FACTORS INFLUENCING ABSORPTION OF WATER 281 



general low temperature had the least affect on absorption of water by the 

 roots of species native to northern latitudes. 



TABLE 27 EFFECTS OF VARIATIONS IN THE SOIL WATER CONTENT UPON THE RATE OF TRAN- 

 SPIRATION OF TOBACCO PLANTS 



(Transpiration values are the averages from several plants. The two experiments were not 



performed concurrently.) 



Sunflower is an example of a species in which the rate of absorption is 

 markedly affected by physiologically low temperatures (Fig. 74). The rela- 

 tion as shown in this figure is that 

 of soil temperature to transpiration 

 rates, but the volume of water tran- 

 spired can often be taken as an ap- 

 proximate measure of the volume 

 absorbed (See, however, discussion in 

 Chap, XVIII on relative rates of 

 transpiration and absorption). The 

 plants for which these results were 

 obtained were growing in a loam soil 

 with a water content of 28 per cent. 

 As shown in this figure, the transpira- 

 tion of sunflower plants is practically 

 constant for a soil temperature range 

 of about 55° F. to about 100° F. 

 (about i3°-38° C), but below this 

 range the rate drops very rapidly. 



30 



40 



50 60 70 80 



SOIL temperature: ctj 



100 



Fig. 74. Relation between rate of 

 transpiration of sunflower plants and 

 soil temperature. Each curve repre- 

 sents the results of a different experi- 

 ment. Data of Clements and Martin 

 (1934)- 



Other experiments by the same in- 

 vestigators showed that the absorption rate approached zero at 0° C. 



Cotton 



