THE PENNSYLVANIAN PERIOD. 605 



growth is much weakened, however, by the fact that the vegetation 

 of undrainecl swamps and bogs assumes many of these xerophytic 

 features, which, in such cases, obviously become pseudoxerophytic. 

 A satisfactory explanation of this phenomenon has not yet been found, 

 nor has its extent and its limitations, either in respect to the nature 

 and degree of swampiness necessary to produce it, or in respect to 

 the kinds of plants or the parts of plants affected, been so thoroughly 

 worked out as to permit a close application to the problem in hand. 

 It is obvious that to fit the case, marsh or bog conditions must have 

 produced pseudoxerophytic adaptations in the great mass of the 

 tree growth, but not in the ferns and sphenophyllous plants. The 

 effects must apparently also be extended to all plants brought into 

 the depositing basins by drainage, whether these were true swamp 

 plants or not, unless a closer study of the Coal flora shall show that 

 the xerophytic aspect was not assumed by all species of the trees 

 involved. It is clear that a more critical study of the problem on 

 all sides is necessary before a final conclusion can be reached. This 

 will become the more apparent when we bring into consideration, 

 as we must a little later, the extraordinary phenomena of glaciation 

 in low latitudes, alternating with coal deposition, though the flora 

 then involved was not identical with that here considered. It bears, 

 however, on the question whether thick coal-beds necessarily mean 

 rapid growth. It will be found that the coal-beds that lie between 

 the glacial beds, attained the usual thicknesses. 



The influence of increased carbon dioxide on plant growth. — It has 

 been commonly believed that a moderate increase of carbon dioxide 

 in the air would promote a more luxuriant plant growth, and some 

 experiments seem to confirm this. 1 In recent experiments by Brown 

 and Escombe, 2 on the contrary, where an increase of carbon dioxide 

 to the extent of three or more times the normal amount was used, the 

 growth of the plants employed (angiosperms) was appreciably decreased, 

 the structure was adversely modified, the growth of the internodes 

 was checked, the leaves became smaller, and there were changes of a 



1 E. C. Tealoresco, Influence de l'Acide Carbonique sur la Forme et la Structure 

 des Plantes. Rev. Gen. de Botanique, Vol. II, 1899. 



2 The Influence of Varying Amounts of Carbon Dioxide in the Air on the Photo- 

 synthetic Process of Leaves and on the Mode of Growth of Plants. Proc. Roy. Soc, 

 London, LXX, 1902, p. 379; On the Influence of an Excess of Carbon Dioxide in 

 the Air on the Form and Internal Structure of Plants, ibid., p. 413 ' '" 



