2-'0 THE PLANT WORLD. 



calculating evaporation from measurements of these factors, 

 our knowledge along this line is at present so incomplete 

 that such formulas are not of great practical use. Further- 

 more, even though we could calculate the evaporation from 

 data of humidity, temperature and wind, the direct measure- 

 ment of evaporation would still be the simplest and most 

 satisfactory method for the physiologist; the atmometer* is 

 much more simple and more easily operated than the in- 

 struments for recording the above factors, the data obtained 

 are much more readily handled, and, finally, the evaporating 

 power of the air is the directly controlling factor in watei 

 loss from plants. The atmometer may be regarded as an ar- 

 tificial plant, of course without the plant's** mechanisms for 

 regulating the water loss from the interior. 



I'he effect of evaporation as a climatic factor in influ- 

 encing the behavior of a number of different species growing 

 in soil which was kept nearly at its optimum moisture content, 

 was studied in an experiment carried on at the Desert Lab- 

 oratory, at Tucson, Arizona, during the summer just past. 

 Narrow beds were prepared in the open ground and were 

 watered by lateral sepage from irrigation trenches which 

 were filled once or twice daily- The water stood in the 

 trenches only a few hours at a time and the soil, a heavy 

 clay, did not at any time become waterlogged. Among the 

 plants tested were the garden nasturtium {TropacoJnm) , 

 morning glory, marigold, sun flower {Helianthiis an nuns)., 

 mustard, castor bean, muskmelon,. teasel (Dipsaciis sylvt's- 

 ris) , and Jimson weed {Datura stramonium.) 



*On account of seniority and correct etymology, the term atmometer 

 Is here used in pkice of the hybrid term, I'mpdrlriuirr, crdporonitirr, atid 

 eraporameter (only the latter of which is etymologically even possible.) 

 Although atome^e"- was adopted by the International Metiorological Congress 

 at Venice in 1874, evaporimeter, etc., are largely used in America. 



**On the matter of regulatory responses, see tlie author's paper. The 

 relation of desert plants to soil moisture and to evaporation. Publication 

 No. 50 of the Carnegie Institution of Washington, 1906. Also, Relative tran- 

 spiration in cacti. Plant World, 10: 110-114. May, 1907. 



