375] B. E. Livingston a Wa 
above, was carried out in an arid region, with high transpira- 
tion rates, but the results of Renner were obtained in a very 
moist summer in Munich, so that it appears to be fairly well 
established that this phenomenon is general in plants. Of 
course, incipient drying is more pronounced with high atmos- 
pheric evaporating power and intense sunshine than with 
aerial surroundings of less aridity, and it is less pronounced 
in plants with low transpiring power than it is in less xero- 
philous forms. 
From Renner’s experiments, and also from those of eee 
ston and Hawkins,® it appears that the rate of absorption of 
water by plant roots is determined by two conditions, which 
may be called, respectively, the absorbing power of the roots 
(internal) and the supplying power of the soil, or other 
medium in which the roots lie (external). It also appears 
that the internal one of these conditions (absorbing power of 
the roots) is at least partly controlled by the degree of incipi- 
ent drying occurring in the plant, which, in turn is partly 
dependent upon the rate of transpiration. Other conditions 
being unchanged, the plant takes up more water from the 
soil when the transpiration rate is high than when it is 
lower. If incipient drying becomes sufficiently pronounced 
its presence is made evident, first by loss of turgor in the 
plant, then by temporary wilting® (from which the wilted 
tissues may recover when transpiration is subsequently de- 
creased), then by permanent wilting’ (from which the plants 
5 Livingston, B. E., and Hawkins, Lon A., “The water relation 
between plant and soil.” Carnegie Inst. Wash. Pub. 204: 5-48. 
Washington, 1915. 
* Brown, W. H., “ The relation of evaporation to the water content 
of the soil at the time of wilting.” Plant World 15: 121-134. 1912. 
‘Briggs, L. J., and Shantz, H. L., “The wilting coefficient for 
different plants and its indirect determination. U. 8. Dept. Agric. 
Bur. Plant Ind. Bull. 230: 1912. Caldwell, J. S., “The relation of 
environmental conditions to the phenomenon of permanent wilting in 
plants. Physiol. Res. {: 1-56. 1913. Shive, J. W., and Living- 
ston, B. E., “The relation of atmospheric evaporating power to 
soil moisture content at permanent wilting in plants. Plant World 
17: 81-121. 1914. 
12 
