40 MISC. PUBLICATION 257, U. S. DEFT. OF AGRICULTURE 



SOIL TEMPERATURE 



In addition to air temperature, the temperature of the soil is an im- 

 portant factor in the transpiration of trees. Yendo and Yamashita 

 (244) showed that in the mulberry the transpiration stream began in 

 March as soon as the soil temperature had reached 5° C; but Ikeda 

 (112), working on the same problem, recorded 4° as the critical soil 

 temperature for the flow of the transpiration stream in this species. 

 Cribbs, also in the studies already referred to (40), concluded that the 

 temperature of the soil was one of the most important factors in the 

 transpiration of Tilia, since the transpiration curve ran very nearly 

 parallel to that of soil temperature. 



Humidity and Factors Influencing It 



Of all the characteristics of the atmosphere which influence tran- 

 spiration, by far the most important is the water-vapor content. The 

 amount of water given off by plants or by any moist surface to the 

 air will depend on the amount of water vapor already present in the 

 air ; and that transpiration varies inversely with the relative humidity 

 is a fact so well known that it seems hardly necessary to call attention 

 to it. Most of the numerous papers published on this subject deal 

 with herbaceous plants, but there is a sufficient literature on trees to 

 give a fairly clear concept of the factors and problems involved. 

 Furthermore, there is very little reason to suppose that the principles 

 concerned differ with the type of vegetation. 



Plants frequently transpire very slowly even in a saturated atmos- 

 phere and, according to Darwin (4%), leaves may continue to give 

 off water into supersaturated air up to 5 percent above saturation, 

 when transpiration becomes nil. But Seybold (189, p. 92) believes 

 that transpiration must cease when the vapor-pressure potential is 

 zero and states that transpiration is found in a saturated atmosphere 

 only where the temperature conditions have not been carefully watched. 

 Only when the leaf temperature is higher than that of the environ- 

 ment can transpiration into a saturated atmosphere occur. If the 

 leaf has the same temperature as the external air, then transpiration 

 into a saturated atmosphere is impossible. 4 



Thus, Dole (52) found the greatest transpiration in Finns strohus 

 when the humidity was lowest but, as previously stated, in such 

 experiments one must be careful to watch both the light and the 

 temperature as well as the other factors that commonly change with 

 the humidity. 



Cribbs (40), who found the relative humidity very important in 

 his studies on the transpiration of Tilia, showed that Tilia transpired 

 more as the general dryness of the habitat (site) increased. 



AIR TEMPERATURE 



Various authors have tried to devise formulas that would take into 

 account the principal contributing factors associated with humidity, 

 but up to the present none of these formulas is very applicable under 

 practical field conditions. Of the many factors which influence 



* This whole question of the relation between the water content and osmotic pressure of cells on the one 

 hand and (1) the vapor pressure (relative humidity) of the surrounding atmosphere as well as (2) the cor- 

 responding water relations of the immediate environment (soil, air, or biotic) on the ether has been discussed 

 at considerable length with special reference to the ecology (xerophytism) of herbaceous vegetation by 

 Walter (224) and Fukuda (62) under the name of the "Hydratur" of plants. 



