210 THE BIOLOGY OF FLOWERING PLANTS 



might demonstrate whether Hmiting effects on growth rate 

 are produced at minimal transpiration values. Until we 

 have such evidence discussion of this function is rather idle. 

 It is, however, difficult to see by what means salts in sufficient 

 quantity could possibly be transported to the leaves and 

 growing points of, say, a beech tree in the absence of the 

 transpiration stream. The evidence available is, however, 

 definitely unfavourable to the view that lowering of tran- 

 spiration, even by a very considerable amount, has an 

 adverse effect on the supply of salts ; Stahl's interpretation 

 of sleep movements and of various structural features of 

 rain-forest leaves must therefore be abandoned. 



Cooling. — The second suggested function of transpiration 

 concerns the process directly and assumes that it is essential 

 in lowering leaf temperature in the sun. We have abundant 

 evidence that leaf temperature in the sun may rise very 

 high. In Blackman and Matthaei's experiments (1905) a 

 leaf of the cherry laurel, placed in direct sunlight at noon 

 in July, showed a temperature of 39*8° C. when vertical, and 

 of 44*6° C. when at right angles to the sun's rays. The air 

 temperature in the sun was 30* 5° C, so that there was a 

 minimum excess of 9° C. and a maximum of 14° C. The 

 temperatures reached were such as to have a rapid and pro- 

 gressive retarding effect on photosynthesis and respiration, 

 and to produce intense transpiration. In a leaf enclosed in 

 a glass chamber the temperature rose to 51° C, an excess of 

 20° C, and the leaf quickly showed visible signs of damage 

 in the appearance and spread of brown spots of killed tissue. 

 Blackman remarks, '' This heating up of the leaf will, no 

 doubt, partly be due to checking of transpiration in the en- 

 closed space, but the effect comes on so quickly that it must 

 chiefly be due to " the greenhouse effect," the imprisonment 

 of the reflected dark heat-rays by the glass plate which is 

 almost impervious to them." The temperature of a leaf in 

 the sun must of course be lowered by transpiration, but 

 such experiments do not tell us to what extent. Neither 

 do comparisons of the temperatures attained by insolated 

 leaves of succulents (supposed to transpire relatively little) 



