82 THE BIOLOGY OF FLOWERING PLANTS 



temperature in the sun. The consequence, then, of the 

 free exposure of a large surface, which must be an evapo- 

 rating surface because it is permeable to carbon dioxide 

 and oxygen, is that normal foliage gives off very large 

 quantities of water vapour — it transpires ; in fact, as we have 

 seen, water lost in transpiration makes up the larger part of 

 the water taken in by the roots. 



Photosynthesis and transpiration take place in the same 

 organ. Essential processes of each are subject to the laws 

 of gaseous diffusion. They go on concurrently ; yet they 

 may be interfering processes, since excessive transpiration 

 may hmit assimilation, both directly by diminishing turgor, 

 and because of the peculiarities of structure shown by 

 plants in arid stations. The two processes are linked to- 

 gether, and for that reason, although their roles are utterly 

 different, we may best treat them together. 



The function of photosynthesis is well understood, 

 but the same cannot be said of transpiration. Two roles 

 have been assigned to it : (i) It undoubtedly tends to reduce 

 the temperature of a leaf exposed to the sun. The Italian 

 peasant puts his drinking-water in a porous earthenware 

 jar in the sun, if he wishes it cool ; the rapid evaporation, 

 using up energy in converting liquid water into vapour, 

 lowers the temperature of the water. The same thing 

 happens with the leaf. This may be important in plants 

 exposed to strong solar radiation, when temperatures so 

 high as to be dangerous may be reached. 



(2) Transpiration entails a constant water current from 

 root to leaf through the wood of the plant ; this is called 

 the transpiration current. It has been supposed that this 

 current is responsible for maintaining a sufficiently rapid 

 supply of the mineral salts, absorbed in small quantities by 

 the roots. It is difficult to get exact evidence for this view, 

 and some of the evidence available is not favourable, as we 

 shall see later. Some American investigators, e.g. Barnes 

 (1910), beheve that transpiration is an unavoidable evil, 

 without benefit to the plant. Yet it is difficult to imagine 

 an adequate supply of salts being delivered to the leaves of 



