io8 THE BIOLOGY OF FLOWERING PLANTS 



stomata show a temporarily increased opening, thus confirm- 

 ing earlier work by F. Darwin (1898). Closure takes place 

 only when wilting is very pronounced (see Fig 12). This 

 temporary opening probably takes place when the epidermal 

 cells, with their lower osmotic pressure, have lost so much 

 water as to become flaccid ; the still turgid guard cells have 



10 

 AM 



Fig. 12. — Water loss and stomatal aperture; the heavy line shows 

 the net water loss, the light line the per cent, opening of the stomata, 

 during wilting. (After Knight, modified.) 



now a smaller resistance acting against them and so bend 

 further apart. Delf (1912) found a very rapid closure to 

 take place within 7-15 minutes after leaves of succulents 

 were detached from the stem. Even complete wilting does 

 not always lead to stomatal closure. Marsh herbs and 

 shrubs, such as the water plantain, brooklime, willow, and 

 alder, were found by Stahl (1894) to have open stomata in 



