1 68 THE BIOLOGY OF FLOWERING PLANTS 



full open at 8 a.m.; they commence to close about 12.30 p.m., 

 and are completely closed at 6.30 in the evening. Fig. 16 

 shows graphs for the two processes, and at first sight there 

 is a striking similarity, indicating a fairly exact corre- 

 spondence. When we compare them in detail, however, 

 we find (i) that after the stomata are full open at 8 a.m., the 

 transpiration rate increases for two hours ; (2) that the 



6 

 AM 



12 

 N 



6 

 RM. 



12 

 M 



Fig. 16. — Transpiration and stomatal movement; the continuous 

 line represents transpiration of a shoot of Verbena ciliata, the broken 

 line represents extent of opening of stomata in microns. (After Lloyd, 

 modified.) 



transpiration rate begins to decrease a full hour before 

 the stomata begin to close, and has already fallen to 70 per 

 cent, of its maximal value when closure begins ; (3) that the 

 check in the fall of the transpiration rate does not corre- 

 spond to any check in the closure of the stomata ; (4) that 

 the transpiration rate continues to fall after the stomata 

 cease to close, and later starts to increase with the stomata 



