56 



BULLETIN 700, U. S. DEPARTMENT OF AGEICULTUEE. 



EFFECT OF EVAPORATION AND TEMPERATURE ON THE PRODUCTION OF DRY 

 MATTER PER UNIT OF LEAF ARKa. 



Quite as significant as the difference in water requirements per 

 unit of leaf area in the respective types is the effect of climatic con- 

 ditions on the efficiency of the leaves as manufacturing agents. This 

 has been calculated for wheat and brome grass, the same specimens 

 being employed as were used in deriving the water requirement data. 

 The summations are given in Table 16 and in figure 31. 



Table 16. — Dry matter produced per millimeter of leaf length of wheat and 

 hrome grass in the type stations. 



Type. 



Dry weight. 



Wheat. 



Brome 

 grass. 



Per cent 

 difference 



in dry 



weight of 



wheat and 



brome 



grass. 



Evapora- 

 tion 



summa- 

 tion. 



Physio- 

 logical 

 temper- 

 ture 

 eificiency. 



Oak-brush, 

 Aspen-fir.. 

 Spruce-flr.. 



Grams. 

 0. 00161 

 .00182 

 .00136 



Grams. 

 0. 00040 

 . 00054 

 .00042 



403 

 338 

 324 



4, 550. 

 2, 780. 3 

 4,251.3 



2, 706. 2 



1,560.5 



730.5 



Both curves in figure 31 representing the dry matter per unit of 

 leaf area show a maximum for the central type, the greatest con- 

 cavity upward occurring in the curve for wheat. This species also 

 shows a slightly greater production in the oak-brush type than in 

 the spruce-fir type. In brome grass the reverse occurs, but in neither 

 instance is the difference particularly marked. 



It is significant that the curve representing the production of dry 

 matter is opposite in slope to the curve showing the evaporating 

 power of the air. The data indicate that evaporation decreases the 

 rate at which the leaves manufacture food material, and the simi- 

 larity in the production of dry matter in the case of both species in 

 the three types may thus be accounted for by corresponding simi- 

 larities in the evaporation conditions. 



Another interesting parallelism is derived by dividing the dry 

 matter per unit of leaf length produced by wheat by the quantity 

 produced by brome grass. In this instance the curve is seen to fall 

 from the lowest to the highest type in the same general direction 

 as the physiological temperature efficiency curve. This apparent 

 correlation between temperature and the efficiency of the leaves 

 as manufacturing agents is of value, of course, only if it may be 

 assumed that the physiological index affords a reliable expression 

 of the relation between the temperature and the plants here dealt 

 with. 



