Oct. IS, 1914 Water Requirement of Plants 57 



table (Table XXXIV) , which places the water requirement of all crops 

 upon the basis of years similar to 191 1 and 191 3. When the water- 

 requirement measurement of any particular crop extended over more 

 than one year, the mean value of the several water-requirement deter- 

 minations is given in Table XXXIV. 



SUMMARY 



This paper deals with the measurement of the water requirement of 

 plants at Akron, Colo., in the central portion of the Great Plains. The 

 term "water requirement" is here used to express the ratio of the water 

 absorbed by a plant during its period of growth to the dry matter pro- 

 duced. The plants were grown to maturity in large galvanized-iron pots 

 having a capacity of about 1 15 kg. of soil. Each pot was provided with a 

 tight-fitting cover having openings for the stems of the plants, the annular 

 space between the stem of the plant and the cover being sealed with wax. 

 The loss of water was thus practically confined to that taking place 

 through transpiration, and the entrance of rainfall was almost wholly 

 prevented. The pots were weighed two or three times weekly to deter- 

 mine the amount of water required to maintain normal weight. Water 

 was delivered from 2-liter calibrated flasks through stoppered openings in 

 the middle of the cover to a 5-inch flowerpot sunk in the soil immediately 

 beneath the cover. 



To protect the plants from birds and severe wind and hail storms, it 

 was found necessary to conduct the experiments in a screened inclosure. 

 Pyrheliometric measurements showed that the inclosure reduced the 

 radiation about 20 per cent. Water-requirement measurements con- 

 ducted simultaneously with the same plants inside and outside the 

 inclosure showed that the inclosure also reduced the water requirement 

 approximately 20 per cent. 



Rich surface soil was used in the pots, and the pots were also fertilized 

 to insure an adequate supply of plant food. Six pots of plants of each 

 variety were used, and the water requirement of each pot was deter- 

 mined independently, in order to provide a basis for the determination 

 of the probable errors of the experiment. 



The detailed results given in the paper comprise measurements of 44 

 species and varieties in 1912 and 55 in 1913. The writers' 191 1 measure- 

 ments have also been included in the summary table. The years 1911 

 and 1 91 3 were similar in character, and the same plants grown during 

 both years gave practically the same water requirement. The 3'ear 

 191 2 was cooler and the evaporation and light intensity were much 

 lower. These conditions had a marked influence on the water require- 

 ment, the mean water requirement in 1912 being only jj per cent of 

 that in 191 1 and 1913. In order to place all of the determinations 

 upon a comparative basis, the 191 2 measurements have accordingly 

 been increased 30 per cent in the summary table (Table XXXIV). 



