54 



BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. 



TABLE 15. Water requirements per millimeter of leaf length of wheat and 

 1)rome grass in type stations. 



Wheat uses nearly twice as much water per millimeter leaf length 

 in the oak-brush type as in the aspen-fir type, and more than twice 

 as much as in the spruce-fir type (fig. 30). In other words, water 

 appears to be used most conservatively by a unit of wheat-leaf area 

 in the type showing the lowest physiological temperature efficiency 

 and temperature summation above 40 F., and most extravagantly 

 in the type of highest temperature efficiency. Hence the curve rep- 

 resenting the water requirement of wheat and the temperature 

 summation curves fall from the type lowest in elevation to that of 

 highest altitude in the same general way. 



In the case of brome grass the water requirement of the leaves is 

 found to be practically the same in all types in spite of the difference 

 in the climatic conditions and in the stage of development of the 

 plants. The reason for this dissimilarity between the two species 

 is not entirely clear, but it may be related to the fact that mountain 

 brome grass does not naturally inhabit the oak-brush type, though, 

 indeed, the specimens observed appeared to develop normally. 



In all instances a given leaf area of brome grass has a lower water 

 requirement than wheat. Notwithstanding this fact, however, the 

 water requirement per unit of dry matter for the plant as a whole, 

 as previously shown, is greater for brome grass than for wheat. 

 This is largely accounted for by the fact that the aerial part of 

 mountain brome grass consists essentially of leaf blades, while a 

 large proportion of the aerial dry matter of wheat is made up of 

 stems and heads, the transpiration from which is low as compared 

 with leaf surface. This relation between the water requirements of 

 leaves of the two species is further shown by the curve representing 

 the water requirement per millimeter of leaf length of wheat ex- 

 pressed as a percentage of the water requirement per millimeter of 

 leaf length of brome grass. These percentages, which are 312 in the 

 oak-brush type, 195 in the central station, and 127 in the spruce-fir 

 tvpe, indicate that wheat becomes relatively more efficient in the 

 use of water as compared with brome grass as the temperature falls. 



