THE ELEMENTS OF A PALEOGEOGRAPHIC PROBLEM 31 



water. Succulent plants occur in deserts, but also in purely aquatic habitat, 

 and when only an impression of the fossil form is preserved the interpretation 

 is difficult or impossible. The best general-source books are Schimper's 

 Plant Geography and Clement's Plant Succession.^ 



Spalding has drawn attention to the fact that a desert environment is a 

 most complex conception. Ranging from very moist along stream-courses, 

 where willows and arrow-leaves may abound, through less damp soil to 

 true desert and the high, dry debris slopes of neighboring mountains, with 

 cactus, greasewood, chaparral, and sagebrush. There is but one common 

 factor — the hot, 'dry winds and the intense insolation. All plants of arid 

 or semiarid regions, even in the damp parts, have coriaceous, heavy, or 

 otherwise xerophilous leaf-structure.^ 



Nathorst, in discussing the vegetation of arctic regions as an index of 

 climate, quotes remarks by Gotham showing that the wood of Cretaceous 

 trees found in Spitzbergen possesses more definite rings of growth than 

 those of equal age in Europe, and considers this as an evidence that the 

 trees were grown in place, in the region of more accentuated climate, and 

 not drifted in from the south, where the climate was more equable.^ 



Saporta showed that as general humidity increases the proportion of 

 monocotyledons increases and of dicotyledons decreases. Lowering the 

 temperature has the same effect. A dry and warm country has more dicoty- 

 ledons than a warm and moist or cold and moist country. He also showed 

 that an abundance of Leguminosse suggest warm and dry conditions, and 

 that under the same conditions there will be a feeble development of ap- 

 pendicular organs— coriaceous leaves with frequently spiny margins and a 

 great complication of the nervation. How uncertain the revelation by 

 plants may be, however, is shown by Schimper, who, in his Java Flora, 

 demonstrates that xerophilous adaptations of similar character are found in 

 xerophiles, halophytes, the Java alpine flora, and evergreen woody plants 

 of colder climates. Obviously the interpretation from any but the Tertiary 

 plants must be of the most tentative character. 



Bailey and Sinnott* have shown a remarkable relation between the form 

 of leaves and climatic conditions. In their summary they say: 



"There is a clearly marked correlation between leaf margins and environment 

 in the distribution of dicotyledons in the various regions of the earth. Leaves 

 and leaflets with entire margins are overwhelmingly predominant in the lowland 



^Clements, F. E., Plant Succession: An Analysis of the Development of Vegetation, Car- 

 negie Inst. Wash. Pub. No. 242, 1916. 



"Spalding, V. H., Present Problems in Plant Ecology: Problems of Local Distribution in 

 Arid Regions, Amer. Nat., vol. 43, 1909. Reprinted in Annual Report Secretary Smith- 

 sonian Institution for 1909, p. 453, 1910. 



' Nathorst, A. G., On the Value of the Forest Floras of the Arctic Regions as Evidence of 

 Geological Climate, Annual Report Secretary Smithsonian Institution for 191 1, p. 

 335. In this article many illustrations are given of the use of trees as indices of climate. 



^ Bailey, I. W., and E. W. Sinnott, The Climatic Distribution of Certain Types of Angio- 

 sperm Leaves, Amer. Jour. Bot., vol. iii, p. 23, 1916. 



