Porometer in Stomata l Investigation . 59 
(2) Errors arising from the effects of the passage of the air-stream 
through the leaf are, from their nature, unavoidable, but a few experiments 
have been carried out in reference to them. 
One effect of the air-stream is to replace the moist air in the inter- 
cellular spaces by drier air from without, which will result in an increase of 
transpiration from the leaf. If this increase is sufficiently large, it might 
conceivably result in wilting and stomatal closure, but if the air-stream 
is passing for only a short period — the usual procedure — the effects must be 
practically negligible. 
An attempt was made to determine experimentally any after effect 
resulting from continued passage of the air-stream through a leaf. Two 
or more leaf-chambers were attached to different leaves of the same plant, 
and porometer readings taken at intervals. The leaves were considered 
as mutual controls, the justification for which will appear at a later stage 
in this paper. Except when readings were actually being taken the leaf- 
chambers were in direct connexion with the outer air, so that there was 
no undue pressure upon the leaves and no artificial air movement within 
them. Having by this means determined the direction and slope of the 
curves of stomatal movement of each leaf during a period of two hours 
or so, air was drawn through one leaf at constant pressure for an hour 
or more, the periodic readings being continued for all the leaves. In 
some experiments the air-stream was then transferred to other leaves, 
the first leaf now acting as a control. By this means it was expected that 
any effect the air-current might have upon the stomata would be re- 
flected in the readings during, and after, the period in which the stomata 
were subjected to a continuous stream. 
The plants used were Ficus elastica i Eucharis M as ter si, and Begonia 
(‘President Carnot’), and the results were not the same for the three. 
Ficus and Eucharis were quite unaffected by the air-stream, whereas in 
the case of Begonia the stomata showed a distinct and sometimes con- 
siderable closure as a result of drawing air through, and when the stream 
was stopped there was a tendency for the curve to return to its former 
slope. Figs. 1 and 2 show curves obtained in experiments with Begonia 
and Eucharis respectively . 1 
Thus, although the conditions of the experiment were somewhat 
severe as compared with the ordinary porometer experiment, it is obvious 
that stomata may be affected by the air-stream used in the latter. The 
resulting error will be reduced to a minimum by ensuring that between 
two readings the air-current is stopped, and that, in addition, the time 
occupied in taking a reading is reduced to a minimum. 
1 These and all other curves in this paper were obtained by plotting the reciprocals of the 
time between successive bubbles, not the square roots of the reciprocals, which was the method 
mainly adopted by Darwin and Pertz. 
