375] B. E. Livingston 177 



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 Eenner's experiments, and also from those of Living- 

 ston and Hawkins, 5 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 6 (from which the wilted 

 tissues may recover when transpiration is subsequently de- 

 creased), then by permanent wilting 7 (from which the plants 



5 Livingston, B. E., and Hawkins, Lon A., "The water relation 

 between plant and soil." Carnegie Inst. W\ash. Pub. 204: 5-48. 

 Washington, 1915. 



6 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. 



7 Briggs, L. J., and Shantz, H. L., "The wilting coefficient for 

 different plants and its indirect determination. U. S. 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 : 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. . 



