1914] OTIS—TRANSPIRATION OF WATER PLANTS 493 



4. The amount of evaporation from a water surface occupied 

 by emersed water plants depends upon the following factors: (a) the 

 species of plant; (b) the density of plant stand; (c) the amount of 

 plant surface exposed to the evaporating power of the air ; (d) the 

 height of the plant growth above the level of the water; (e) external 

 factors (physical factors, like wind, temperature, relative humidity, 

 etc.); (/) internal factors (chemical and physiological phenomena 

 within the plant) . 



5. Transpiration from emersed water plant surfaces occurs 

 both by day and by night, but transpiration by day is greatly in 

 excess of that by night. 



6. For different species there is no constant ratio between rate 

 of transpiration and the area of surface exposed. 



7. The rate of evaporation from the transpiring surface of an 

 emersed water plant during the day may equal, and in some cases 

 may exceed, that from a free water surface of the same area; but at 

 night the evaporation from a free water surface is usually greatly 

 in excess of that from a transpiring plant surface. 



8. Wind, temperature, and relative humidity undoubtedly have 

 a great influence on the rate of transpiration of emersed water plants, 

 but these factors alone do not explain the great difference existing 

 between transpiration of emersed water plants and the evaporation 

 from a free water surface. 



9. These data are of economic importance in indicating what 



plants should be grown in and what plants should be excluded from 



storage reservoirs in regions of small rainfall and scant water 

 supply. 



In conclusion I desire to extend my hearty thanks to Professor 

 F. C. Xewcombe, who has directed this work throughout and who 

 has been a constant inspiration and help, and to Mr. Gardner S. 

 Williams and the Eastern Michigan Edison Company for their 

 aid in furnishing the tanks and pontoons and for their suggestions 

 on methods of construction. 



University of Michigan 

 Ann Arbor, Michigan 



