EVAPORATION AND CAUSAL FACTORS. 



65 



and nasturtium were wetted experimentally the stomata opened. 

 Closure, however, did not always follow the drying of this moisture 

 as rapidly as it did in this series. 



Wind carries fine particles of dust to coat the leaves, thus reducing 

 light penetration and chlorovaporization and indirectly affecting 

 stomatal movement. Dust particles are also found wedging open 

 many of the stomata after a violent wind. A stoma of wheat kept 

 from closing in this manner is shown in plate 1. In one leaf of sugar- 

 beet, 17 per cent of all the stomata observed were wedged open by 

 dust particles. This is an unusually large proportion, but at many 

 times sufficient stomata in each leaf are found permanently held open 

 in this manner to produce a serious effect upon the plant during 

 times of stress. High winds which arise suddenly seem to force 

 currents of air through the leaves before the stomata close, and 

 wedge the grains into the slit in this manner. This is also the most 

 plausible explanation of the immediate great increase of transpiration 

 which occurs at once upon the sudden appearance of a high wind. 

 Under ordinary circumstances, however, wind probably only removes 

 shells of nearly saturated air from the outer ends of the stomatal 

 pores (Dixon, 1914). 



The attempt to correlate evaporation from atmometers and free 

 water-surfaces with transpiration has met only with negative results. 

 A porous cup can hardly be expected to respond more freely to its 

 environment than a free water-surface or saturated blotting paper, 



\c 



7 8 9 10 11 NOON 



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FIG. 35. "Relative transpiration" based on evaporation from blotting-paper 

 atmometer (A) and from white-cylinder porous cup (B), compared 

 with stomatal movement in onion (C). 



and hence the results from a white-cylinder atmometer have no 

 advantages over those from the other types. Until an atmometer is 

 devised which responds in the same manner and degree to each of the 

 factors concerned, the ratio of transpiration to evaporation is mean- 

 ingless. Thus, in series 34, "relative transpiration," calculated from 

 the data obtained by the blotting-paper atmometer, shows a certain 

 degree of correlation with stomatal movement, while that calculated 

 from the evaporation from the porous cup shows practically none 



