statement is based 

 on the present geographical distribution of recent 

 plants with which the numerous fossils discovered on 

 the west coast of Greenland are most nearly related. 

 It is, of course, impossible to say to what extent the 

 fossil species differed from their surviving relatives in 

 the power of resisting unfavourable conditions, but 

 the fact that many of the Greenland fossils are ver}- 

 closely related to plants now confined to tropical and 

 subtropical countries carries more weight than if the 

 evidence rested on one or two isolated cases. The 

 occurrence in Lower Cretaceous rocks on the west 

 coast of Greenland of fossil ferns very similar to living 

 species of the common tropical genus Gleichenia 

 affords one of several instances of the vicissitudes and 

 changing climates to which groups of plants have 

 been exposed. 



The preser\-ation of plants as petrifactions affords 

 valuable data in connection with climatic questions,* 

 but it is unfortunately only in comparatively rare in- 

 stances that the relics of ancient floras retain their 

 tissues in a state that admits of microscopical investi- 

 gation. In some of the Lancashire and Yorkshire 

 coal seams there are calcareous nodules containing 

 numerous petrified fragments of the stems, leaves, 

 roots, seeds, and spores of plants that flourished in the 

 Carboniferous forests, and by cutting transparent sec- 

 tions of these blocks of stone the tissues can be 

 examined as thoroughly as in thin sections of existing 

 plants. For the most part the trees and smaller plants 

 of the Coal period were converted into coal. A mass 

 of vegetable debris accumulated on the swampy site 

 of a forest, and after submergence and sealing-up 

 under superposed sediments it passed by slow degrees 

 into more or less homogeneous coal. In a few places 

 patches of this peatv material were petrified bv the 

 deposition of carbonates of magnesium and calcium 

 derived from mollusc shells and so preserved as 

 samples of the coal-forming vegetable detritus. From 

 thin slices of these patches it has been possible not 

 merely to glean information as to the affinities of 

 Palaeozoic plants, but to learn something at least of 

 the conditions under which they lived. It is note- 

 worthy that in addition to the plant-containing nodules 

 embedded in the coal itself, others are found in the 

 roofs of the seams, and in these are occasionally pre- 

 served pieces of stems and other organs associated 

 with numerous marine shells. It is probable that 

 while the fragments preserved in the nodules from 

 the seams formed part of the debris accumulated on 

 the actual site of the forest, those from the roof- 

 nodules are waifs and strays drifted by sea-water from 

 the vegetation of higher ground. It has often been 

 stated, though without adequate reason, that the Coal 

 period was characterised by tropical conditions. It is 

 at least certain that the conditions were favourable 

 to luxuriant growth, and it is by no means unlikely 

 that the atmosphere was richer in carbon in the form 

 of carbon dioxide than it is to-day. The flora that has 

 left scanty records in the roof-nodules differs in certain 



