6r. R. Wieland— Polar Climate in Time. 425 



ties, now living in tropic or subtropic regions, is a habit acquired 

 in an original boreal home, and never broken. 



As bearing on the general effect of light and darkness on 

 plant growth, the elaborate experiments of MacDougal* are of 

 high importance. He shows that etiolated tissues tend to 

 remain in a primitive condition, though there may be trans- 

 ference of light effects to etiolated parts, that light shortens 

 the meristematic period and induces the formation of perma- 

 nent tissue, and that aplastic material is not so readily laid 

 down in the absence of light. These facts are all suggestive 

 of a chemico-physical basis of growth and development, and 

 in complete accord with the ideas proposed here. 



But with regard to the effect of the Arctic night on plants 

 there have been probably no experiments in detail, although 

 it is well known that the women of Disco Island find no diffi- 

 culty in growing in their homes ornamental plants from far 

 southern latitudes. If it were attempted, however, to demon- 

 strate experimentally the effect of conditions representing hypo- 

 thetical climates of the past, it would be necessary to use plants 

 which have changed greatly and could therefore yield only 

 relative results. Thus the entire cast of vegetation in the 

 Carboniferous, of course conformed in transpiratory and other 

 delicate functional structures to the moist hot climate, diffuse 

 sunlight, and superabundance of carbonic acid characterizing 

 that period. Hence while duplications of these climatic features 

 may be readily made in the laboratory, their effect on plants 

 now living affords only a general idea of the actual effect of 

 the conditions of Carboniferous vegetation. f 



As in the case of lights heat and moisture were also similarly 

 variable to the utmost degree in the polar areas. Even if the 



* Memoirs of the New York Bot. Garden, vol. 11, Jan. 20, 1903. 



f A series of experiments, apparently requiring further extension-^espe- 

 cially with reference to Pteridophytes, — has recently been made by H, F, 

 Brown and F. Enscome (Proc. Eoy. Soc, Ixx, p. 397-413, pi. 5-10) to deter- 

 mine the probable effects of varying amounts of atmospheric carbon dioxide 

 on the photosynthetic processes of leaves and on the mode of plant growth. 

 These experimenters subjected plants during periods of several months to 

 an artificially conditioned atmosphere containing as much as 11 '47 percent of 

 carbon dioxide, and found that this resulted in diminished leaf surface, in- 

 creased starch and chlorophyll content, deeper green, and various stem modifi- 

 cations. Fructification was apparently checked. Without attaching undue 

 importance to experiments which leave many categories of inquiry unsatis- 

 fied, we may saj^ that it is demonstrated that variations in the amount of 

 atmospheric carbon dioxide do affect plant growth profoundly. And the 

 effect on animals is equally or even greater, considered as either direct or 

 indirect. At the poles, in addition to the direct chemical effect due to 

 increase or decrease in the amount of atmospheric carbon dioxide, there is a 

 further important physical effect. Dr Arrhenius (Philosophical Magazine, 

 S. 15, vol. xli. No. 251, April, 1896, pp. 237-279) estimates that the addition 

 of but two to three per cent of carbon dioxide to the atmosphere would be 

 sufficient to give to the Arctic regions the genial climate indicated by their 

 Tertiary flora. 



