TRITICUM 117 



and up to 8,000 feet elevation in the Central Rocky Moun- 

 tains. In this last-mentioned section wheat will yield a crop, 

 except in unusual years, where the mean temperature for the 

 year is not below 38°F., and that for the summer season is 

 not below 58°F. Winter wheats are able to resist low tem- 

 peratures for longer periods than spring wheats. 



Plants differ widely in their water economy. Some re- 

 quire much more water than others to produce a unit of 

 dry matter. The water requirement of a plant is defined as 

 the number of units of water absorbed by the plant in the 

 production of a unit of dry matter. The following data are 

 taken from Briggs and Shantz: 



WATER-REQmREMENT DETERMINATIONS AT AkRON, COLORADO, IQII, I912, 



AND 1913, Based on the Production of Dry Matter 



Plant Mean of genus 



Proso 293 



Millet 310 



Sorghum 322 



Corn 368 



Wheat 513 



Barley 534 



Buckwheat 578 



Oats 597 



Rye 685 



Beet, sugar 397 



Potato 636 



Pea, Canada field 788 



Alfalfa 831 



If the water requirement of proso millet is regarded as i, 

 the water requirement for the following crops is as follows: 

 millet 1.06; sorghum i.io; corn, 1.26; wheat 1.76; barley 

 T.83; oats 2.04; rye, 2.34; rice, 2.42. 



The water requirement of a plant is dependent upon a 

 number of conditions, chief of which is the fertihty of the 

 soil. The water requirement is greater in an unfertile than 



