292 PLANT PHYSIOLOGY 



Exact quantitative determinations that have been carried 

 out by means of potometers have shown that at a lowered tem- 

 perature the rate of water absorption by the roots rapidly slows 

 down. This reduction, however, is not equal in different plants. 

 The thermophilous plants of the tropics and subtropics show a 

 reduction in their absorbing capacity more rapidly than do plants 

 of temperate zones. The reduced absorption caused by low 

 temperatures is induced not so much by the slowing down of the 

 diffusion rate, which changes but sHghtly under the influence of 

 temperature, as by the altered properties of the protoplasm. 

 At low temperatures, the viscosity of the fluid protoplasm 

 increases. It may even congeal, as a solution of gelatin does at 

 room temperature. Congealing slows down markedly the rate 

 at which water passes through the protoplasm. Experiments 

 have shown that not only the water absorption of a whole root 

 system, but also plasmolysis and deplasmolysis connected with 

 the passing of water through the protoplasm, are strongly 

 retarded by a low temperature. At 0°C., this process is only 

 one-quarter to one-seventh as fast as at 20°C. 



The retardation in water absorption by a cold soil explains 

 many pecuHarities in the life of plants growing in temperate 

 zones. In autumn, when the temperature of the air is still 

 quite high during the daytime and transpiration is considerable, 

 the roots absorb water slowly from the already cooled soil. As 

 a result, a considerable water deficit may exist within the plant. 

 The usual response by a plant to these conditions is a reduction 

 of its evaporating surface. This is effected by abscission of 

 leaves through the formation of an abscission layer across the 

 petiole (see Art. 103). According to Schimper, cold soil is 

 physiologically dry, though it may be saturated with water. 

 This explains many peculiar structures in bog plants, which 

 evidently lessen transpiration, since bog soils are cold and thaw 

 slowly in the spring. Many bog plants, Uke Oxycoccus, 

 Andromeda, and Ledum, have leathery leaves with somewhat 

 involute margins. Their stomata, through which water is given 

 off, are depressed. This structure helps to decrease transpira- 

 tion. Other bog plants, for instance, Eriophornm, have very 

 narrow, almost awl-shaped leaves. 



The influence of a low soil temperature on the absorption of 

 water by the plant, however, must not be overestimated. In 



