148 TEXTBOOK OF PLANT PHYSIOLOGY 



modifications. Frequently, only branches instead of whole plants 

 are employed. It is often expedient to construct a potometer that 

 can be weighed, in order to determine the water lost by the plant. 



Exact quantitative determinations which have been carried 

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

 perature the rate of water absorption by the roots rapidly slows 

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

 The thermophilous plants of the tropics and subtropics show a 

 reduction in their suction power more rapidly than do plants of 

 temperate zones. The impeded absorption, caused by low tem- 

 peratures, is induced not so much by the slowing down of the dif- 

 fusion rate, which changes but slightly under the influence of tem- 

 perature, as by the altered properties of the protoplasm. At low 

 temperatures, the viscosity of the liquid part of the protoplasm 

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

 room temperature. Such congealing slows down markedly the 

 rate with which water passes through the protoplasm. Experi- 

 ments 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 times as fast as at 20° C. 



The retardation in water absorption by a cooled soil explains 

 many peculiarities 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 only from the already cooled soil. As a result, a 

 considerable water deficit may arise within the plant. The usual 

 response by a plant to these conditions is a reduction of its evap- 

 orating surface. This is effected by abscission of leaves through 

 the formation of an abscission layer across the petiole. According 

 to Schimper, cold soil is physiologically dry, though it may be sat- 

 urated with water. This explains many peculiar structures of bog 

 plants, which evidently are intended to lessen transpiration, since 

 bog soils are cold and thaw slowly in the spring. Many bog 

 plants, as Oxy coccus, Andromeda, and Ledum, have leathery leaves 

 with somewhat involute margins. Their stomata, through which 

 water is given off are depressed in tubes. This structure helps to 

 decrease transpiration. Other bog plants, for instance Eriophorum, 

 have almost awl-shaped leaves. 



