54 J.L.MONTEITH 



Evening: 19% 



Following morning: dewed 10% 



undewed 17% 



The measurements were made during September 1954 in Jerusalem after 

 several months without rain. A further experiment showed that dew 

 absorption increased the leaf water content of Ceratonia siliqua saplings 

 exposed to drought by 10% during the night, whereas the increase for 

 irrigated saplings was 7%. These figures suggest that the absorption of 

 dew by plants growing in dry soil may usefully supplement the absorption 

 of soil moisture, but Waisel emphasised that dew absorption was never 

 sufficient to restore full leaf water content. Stone (1957b) showed that when 

 seedlings oiPinus and other forest species grew in soil with moisture at the 

 wilting point, hfe could be prolonged for several weeks by spraying the 

 foHage with artificial dew. 



The effect of dew on photosynthesis has not yet been studied in the field 

 but there is convincing laboratory evidence, recently reviewed by Stocker 

 (i960), that when leaves subjected to severe water stress are subsequently 

 wetted, the photosynthetic rate increases rapidly. As an example of 

 recovery from sub-lethal stress, Montford and Hahn (1950) found that when 

 leaves of Caltha palustris were dehydrated to 59% of their normal water 

 content, photosynthesis increased within two hours of wetting to 40% of 

 the value for controls. Synthesis of similar laboratory studies with field 

 measurements of the Waisel type is needed to estabhsh the significance of 

 dew absorption for plant metabohsm. 



On the available evidence it is unhkely that dew is of great physiological 

 importance, either in humid cHmates or in the desert. Where rainfall is 

 marginal, there may be a few species which grow and reproduce more 

 vigorously when their leaves are frequently wet at night. At least three 

 plants of economic importance -cotton, pineapple and melon- are reported 

 to yield well in climates where there is little rain but abundant dew, but a 

 causal relation between dew and growth, exploitable in arid zone agricul- 

 ture, has yet to be estabUshed. 



REFERENCES 



AiTKEN, J.(i885) On dew. Trans. Roy. Soc. Edinb. 33, 134-186. 



Angus, D.E. (1958) Measurements of dew. UNESCO Arid Zone Research,XI,T,oi-303. 



Arvidsson, I. (195 1) Austrocknungs- und Diirreresistenzverhaltnisse einiger Reprasen- 



tanten olandischer Pflanzenvereine nebst Bemerkungen uber Wasserabsorption 



durch oberirdische Organe. Oikos, Suppl. i. 



