. Climate and plant growth in certain vegetative associations. Growth (Plants); Crops and climate; Plant ecology. 54 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. Table 15.—Water requirements per millimeter of leaf length of wlieat and brome grass in type stations. Per cent difference in water require- ments between wheat and brome grass. Water requirements per millimeter leaf length. Wheat. Brome grass. Physio- logical | temper- ature efficiency. ] Degrees. Oak-brush I 2,002 Aspen-fir 1,404 Spruce-fir 991 312 195 127 .526 .407 Grams. .273 .319 Index. 2, 1,
. Climate and plant growth in certain vegetative associations. Growth (Plants); Crops and climate; Plant ecology. 54 BULLETIN 700, U. S. DEPARTMENT OF AGRICULTURE. Table 15.—Water requirements per millimeter of leaf length of wlieat and brome grass in type stations. Per cent difference in water require- ments between wheat and brome grass. Water requirements per millimeter leaf length. Wheat. Brome grass. Physio- logical | temper- ature efficiency. ] Degrees. Oak-brush I 2,002 Aspen-fir 1,404 Spruce-fir 991 312 195 127 .526 .407 Grams. .273 .319 Index. 2, 1, 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 f
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