66 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



(fig. 35). There is, in consequence, no warrant for the view that lack 

 of agreement beween stomatal movement and "relative transpira- 

 tion" based upon evaporation from the porous-cup atmometer 

 indicates that stomata are non-regulatory. Nor, on the other hand, 

 can the lack of close agreement between " relative transpiration" 

 based upon the blotting-paper type of atmometer be attributed to 

 incomplete regulation by the stomata with as much justice as that 

 this evaporimeter fails to respond in the same manner and degree 

 to the various factors as the plant, aside from the effect of stomatal 

 movement. The only conclusion permissible is that the blotting- 

 paper atmometer represents with somewhat greater fidelity the 

 water-loss from the plant when it responds freely to its environment 

 than does the porous cup. 



WATER-CONTENT. 



Water-content acts upon stomatal movement by changing the rate 

 at which water is supplied to the leaves. When humidity is high, 

 therefore, a moderately low water-content may not produce any 

 significant difference in movement as compared with that found in a 

 plot of highly watered plants. But if humidity becomes low, tem- 

 perature high, and sunlight intense, the same water-content may be 

 critical and produce striking differences in movement. The Great 

 Salt Lake region was peculiarly adapted to studies dealing with the 

 divergence of movement caused by differences of water-content in 

 relation to evaporation, as the rainfall is deficient, permitting the 

 water-content to be readily controlled, and very humid days are 

 found as well as very dry ones. Attempts to conduct similar experi- 

 ments at the greenhouse of the University of Minnesota failed to 

 show anything conclusively, since the humidity could not be lowered 

 materially, or the much greater changes produced by deficient light 

 be discounted. Hence, to be conclusive, such experiments must be 

 conducted in the field in a dry, irrigated region. 



Four plots of alfalfa plants were used in series 32, started at 7 p. m. 

 August 25 and ended 7 p. m. August 26, 1919. The plants of the 

 first plot were water-logged; the second was irrigated with 2 inches 

 of water daily, beginning a week before the start of the series; the 

 third had been irrigated the week before with 4 inches of water; 

 and the fourth had not been irrigated for more than a month. The 

 soil of plot 1 was saturated, plot 2 had a water-content of 32 per cent 

 at the start of the series, plot 3 had 24.3 per cent, and plot 4 had 10.6 

 per cent. The plants of plot 1 presented a wilted appearance through- 

 out the series, those of plots 2 and 3 were normal, while those of plot 

 4 wilted somewhat during the day and recovered at night. The 

 weather conditions during the series were rather favorable, although 

 the temperature rose to 89 F. at 2 p. m. August 26. A haze formed 

 after sunrise and thickened into light clouds at 10 a. m., which floated 



