Leaf Water -content, and Transpiration Rate. 223 
(1878) both found that when transpiration from one surface of a leaf is 
stopped, the transpiration rate of the other surface is greatly increased. 
A possible explanation is that by decreasing the transpiration rate of 
a portion of the leaf, the water-content was increased and therefore the 
transpiration rate of the rest of the leaf was increased. 
The present investigation is concerned primarily with the relation of 
stomatal size to transpiration rate. It was undertaken at the suggestion 
of Professor V. H. Blackman, and I desire to express my gratitude to him 
for helpful advice and criticism. 
Methods. 
For the experiments on stomatal aperture the porometer method 
(Darwin and Pertz, 11) was used and a preliminary investigation was carried 
out to determine the precautions necessary when using the method. The 
results of this have already been reported (Knight, 15). In some of the 
following experiments the self-recording apparatus previously described 
(Laidlaw and Knight, 16) has been used ; in others the method of timing 
bubbles with a stop-watch (Knight, 14). The usual practice was to attach 
three leaf chambers to leaves of different ages and to connect all three to 
the same recording apparatus, thus obtaining an average reading (see 
Balls, 3, and Knight, 15, p. 75). Sometimes two chambers were employed, 
and occasionally only one. 
Darwin has pointed out (9, p. 41 5) that transpiration through stomata 
is most likely to be proportional to the square root of the rate of flow of 
the air through the porometer, and accordingly he uses the square root 
of the porometer readings to represent the aperture of the stomata. The 
same course has been followed in the present work. 
The measurements of transpiration were made throughout by means 
of periodical weighings, the periods generally being of 30 minutes duration. 
Concurrently with the records of transpiration rates the evaporating 
power of the air has also been determined by weighings of an atmometer. 
Several forms of this instrument have been used, including the Livingston 
standard earthenware type and a modification of the Piche paper form. 
The latter was found to respond more quickly to temperature changes than 
the earthenware types. 
By comparing the transpiration and evaporation rates it was possible 
to obtain figures which were proportional to relative transpiration ; but the 
figures given in the following tables do not actually represent relative 
transpiration because the rates of transpiration and evaporation were not 
calculated for unit areas, and also because different atmometers were used 
in different experiments. For the same reasons the transpiration and 
evaporation ratios of different experiments are not comparable. 
The importance of controlling the conditions of air movement during 
