379] B. E. Livingston 181 



for last hour of incipient drying was 1.3 cc. Transpiration for this 

 hour was 0.38 g., and absorption was 0.11 cc. 



(8) Dec. 1, cloudy. Mimosa plant. Transpiration was greater 

 than absorption for the period 7:55 a. m. to 3:55 p. m., incipient 

 drying amounting to 2.97 g. Absorption was greater than tran- 

 spiration for the period 3:55 to 5:55 p. m., the plant gaming in 

 weight 0.27 g. No wilting was noted; the leaves were in night 

 position at end of last hour. Atmometric index for last hour of 

 incipient drying was 0.7 cc. Transpiration for this hour was 1.03 g. 

 and absorption was 0.87 cc. 



These data show very clearly that incipient drying, tempo- 

 rary wilting, and even permanent wilting of most of the 

 leaves, may occur without any resistance at all to water- 

 absorption by roots. These phenomena are here quite inde- 

 pendent of such resistance to water intake as may be offered 

 by unsaturated soils. Furthermore, in the complete absence 

 of environmental resistance to water absorption by the root 

 system, incipient drying may begin with an evaporating 

 power of the air as low as 0.33 cc. per hour from the Living- 

 ston standard white sphere (Coleus, Nov. 4). Consequently, 

 it does not require a high atmometric index to render the 

 transpiration rate larger than the rate of absorption, in the 

 case of some plants at least. The truth of this statement must 

 be much more pronounced when the plant roots are sur- 

 rounded by ordinary, fairly dry soils, which interpose an ex- 

 ternal resistance to water intake. 



Unfortunately, atmometric observations were omitted in 

 the first three tests, so that it is not possible to state what 

 order of atmometric index values produced the wilting phe- 

 nomena recorded for Sept. 20, 21 and 23. It is, of course, 

 certain that these index values were not exceptionally high, 

 however; the index for Baltimore is never high, and there 

 \vas no artificial heat applied to the greenhouse on these 

 days, so that the index value was not artificially raised. It 

 is worth something to note that permanent wilting of most 

 of the leaves of healthy buckwheat plants occurred in an 

 unheated greenhouse in Baltimore on Sept. 20, with clear sky, 

 and on Sept. 23, with partly cloudy sky. 



