DEW: FACTS AND FALLACIES 53 



humidity of the soil atmosphere has led some workers to study the con- 

 densation on artificial surfaces buried within the soil. Such observations are 

 irrelevant to the water economy of plant roots. 



7. DEW AND PLANT GROWTH 



In purely physical aspects of dew formation, it is relatively simple to 

 distinguish fact and fallacy. Stone's (1957a) review showed that the 

 biological picture is less clear because there is so httle relevant evidence 

 from the field. Dew is known to promote the spread of plant disease by 

 providing a suitable environment for plant pathogens (Duvdevani, 1946) ; 

 but when there is no potential disease, many farmers and gardeners beheve 

 that dew stimulates the healthy growth of plants in dry weather. Duvdevani 

 (1957) claimed that the yield of cucumbers exposed to dew was 50% 

 greater than from undewed controls, but experimental details have yet to 

 be published. 



In countries where little rain falls during the growing season, there are 

 two ways in which dew may contribute to plant water economy (Slatyer, 

 i960) : the evaporation of dew from leaves may reduce transpiration in the 

 early morning; and foliar absorption during the night may reduce local 

 water deficits and accelerate growth. For purely physical reasons, the 

 decrease in transpiration after dew is unlikely to benefit mesophytic plants 

 because in cloudless summer weather the ratio of potential condensation to 

 potential evaporation is (very roughly) i : 7 in a humid chmate and i : 14 in 

 an arid climate. Dewfall evaporates within a few hours of sunrise and plants 

 with inadequate soil moisture reserves endure severe water stress for the 

 hottest part of the day. The hfe of a plant not adapted to such stress may be 

 prolonged for a few days wliile dew nights are frequent, but death is 

 inevitable in the continued absence of rain. For desert plants, dewfaU which 

 cannot be absorbed is a luxury : none of the mechanisms by which xero- 

 phytes conserve moisture require the presence of liquid water on the 

 external organs. 



Data for dew absorption (Lehmann and Schanderl (1942); Arvidsson 

 (195 1); Waisel (1958) and others) show wide differences between species, 

 attributable to differences in cuticular permeabihty and in the free energy 

 deficit of leaf cells. Distinguishing carefuUy between the nocturnal uptake 

 of moisture from the soil and from the atmosphere, Waisel found that 

 average saturation deficits* o£Pinus halepensis and Oka enropaea were 



* Now using the term in its physiological sense: loox (saturated leaf weight — 

 actual leaf weight)/saturated leaf weight. 



