222 Knight. — The Interrelations of Stomatal Aperture , 
Bakke (1), Trelease and Livingston (22), and Bakke and Livingston (2), 
using the hygrometric paper method. In addition to this ‘ early maximum * 
of transpiring power, a secondary maximum has also been found by Bakke 
(1), Shreve (21), and by Bakke and Livingston (2) in various plants. 
The regulating mechanism concerned in this retardation was not, in 
Livingston’s opinion, stomatal in nature because of its early appearance 
before any stomatal closure was to be expected. Trelease arid Livingston 
have since demonstrated the correctness of this view by actual porometer 
experiments, which showed that the maximum stomatal aperture was 
reached some three hours later than the maximum of transpiring power. 
A suggested explanation of the checking of the transpiration rate was 
forthcoming as the result of an investigation by Livingston and Brown in 
1912 (18). They found that the water-content of transpiring leaves was 
not constant throughout the day, but that it tended to decrease during the 
morning hours and reached a minimum at about the same time as the 
evaporating power of the air reached a maximum, i. e. some time after 
the maximum of transpiring power. The suggestion offered was that as 
the evaporating power of the air increases a corresponding increase in 
transpiration results, which reduces the water-content of the leaf. The 
decrease in water-content, in turn, tends to lower the transpiration rate, by 
the ‘incipient drying’ of the walls of the transpiring cells, in a manner 
similar to that in which a Piche paper atmometer tends to dry out as the 
result of excessive evaporation. 
Shreve (21) has confirmed the work of Livingston and Brown and 
agrees with their interpretation of the experiments. 
Briggs and Shantz (5) in 1916 concluded, as the result of an extensive 
series of experiments upon the influence of environmental factor on 
transpiration, that the transpiration coefficient of the plants investigated 
changes during the day, and they ascribed the change to stomatal control 
or to lack of water. 
Iljin (13) found that the osmotic pressure of the guard cells was much 
greater than that of the other epidermal cells when the stomata were open, 
but when the stomata were closed the osmotic pressures of all the epidermal 
cells were the same. He also found that the graphs of transpiration rate 
and of the osmotic pressure of the guard cells were parallel, and suggested 
that excessive transpiration reduced the osmotic pressure of the guard cells 
thus producing stomatal closure and inhibition of transpiration. He found, 
however, that the rate at which the stomata closed was not dependent upon 
the rate of removal of water from the plant. Also he frequently found 
open stomata in wilted plants. 
It may be observed here that two investigators quoted by Lloyd 
(19, pp. 60, 61) have made observations which supply evidence supporting 
the theory of the action of incipient drying. Unger (1862) and Corneo 
