TRANSPIRATION AND THE ASCENT 

 OF SAP IN PLANTS. 



CHAPTER I 



THE NATURE OF TRANSPIRATION 



Transpiration and evaporation. Comparison of 

 the transpiration from the upper side and of that from 

 the under side of hypostomatous leaves has shown that 

 transpiration is principally effected through the stomata. 

 The total area of the stomata, however, bears but a very 

 small proportion to that of the whole leaf. For example, 

 in a leaf of Catalpa bignonioides it is but 0*9 per cent, of 

 the total leaf surface. In a leaf of Helianthus annuus 

 the area of the stomata appears to be about 3 per cent, 

 of the whole leaf surface. 



Many years ago, Unger showed that under similar con- 

 ditions the amount of water evaporated from a free surface 

 was 2 - 8 to 13*8 times that transpired from an equal leaf 

 surface. If we take the lower limit, this means that 

 35 sq. cm. of water surface will give off as much water 

 vapour as 100 sq. cm. of leaf surface under similar con- 

 ditions. Of this 100 sq. cm. as much as 3 sq. cm. may 

 be represented as stomata. So we arrive at the surprising 

 result that the combined area of the stomata is at least 

 ten times as efficient in giving ofT water vapour as a free 

 water surface of equal area. 



B 



