Porometer in Stomatal Investigation. 69 
has been used for a variety of plants, whilst other less simple methods have 
been used in one or two experiments on suitable plants. Of these the 
following method provides clear demonstration of the part which inter- 
cellular spaces may play in retarding the air-stream. 
Several chambers (four and five have been used) are fixed in a straight 
line on one leaf, preferably of uniform nature. Small chambers or a large 
leaf must be employed — Encharis Master si and E. amazonica have been 
found to answer well with chambers of 5 mm. internal diameter. The leaf 
is now smeared with vaseline over the whole stoma-bearing surface, except 
for a small area, 1 sq. cm. or so, just beyond the end chamber of the series and 
in line with them. This area serves for entrance to the leaf of the air- 
stream, which is drawn out through the chamber at the other end of the line 
by a constant-pressure aspirator in the usual manner. Each of the inter- 
mediate chambers is connected to a glass tube dipping into water and 
serving as a manometer. 
A stream of air entering the leaf at F and leaving it at A (see Expt. 52, 
p. 70) will encounter the resistance of the stomatal pores at those points, and 
also that of the tissue between them. If this latter resistance is not appreci- 
able, the passage of the air from F to A within the leaf will be easy, and the 
air pressures in the intercellular spaces at F and A will not be widely different. 
If, on the contrary, the resistance offered by the tissues is considerable, 
there will be a pressure-gradient in the leaf along the line FA, which will be 
indicated by the manometers B, c, D, E. In practice it is found that the 
movements of the water-columns in the manometers are slow, owing to the 
pressure having to be transmitted through the stomata, so that air is drawn 
through the leaf continuously for some hours and the manometers read at 
intervals. A rough estimate of stomatal changes is deduced from the rate 
of the air-stream. 
In all experiments in which extensive blocking of the stomata was 
resorted to — generally with species of Eucharis — it has been found expe- 
dient to cut the leaf from the plant and immerse the cut end of the petiole 
in water, on account of the tendency of air to pass through an attached 
petiole from other parts of the plant, when air is drawn from the leaf into the 
leaf-chamber. This can be demonstrated by fixing a chamber to a leaf and 
vaselining the whole of the rest of the surface, when it will still be found 
possible to draw air into the chamber. If the petiole is now severed and 
the end immersed in water, the air-stream stops. On cutting the petiole 
again above the water, air can be again drawn through, showing that the 
previous cessation of the stream was not due to stomatal closure. 
There is little possibility of the experiment being affected by severing 
the leaf, as is shown by the fact that three days after a leaf was detached and 
its petiole immersed, the stomata were still responding to the daily changes 
of illumination. 
