3l6 AMERICAN JOURNAL OF BOTANY {Vol. g, 



system of twelve Livingston's porous-cup atmom6ters which were fed from 

 ah elevated water tanki The bottom of the chamber was also covered by- 

 two large flat trays of water. Under these coriditions it was possible to 

 keep the relative humidity between 70 and 100 percent. The temperature 

 varied from 15° to 31° C. during the daytirtie, usually averaging about 25° 

 C. The atmospheric humidity in the dry chamber was kept low by the 

 use of anhydrous calcium chloride. Under these conditions it was possible 

 to keep the relative humidity between 30 and 60 percent. The tempera- 

 ture; variations in the dry chamber were about the same as in the humid 

 chamber, but often the temperature was slightly higher in the former. The 

 evaporating power of the air was determined by standardized Livingston's 

 dark porous-cup atmometers. The average quantity of water lost per 

 day for seven days was 13 cubic centimeters in the dry chamber as com- 

 pared with 6 cubic centimeters in the humid chamber. 



The cultures growing in the sunlight were placed about 20 centimeters 

 apart on a greenhouse bench. The relative humidity under these condi- 

 tions varied from 25 to 60 percent. The temperature was usually between 

 22" and 26° C, but the extremes were 14° and 30° C. The cultures grown 

 in the shade were on the same greenhouse bench, but were covered with a 

 tent of the same size and shape as the dry and humid chambers, made of 

 two layers of cheesecloth, so as to reduce the sunlight. Within this tent 

 the relative humidity was usually about 5, percent higher than in the open 

 sunlight. The temperature was usually from i to 5 degrees lower than in 

 the open sunlight. The average daily evaporation from standardized 

 atmometers was 29 cubic centimeters in the sunlight and 17 cubic centi- 

 meters in the shade. 



Barley {Hordeum vulgare L.) was used in this experiment. In order to 

 avoid as far as possible error due to individual variation, seed from a pure 

 line of barley was obtained from the department of plant breeding, Cornell 

 University. The seeds, selected for uniformity in size and shape, were 

 sterilized by formalin treatment, and germinated. When the roots were 

 about four centimeters long the seedlings were planted in culture jars. The 

 culture jars used throughout these experiments were quart fruit jars of the 

 "Iinproved Mason" brand which were covered with black paper. Four 

 seedlings were planted in each culture jar. After standing on a greenhduse 

 bench for one week, those cultures which contained four healthy plants were 

 divided into four similar lots of 28 cultures and placed under the following 

 conditions : 



a. Dry chamber 



b. Humid chamber 



c. Sunlight 



d. Shade tent 



These cultures were grown for five weeks, August 4 to September 8, 

 1920. During this time the water lost by transpiration was replaced with 



