52 J.L.MONTEITH 



Normal transfer of water through plants from the soil to the atmosphere 

 requires 



Ha<Hl< Hs 



By imposing the reverse gradient 



Hs<Hl< Ha 



in the laboratory, Slatyer and others demonstrated the absorption of vapour 

 by the leaf cuticle and the simultaneous transfer of water to the roots. 

 Further transfer from roots to soil is theoretically possible but the experi- 

 mental evidence is ambiguous. In Slatyer's experiments with one-year-old 

 Pinns echinata seedhngs, water vapour was absorbed when the atmosphere 

 was held saturated (H=o) or with relative humidity 98-8% {H= -155 m). 

 With strict temperature control ( ± o-ooi°C) there was presumably no leaf- 

 air temperature difference and therefore no condensation of hquid water 

 on the plant leaves. In nature, plant leaves are usually cooler than the 

 atmosphere by i or 2°C during the night, so condensation begins when the 

 relative humidity rises to 96 or 98% (Fig. i). Unless H is exceptionally 

 low (< — 300 m) condensation will precede and preclude cuticular absorp- 

 tion of vapour. The only species Hkely to benefit from vapour absorption 

 are those which can survive extreme water stress. For example. Stone, 

 Went and Young (1950) found that a Pinus coulteri seedlmg growing in soil 

 unwatered for ten months could absorb vapour from air with relative 

 humidity below 90%, implying H< - 1400 m. 



Exposed to bright sunsliine, surface soil may dry to very low values of H, 

 say to -10^ m in equilibrium with air at relative humidity 48%, and 

 subsequent absorption of vapour at night is common. In a study of vapour 

 uptake by Indian soils, Ramdas and Katti (1936) found that the moisture 

 content in the surface layer of a 'black cotton soil' increased from 2-6% dry 

 weight in the afternoon to 7-7% in the early morning. Damagnez (1958) 

 measured absorption rates of 0-15 mm /night and showed that for given 

 atmospheric conditions, uptake was greater at lower soil moisture contents. 

 At Rothamsted, maximum absorption by fallow soil (a clay loam) was 

 0-6 mm/night, measured with the field balance on May 6, 1957. 



Most of the moisture absorbed by a dry soil is held at levels of free energy 

 far below those available to plant roots (Fig. 3). Because tliis moisture cannot 

 subsequently be absorbed by roots, it makes no direct contribution to plant 

 water economy; but plant transpiration may be shghtly reduced for a few 

 hours after sunrise while absorbed moisture evaporates from the soil 



surface. 



Confusion over the relationship between soil moisture and the relative 



