46 J.L.MONTEITH 



form of a pyramid of calcareous stones with base 3x3m and height 2-5 m. 

 During summer months, the amount of water collected daily was between 

 I and 2-5 Htres consistent with present calculations. At Dakar, attempts to 

 exploit this simple method of obtaining water from the atmosphere appear 

 to have been unsuccessful (Lejeune and Savornin, 1958). 

 (3) Hemisphere with base on ground. 



With // = o, p= 1-5, independent of size. 



Two fallacious conclusions have been drawn from the observation that 

 dew on isolated plants (expressed on the basis of ground area) often exceeds 

 condensation on a gauge. The first is that condensation on a field crop can 

 be estimated by multiplying gauge condensation by a 'plant factor' 

 equivalent to p and determined from dew measurements on an isolated 

 plant of the same species. For example, Ashbel (1949) apphed Hiltner's 

 factor for beans {p = 6) to dew gauge measurements in the Negev, obtaining 

 an annual dew total of 10-25 cm. A more acceptable estimate for the region 

 is 10-25 mm (Gilead and Rosenan,i954). Hofmann quotes similar examples 

 and shows that in unsaturated air, p may vary from zero to infinity depend- 

 ing on the variation of relative humidity with height near the ground. The 

 variabihty of 'plant factors' v^^hen the atmosphere is unsaturated makes 

 them worthless in ecological studies of dew. 



The second fallacy is that the net water loss from isolated and heavily 

 dewed plants is less than from plants of the same species in a closed com- 

 munity. Incoming radiation received by an isolated plant during the day 

 always exceeds the radiation falUng on a plane surface covermg the same 

 groimd area, and when there is no physiological control, transpiration is 

 greater than from the equivalent area of a closed canopy. Because potential 

 evaporation is normally much greater than potential condensation, the net 

 water loss from an isolated plant will then be greater, not less, than the 

 loss from a closed canopy covering the same ground area. 



The effect of plant spacing on dew may become important for xerophy tic 

 species when transpiration rates are comparable with potential condensa- 

 tion. Deacon and others (1958) suggested that the natural habit of desert 

 species growing in isolated clumps may represent an optimum spacing for 

 maximum dew, but there is no experimental evidence for this hypothesis. 



5. SOURCES OF DEW 



Controversy over the source of dew is more than two thousand years old, 

 and in 1929 it reached the pages o£ Nature. Theories of rising and falHng 

 dew both received support, but Simpson (1929) stifled further correspon- 



