75 
Porometer in Stomatal Investigation. 
distribution of stomata, and in the experiments upon leaves of different 
ages, when the distribution is allowed for, there is still the same relationship 
between the mean readings. 
Therefore, whilst readings from one chamber are adequate indication of 
stomatal behaviour in a single leaf, yet when the whole plant is considered 
it is advisable to fix chambers to more than one leaf, as suggested by Balls. 
Summary of Results. 
The conditions of the porometer experiment involve the possibility 
of a number of errors which by means of suitable precautions can to a large 
extent be eliminated. 
i. Temporary deformation of the leaf is liable to occur owing to 
the pressure difference employed to draw air through. Such deformation 
may cause undesirable changes in the stomatal pores. Any such effect 
is reduced to a minimum by using small pressure differences, and if a con- 
stant pressure is maintained the effect will be constant. 
3. Some stomata show a tendency to close when air is drawn con- 
tinuously through them. To avoid this, the air-current should be stopped 
when readings are not being taken, by placing the leaf-chamber in direct 
connexion with the outer air. 
3. The stomata of some leaves are sensitive to shock, the handling 
involved in fixing chambers to the leaf causing the stomata to close 
almost completely, but recovery is fairly rapid, and two hours has been 
found sufficient. The mere tapping or shaking of some leaves may 
induce a closure of the stomata. It is therefore advisable that after the 
leaf-chamber is fixed, readings be not taken for two hours, and that the leaf 
be disturbed as little as possible. 
4. The resistance offered by the intercellular spaces to the passage 
of air through a leaf is considerable, and may have a marked effect upon the 
_ porometer readings. In leaves other than amphistomatous ones, it is there- 
fore advisable to reduce as far as possible the length of tissue to be traversed 
by the air-stream, by using chambers of suitable construction. 
5. In the plants investigated, stomata on different parts of the same leaf 
behave similarly under approximately similar conditions, and are open 
to about the same extent at the same time. Thus readings from one 
chamber on a leaf are sufficient indication of the stomatal behaviour of that 
leaf. 
6. Stomata on different leaves in general behave similarly, but the 
agreement is not so close as between stomata on the same leaf. Stomata 
of a mature healthy leaf may open more widely than those of either a very 
young or very old leaf. Thus, when using a plant with several leaves, in order 
to obtain a comprehensive measure of the behaviour of its stomata, chambers 
