7i 
Porometer in Stomatal Investigation. 
E was connected to the aspirator with a pressure difference of 15 cm. 1 
Readings were taken with each of the chambers open in turn, but those 
from A were discarded, as indicated above. Only one set of readings 
is given here, which are typical of those throughout the experiment. 
Table II. 
Path of air-current. 
D to E 
C to E 
B to E 
Length within leaf. 
1*9 cm. 
4-2 „ 
6-4 „ 
Time between bubbles. 
12-9 sec. 
15*6 „ 
i8'7 » 
A variety of methods having been used to demonstrate the resistance of 
the intercellular spaces, some consideration of the effect of this resistance 
upon porometer readings is necessary. 
In fixing a porometer chamber to a leaf a large number of stomata are 
of necessity blocked by the adhesive, and thus in a hypostomatous leaf the 
stomata by which the air enters are some distance from those by which 
it passes from the leaf into the chamber, and an extra resistance is therefore 
encountered. In an amphistomatous leaf the blocking of the stomata is of 
less account, as the air may pass from one surface to another, although, 
as has been shown (p. 68, Dracaena\ this direct path is not always traversed. 
For accurate work with hypostomatous leaves it is thus important to 
reduce the length of path within the leaf by blocking as few stomata as 
possible. 
In their original paper on the porometer (loc. cit.), Darwin and Pertz 
described the leaf-chamber used by them as having a flange at the mouth to 
facilitate its attachment to the leaf. Balls (7), in his work on the Stomato- 
graph, advocates a form of chamber which also involves blocking the 
stomata over a very considerable area. These forms may be suitable for 
amphistomatous leaves, but if used for others must introduce much inter- 
cellular space resistance, and tend to mask stomatal changes, if not actually 
to introduce errors. 
The form of chamber which has been generally found efficient is a piece 
of glass tubing of the required bore, tapering at one end to take the rubber 
connexion, and with the other end cut off square and ground flat, leaving to 
be attached to the leaf a surface equal in width to the thickness of the walls 
of the tubing, i. e. 1 to 1 mm. 
Darwin and Pertz, after experimenting with many adhesives, finally 
decided in favour of glue, whilst Balls has recommended paraffin. In 
the present work glue has been found quite satisfactory, but the consistency 
needs to be carefully adjusted, depending upon the plant used and the 
1 In practice, readings were also taken using A, B, c, and d respectively as the exit chamber, 
and only after the experiment, when measurements of areas had been made, was it possible to determine 
which set of results was significant. 
