12 



substances from the leaves encounters the opposite set of 

 conditions. This movement takes place almost entirely by 

 osmose and diffusion and is a much more complicated process, 

 both chemically and physically, taking place in living cells 

 only. The cooling of the liquid would entail alterations in 

 its j)ower of carrying substances in solution and would also 

 alter its physical relations to the atmosjiheric gases present. 



It may be said, in conclusion, that the facts disclosed as to 

 the actual temperatures in the soil, the diurnal and seasonal 

 changes therein, and as to the differences in temperature of 

 the aerial and underground portions of plants can not fail to 

 be of very great importance in the physical and chemical pro- 

 cesses, upon which growth, cell division, nutrition, and projDa- 

 gation dej)end. The determination of the effect of differences 

 in temperature between the roots and aerial shoots has received 

 but little consideration from the physiologist and the geogra- 

 pher. A careful analysis of the conditions and results of 

 experimental observations carried on with plants under arti- 

 ficial conditions, with the roots and shoots under abnormally 

 similar temperatures, would no doubt result in the detection 

 of many mistaken conclusions, especially in regard to absorp- 

 tion, trajislocation, and transpiration. 



That soil temperatures and the relations of these tempera- 

 tures to those of the air must be of very great importance in 

 the cultivation of economic plants is self-evident, es^Decially 

 in species in which the desired useful portion is formed under- 

 ground and receives storage material formed by the activity 

 of the aerial organs. Thus, in the case of such j^lants as the 

 l^otato, certain mineral substances are absorbed from the soil at 

 a comparatively low temperature, carried aloft into the heated 

 leaves, where they participate in activities resulting in the 

 formation of sugars, starches, and other carbohydrates, per- 

 haps some nitrogenous substances as well, and then these 

 complex bodies are slowly diffused downward, with many ac- 

 companying chemical and physical modifications, to under- 

 . ground cool storage organs, where a condensation occurs and 

 the products are stored in insoluble form in the tuber. 



