44 Irrigation and Drainage 



ally controlled by changes in the transpiring surfaces of plants, 

 and if this control is sensitive, then there would also be a ten- 

 dency to cause the amount of water necessary to produce a pound 

 of dry matter in a given species of plant to remain nearly con- 

 stant under wide ranges of climatic conditions. That most land 

 plants are provided with organs which modify the rate of trans- 

 piration has been long established ; but how narrow the limits 

 of control are remains to be demonstrated. It is fundamentally 

 very important that such facts as these should be established, for 

 they are needed in order that we may know how much land under 

 a given crop a given quantity of water will irrigate. 



We have, at this writing, just completed a set of observations 

 bearing upon this fundamental problem, and although they are 

 not sufficiently extended to be demonstrative, they are yet very 

 suggestive, and will be of interest here. 



If it is true that plants lose little moisture except through 

 their breathing pores, and if these are closed during those times 

 when there is not sufficient light to allow carbonic acid gas to be 

 decomposed by the plant, then during the night, and perhaps, 

 also, during cloudy weather, plants should lose but little moisture 

 through their surfaces. To test this question, one of the small 

 cylinders in the plant- house, containing four fully grown stalks 

 of maize, was hung upon the scales, to be weighed hourly dur- 

 ing the day ; and by the side of it was^ set a Piche evapo- 

 rometer having an evaporation surface of 27 square inches, also 

 to be read hourly. Below are given the results of these obser- 

 vations : 



During the day, from 8:15 A. M. until 6:15 P. M., it was some- 

 what cloudy most of the time, but the clouds were not heavy, and 

 there was a little sunshine through a haze from 11:15 A. M. until 

 2:15 P. M. From 8:15 A. M. until 6:15 p. M. the corn and soil 

 lost 3 pounds of water, and there was evaporated from the evaporo- 

 meter 31.5 c. c. or 1.2 cu. in. From 6:15 P. M. until 6:45 A. M. 

 the next morning, the corn had not lost enough to show on the 

 scales, which are sensitive to one -half pound ; and the evaporo- 

 meter showed a loss of 2.3 c. c., equal to .14 cu. in. The next 

 day was bright and sunny the whole time, and from 6:45 A. M. 



