Porometer in Stomata l Investigation. 63 
Therefore to avoid errors from this source it is advisable to touch 
or shake the leaf as little as possible after affixing the leaf-chamber. It 
was with this end in view that a three-way stopcock was inserted between 
the leaf-chamber and the aspirator bottle in the apparatus used for the 
porometer readings (6) ; for by this means the chamber can be connected 
with the outer air with the minimum risk of shaking. 
It is obvious that Lloyd’s alcohol method of measuring stomata is not 
applicable to plants with sensitive stomata, since the manipulation required 
by his technique would be sufficient to cause change of aperture. 
Effect of Intercellular Spaces. 
The air drawn through the leaf in a porometer experiment must 
enter the leaf through stomata situated outside the leaf-chamber, must 
pass through a length of intercellular spaces, and finally leave the leaf 
through the stomata covered by the chamber. The rate at which this 
air moves is determined by the pressure difference, and by the resistance 
encountered, part of which is due to the stomata and part to the intercellular 
spaces. 
Any change in the size of the stomatal pores will be reflected in 
the resistance they offer to the air-stream, and therefore in the speed 
of the air, if the size of the intercellular spaces remains constant. In inter- 
preting the rate of air-flow as an indication of stomatal aperture, this 
constancy is necessarily assumed, but it would seem probable that when 
a leaf wilts, for instance, the intercellular spaces will be appreciably smaller 
than when the tissue is turgid. But in a normal daily cycle of changes, the 
variation is likely to be small, and the influence the spaces have upon the 
readings will depend upon the relative resistance to air-flow of stomata and 
spaces respectively. If the resistance is mainly due to the stomata, changes 
in the intercellular spaces will have but little effect upon the readings, but if 
a considerable proportion of the resistance is due to the spaces, then their 
changes will be reflected in the readings, and, in addition, changes in the size 
of the stomata will be appreciably masked by the part played by the spaces, 
and the sensitiveness of the method will be impaired thereby. 
The following series of experiments constitutes an attempt to estimate 
the relative importance of the parts played by the stomata and intercellular 
spaces in resisting the air-flow. From the nature of the problem, an 
accurate estimate of the relationship is almost impossible, especially as it is 
probably different for every plant and is continually changing, so that 
no quantitative generalization has been made. 
Four methods have been employed : 
j. Two chambers were fixed to the same leaf some distance apart, and 
periodical readings taken from each, preferably till a fairly constant value 
was reached in both. Vaseline was then smeared on that portion of the 
