ii DEVELOPMENT AND ENVIRONMENT 41 



chamber, within which the pressure is higher or lower 

 than that of the atmosphere. The air-flow, restoring 

 equilibrium of pressure, ca*n thus take place only through 

 the stomata and mesophyll tissues, while the square root 

 of the velocity of this flow is approximately directly 

 proportional to the stomatal aperture. 



The author's Stomatograph 31 consists of a sensitive 

 electric air-pump, which maintains a constant pressure 

 gradient between the chamber and the outside air, 

 signalling the time of completion of every stroke and 

 hence the stomatal aperture on a chronograph drum. 

 The pump, with its operating battery and a relay, is 

 contained in a small dust-proof box, placed below the 

 plant under examination. From this box issues 

 the tube leading to the leaf-chamber, and also a 

 telegraph wire which leads from the relay to the chron- 

 ograph ; the latter may be at any convenient distance, 

 preferably in the field laboratory. The instrument is 

 independent of any meteorological change or disturbance. 



In the usual simple experiments, we find that 

 desiccation, darkness, and poisons cause closure, while 

 sunlight causes the stomata to open to their maximum 

 aperture. 



The records obtained from normal leaves in field 

 conditions, which in one case extended over five con- 

 secutive days with the same leaf (Fig. 39), reveal a most 

 fascinating interplay of factors. During the night the 

 stomata are completely closed ; they begin to open 

 slowly at dawn, and continue to do so until a moderate 

 aperture has been attained, even in diffuse light. When 

 direct sunlight strikes the leaf, the aperture is rapidly 

 increased, till maximum is reached at about 9 a.m. 

 This maximum aperture is maintained for a varying 

 period, the length of which is undoubtedly determined by 

 the water- absorption of the root, provided that sunshine be 

 continuous. Immediately after irrigation it may last two 



