138 Dr. F. Darwin and Miss D. F. M. Pertz [June 15, 



The pressure within the chamber C being thus diminished, air is sucked 

 through the stomata into the chamber, and the water column falls to its 



Fig. 1. — Porometer attached to a leaf, see the text for explanations, 

 original level B. By again applying suction, the column is again raised, and 

 the observation can be repeated as often as may be necessary. The time in 

 which the column falls, say from A to B, is recorded. We thus get a series 

 of readings of the rate of flow at the mean 

 pressure \ (A + B). The mean is generally 

 20 cm. of water, the fall of the meniscus 

 being timed either from 23-17 cm., or 22-18 

 or 21-19, as may be most convenient. The 

 tube is commonly of such a bore that 1 cm. 

 in length = 01 e.c. 



Fig. 2 shows a chamber fixed to an 

 enormously exaggerated leaf ; the arrows 

 show the air entering the leaf outside and 

 emerging from the leaf inside the chamber. 

 It is obvious, if successive readings are 

 * IG ' z ' made at a known mean pressure, that a 



diminution in the aperture of the stomata will give a slower fall of the water 

 column (fig. 1) from A to B. And as a matter of fact it is found that such 

 diminution of flow is produced by the well-known causes of stomatal 

 closure, such as darkness or faint illumination, withering, 



