WATER UTILIZATION BY TREES 49 



moisture but to the inability of the plant to conduct water rapidly 

 enough. The drop in transpiration occurring when the soil moisture 

 reached the wilting coefficient was more permanent and was accom- 

 panied by visible wilting. The soil moisture in a region of ample 

 precipitation is thus more likely to become a limiting factor in sand 

 than in clay soils, since it is here much easier for the moisture content 

 of sandy soils to get as low as the wilting coefficient. 



Hendrickson (93) f however, found in various Prunus orchards in 

 California that the stomata were wider open on moist soils than on 

 dry soils, indicating (not proving) that transpiration was taking place 

 more rapidly. 



A significant paper by Stanfleld (206) in this connection illustrates 

 the importance of the interpretation of data. He studied the osmotic 

 pressure of Pinus ponderosa leaves from trees 4 to 6 feet high near 

 Boulder, Colo., in an attempt to find whether the osmotic pressure 

 was correlated with the evaporation rate (as measured by atmometers) 

 as well as with the soil moisture and soil temperature. He found 

 that the lowest relative humidity was accompanied by the highest 

 osmotic pressure, and he seems to have concluded that the two are 

 correlated. Since, however, the work of other research workers has 

 shown that high transpiration rates are correlated with low relative 

 humidities, it would necessarily follow that high transpiration rates 

 are correlated with high osmotic pressures, which is a conclusion 

 contrary to the results of most workers. 



When Stanfleld 's data are carefully analyzed, however, it is found 

 that the contradiction is more apparent than real. It is unfortunate 

 that he did not measure the transpiration rates of the trees studied, 

 for if he had he would probably have found a low transpiration rate. 

 His results simply mean that in the comparatively dry atmosphere 

 of Colorado the trees studied reduced their transpiration, and this 

 reduction of transpiration was associated with the high osmotic pres- 

 sures of the needles. The highest osmotic pressure was found in 

 July, which is also the season of lowest relative humidity. Stanfleld 

 also reported that there was no correlation between the osmotic pres- 

 sure and the soil moisture; but if his data are examined, closely, it 

 will be noted that the highest osmotic pressure during the time of his 

 studies occurred either in July when the soil moisture was at a mini- 

 mum, or in May when the soil temperature was lowest (and the soil 

 moisture, consequently, unavailable). The literature is very often 

 confused in this fashion by workers who apparently have too narrow 

 a knowledge of the field and of the problems involved to interpret 

 their results correctly. 



Experimental work in this field is perhaps more conclusive. Bader 

 (5) found that transpiration in various species of Pinus was directly 

 related to soil moisture, which, along with light, was the chief con- 

 trolling factor. In transpiration intensity the pines studied fell into 

 three groups: 



(1) Highest: P. excelsa Wall., P. cembra, P. peuce Griseb. 



(2) Middle: P. sylvestris, P. montana Mill. 



(3) Lowest: P. austriaca Hoss., P. strobus. 



The morning drop in transpiration occurs more quickly in the more 

 rapidly transpiring species. 



121727°— 37 4 



