252 Physiologie. 



Arizona, so that the writers are unable to compare their conditions 

 with those of more humid or cooler regions. From their experience 

 with cloudy weather, they are inclined to the prediction that the 

 diurnal decrease in leaf moisture here established for high evaporation 

 rates may fall to occur in regions of low evaporation when accom- 

 panied by relatively high rates of soil moisture supply. 



These studies also indicate that some non-succulent, small-leaved 

 xerophytes (such as Covülea and Prosopis) fail more or less com- 

 pletely to exhibit a diurnal fall in foliar moisture under conditions 

 of evaporation which render it manifest in the common type of 

 thin-leaved plants (such as Martynia, Sida, Physalis), as well as in 

 such pronounced succulents as the Portulaca-Uke Trianthentum of 

 this paper. It is suggested that these exceptional small-leaved 

 xerophytes may actually show a somewhat higher leaf moisture 

 content by day than by night, but this proposition is uncertain. 



While the other logically possible cause of this diurnal decrease 

 in relative water content of foliage leaves, namely, a diurnal increase 

 in materials other than water within the tissues, remains still to be 

 considered in a thoroughly adequate way, the tindings fail to 

 adduce evidence in favor of this as the true cause of the observed 

 phenomena, and do furnish several lines of indirect opposing 

 evidence. It may be stated, therefore, that, so far as evidence is 

 at hand (including indirect considerations of the literature), it is 

 probable that the cause of this diurnal minimum in foliar moisture 

 rests in the phenomenon of incipient dr3'ing, brought about whenever 

 the ratio of water loss to water supply in the leaves is rendered 

 less than unity. It may thus be suggested that, although the writers' 

 tests with Physalis would lead to the conclusion that the external 

 factor which controls this diurnal fall of leaf moisture is evaporation 

 intensity simply, the true Controlling condition is more probably 

 the ratio of water supply to water loss. Thus, the structure of the 

 plant (including all ot its various „adaptations" to dry habitats), the 

 moisture conditions of the soil, intensit}^ of evaporation and of solar 

 illumination appear to make up the Controlling environmental 

 complex. 



It seems highly probable from the present studies that the 

 diurnal non-stomatal retardation of the escape of water vapor from 

 green leaves in sunlight (as first described in Publ. 50, Carnegie 

 Inst., and there attributed to the influence of temperature or evapo- 

 ration intensity) is but the effect of a lower vapor tension within 

 the internal atmosphere of the leaves and over their surfaces, this 

 lower vapor tension being brought about by the increased surface 

 tension and decreased evaporating surface which accompanies a 

 lowered water content of the internally and externally exposed 

 cell walls. 



In conclusion, it may be suggested that we have here, in the 

 diurnal minimum in the water content of foliage leaves, a criterion 

 that may be of some importance to scientific agriculture, at least 

 in the arid regions of the globe. By this criterion it raa-j be possible 

 to determine indirectly, and somewhat simply, the Status of the 

 water relations of the plant, and indeed to foresee the need of 

 increased soil moisture, long before the usual criterion of cessation 

 of growth or actual wilting l3ecomes manifest. Jongmans. 



Ohlweiler, W, W., The relation between the density 



01 



