THE MAGNITUDE OF TRANSPIRATION 165 



The Magnitude of Transpiration. — The magnitude of transpiration^ 

 whether computed on the basis of a unit area of leaf surface, an entire plant, 

 or an acre of forest or crop plants, shows a great variation depending both on 

 the plant species and environmental conditions to which they are exposed. 

 The quantity of water transpired may be computed and compared for hourly, 

 daily, seasonal, or yearly periods. Because of the great fluctuations in trans- 

 piration rates with environmental conditions the citation of isolated values of 

 transpiration rates is usually of very little physiological significance. 



Transpiration rates in plants of temperate regions range up to about 5 g. 

 per dm. 2 of leaf surface per hour.^ Usual rates, under favorable conditions 

 for stomatal transpiration, will generally fall within a range of 0.5 to 2.5 g. 

 per dm. 2 per hour. At night, or during periods unfavorable to stomatal 

 transpiration (dry soil conditions, etc.) the rate may fall to o.i g. per dm. 2 

 per hour, or even less. Under favorable conditions many herbaceous plants 

 will transpire several times their own volume of water in the course of a 

 single day. 



It is impossible to calculate accurately the transpiration rate of large 

 plants such as trees from the known rate of transpiration of some of the leaves 

 and an estimate of the area of all of the leaves on the tree. In the first place it 

 is difficult to arrive at any accurate estimate of the aggregate leaf area of a 

 large tree. In the second, there is a great variability in the transpiration rate 

 among the different leaves. The rate of transpiration of the individual leaves 

 varies depending upon their position, exposure, age, and internal physiological 

 conditions. For example, Huber (1923), found that low twigs of a red- 

 wood tree transpired six times as rapidly as similar twigs from the height of 

 12 meters. Neither can it be assumed, as mentioned in the discussion of the 

 potometer method, that the transpiration rate of leaves of branches which 

 may have been removed from a tree and placed with their cut ends in water 

 will be the same as their rate of transpiration before the branches were 

 detached. 



Hence attempts to determine the transpiration rate for an entire tree from 

 values determined for a few of the leaves on that tree can only result in 

 approximations. The same comments in general apply, although perhaps 

 not so emphatically, to calculations of the total transpiration of fields of cul- 

 tivated crops and natural vegetation areas based upon transpiration as deter- 

 mined for a limited number of individual plants. All such computations are 



^ If transpiration is measured for one minute intervals rates much in excess 

 of this may be observed, but such rates are maintained only for relatively short 

 periods of time. 



