i64 THE BIOLOGY OF FLOWERING PLANTS 



temperature, fall of atmospheric humidity, insolation and 

 wind increase the drought which dries the washing, so they 

 increase the transpiration from the leaf. The leaf is not, 

 however, as simple a thing as a towel hung out to dry ; 

 these factors have not only a direct effect on its transpiration, 

 they may also influence it indirectly by affecting the condition 

 of the leaf. The leaf reacts in various ways to changes of 

 illumination ; it behaves differently when turgid and when 

 flaccid. Changes in transpiration rate due to such altera- 

 tions may be looked on as indirect effects of the external 

 conditions, or as a process of regulation by the leaf. The 

 atmospheric conditions may have a further indirect effect 

 on transpiration from their influence on soil moisture, and 

 thus on water supply. 



Methods of Investigation. — In investigating the effect 

 of external factors on transpiration, two methods may be 

 employed. Records may be made of temperature, humidity, 

 wind, etc., and plotted along with the graph representing 

 transpiration so that their relative effects are exhibited. It 

 is, however, more convenient to use for comparison a record 

 of the evaporation from a standard water surface. This 

 sums up the influence of the various atmospheric conditions, 

 and enables us to see any differences between the effects of 

 these on transpiration and on evaporation. The evaporation 

 record may be supplemented by suitable data for par- 

 ticular factors. The water surface generally employed is 

 the saturated surface of a porous porcelain filter candle. 

 This is filled with water, and a glass tube, passed through 

 the cork which closes it, dips in a bottle of water acting as a 

 reservoir. The evaporation may be determined by weighing 

 or, with a suitably graduated bottle, by direct reading. 

 Such an instrument, devised by Livingston, who has also 

 worked out methods for its standardisation, is called an 

 atmometer. 



If the amount of transpiration is divided by the amount 

 of evaporation, both being referred to unit area, a value is 

 obtained which is called relative transpiration. In it the 

 direct evaporating effect of the atmosphere is supposed to 



