100 Plant Physiology 



or filter tube, the whole being filled with water. This 

 filter tube may be shellacked to a known surface, adopted 

 as a unit, and all other instruments may be standardized 

 with respect to this. 1 Under different conditions the 

 curve of water-loss from this instrument may not be 

 comparable to that from a free water-surface; but the 

 effects of conditions upon it are supposed to be more 

 nearly comparable to the effects upon plant surfaces. 



The evaporimeter has also been serviceable in contrast- 

 ing transpiration and evaporation in unit areas, thus 

 relative transpiration may be taken as the ratio of trans- 



* R 



piration to evaporation, conveniently expressed as . The 



E 



extent of variability respecting this ratio has been regarded 

 ^, fair indication 6f what is conveniently termed physio- 

 logical checking of transpiration. In studying this regu- 

 latory check upon transpiration Livingston finds that in 

 certain desert plants it is especially operative between 

 6.30 A.M. and 1 P.M., and especially pronounced at tem- 

 peratures from 79 to 90 F. 



A proper study of the relation of certain horticultural 

 plants to evaporation factors promises to yield much data 

 of practical value. 



Various observers have compared the loss of water from 

 leaves with the loss from an equal surface area of soil. 

 Nobbe has shown that evaporation from the surface of 

 the soil may be 1.6 to 5 times the amount lost from an 

 equal leaf area. In these cases the ordinary crop plants 



1 Transeau (Bot. Gaz., 49 : 459, 1910) has recently constructed an 

 instrument which seems to possess some advantages in the way of sim 

 plicity and rate of evaporation. 



