60 JOURNAIv OF FORIJSTRY 



iological functions of standard plants developed under different cli- 

 matic conditions. 



The plants experimented with under the different types of climate 

 and in different kinds of soil were peas, wheat, and brome-grass, all of 

 which were grown in suitable potometers. Water was added in mea- 

 sured amounts, as needed, to keep the soil from drying to a point ap- 

 proaching closely its wilting coefficient. 



The measurement of the plants grown in the battery of potometers, 

 at each of the different stations representing the three climatic types, in- 

 cluded the measurements of the stems and leaves at regular intervals 

 throughout the growing season, in order to obtain data on the relation 

 of the environment to the tendency of the plants to elongate their stems 

 and to expand their leaves: also, at the end of the growing season, the 

 dry weight and ash content of the parts above ground were determined, 

 and in some instances the soil was washed from the roots and the dry 

 weight and ash content also determined. In these measurements full 

 consideration is given to possible sources of experimental error. 



The measurement of the physical factors at each of the type stations 

 where batteries of potometers were located included continuous records 

 of air temperature, precipitation, evaporation, relative humidity, sun- 

 shine, and barometric pressure. A continuous record of wind velocity 

 was kept at the upper and lower stations. 



It is well known that the values obtained in the measurements of the 

 physical site factors depend largely upon the kind and character of the 

 instruments used. The author emphasizes this point and presents a 

 comparison of evaporation data derived from a free-water surface and 

 from the porous cup atmometer of various types. In order to obtain 

 an evaporation record comparable with transpiration from the plant for 

 short periods, the instrument with which the evaporation is measured 

 should correspond quickly with temperature changes in the air, as does 

 the plant itself. The author found the atmometer superior to the free- 

 water surface employed, due to its quicker response to temperature 

 changes. 



The data on sunshine intensity were obtained by noting the difference 

 in evaporation between the radio-atmometer and the ordinary white 

 porous cup atmometer. The duration of sunshine was measured with 

 the Marvin sunshine recorder. The difference in the rate of evapo- 

 ration from the black and white porous cup atmometers is not only a 

 measure of sunshine intensity, but it was found to be a fairly good 

 index of sunshine duration. There was but little variation in the slope 

 of the curves of evaporation from the two atmometers except on cloudy 



