1 66 THE BIOLOGY OF FLOWERING PLANTS 



given the graph representing transpiration of a Euphorbia 

 during three days, and the graph of evaporation from an 

 atmometer, as determined by Livingston (1906). It will be 

 seen that there is a general resemblance between the two ; 

 the changing conditions of the passing day have a similar 

 effect on both. Looking closer, however, we see that there 

 is a very important difference. The highest rate of tran- 

 spiration occurs each day betvv'een 10 a.m. and 12 noon, and 

 transpiration then decreases steadily. Evaporation, on the 

 other hand, increases till between 2 and 4 p.m., where the 

 maximum occurs. This means that external conditions 

 are such as to increase evaporation till late in the afternoon,, 

 but that some hours earher a change in the reaction of the 

 plant has led to a diminution of water loss. The same 

 thing comes out in the graph of relative transpiration which 

 is also given. If transpiration and evaporation were 

 affected by external change in the same way and to the same 

 extent, this graph would be a straight line. Actually, it 

 shows that transpiration is increasing more rapidly than 

 evaporation till the maximum is reached, and that subse- 

 quently the increase in evaporating power of the air is not 

 reflected in the transpiration. 



The fact demonstrated by such an experiment is that 

 the conditions of the atmosphere have an important effect 

 in influencing transpiration, but that, at the same time, the 

 condition of the leaf modifies the transpiration. We must 

 now consider in what way this modification is effected by the 

 leaf. 



§ 21. Stomatal Regulation 



It was long thought that regulation depended on altera- 

 tion in the aperture of the stoma, the mechanism of which 

 we have already studied ; but until recently no exact 

 experimental data were available, because no proper method 

 of measuring stomatal aperture, frequently and accurately, 

 was available. We have now two suitable methods. The 

 first is based on the fact that if a piece of epiderm is removed 



