INTRODUCTION. 15 



The physical data were obtained chiefly by means of simple instru- 

 ments. A stop-watch photometer was used for the light data, a 

 cog psychrometer to determine the relative humidity, a compen- 

 sated aneroid barometer for obtaining the pressure, a group of ordi- 

 nary chemical thermometers checking within a tenth of a degree 

 centigrade to determine the temperature at various levels of the soil 

 and air, and a Tycos anemometer to measure the velocity of the 

 wind. The water-content was determined by taking soil samples at 

 the depths described, weighing, drying them on a water-bath, and 

 weighing again after they had become constant in weight. Samples 

 were always taken in duplicate. 



The stop-watch photometer devised by Clements may need a fuller 

 description. It consists of a drum on which a strip of Solio paper is 

 wound and inclosed in a case which is light-proof. This case has a 

 rectangular opening directly over the strip of Solio. This opening is 

 covered and light-proof until an exposure is desired. The case can 

 be revolved one twenty-fifth of the circumference over the drum and 

 paper, exposing a new area on the strip each time. The slide is so 

 arranged that when the opening in the case is uncovered the stop- 

 watch attached to the front of the photometer is started, and stopped 

 when the slide is closed. In this manner the time of exposure can 

 be measured accurately to a fifth of a second in fact, much more 

 accurately than a fifth, for the operator soon learns to snap the slide 

 shut just at the moment that the hand of the watch jumps to the 

 proper fifth of a second. The time is probably accurate to a fiftieth 

 of a second. The slide is attached to a spring which instantly covers 

 the opening the moment it is released. 



The standard was made at noon on a clear day at the time of the 

 summer solstice. This consisted of a Solio strip exposed 1 second 

 at the first area, 2 seconds at the second area, 3 seconds at the third, 

 etc., the last area being exposed 25 seconds. As a rule, several of 

 these were made, one for each clear day during the proper period. 

 The one showing the deepest color was selected as the standard. 

 When a light reading was desired, an exposure was made and the 

 time and number of the exposure recorded. When the 25 exposures 

 were exhausted, the photometer was taken to the dark-room, the 

 strip removed, and each area of exposure compared with the standard. 

 When an area exposed 20 seconds has the same depth of color as the 

 3-second area on the standard, the light at that time was three- 

 twentieths of the maximum for that region, or 15 per cent. 



With the exception of Lloyd, no investigator seems to have used 

 plants growing in a natural environment. In spite of this, many of 

 these workers have tried to apply their data to plants growing natu- 

 rally, without due allowance for the conditions under which their 

 experiments were performed. Gray and Peirce (1917) experimented 



