1310 
a slow closing set in, so that at about 5 o’clock the condition was 
almost equal to that of 24 hours ago. 
At first sight the results obtained in this way, seem satisfactory. 
Undoubtedly the general course of the aperture of the stomata may 
be read from the diagram. Yet the apparatus discussed above meets 
with the same objection as the other recording porometers: the 
uninterrupted flow of air through the leaf. This objection can be 
removed, by introducing between every two observations a period 
of rest, during which the pressure in the porometer is equal to that 
of the atmosphere. For this purpose the following arrangement has 
been added to the apparatus. (fig. 1. upper half). The circuit IIT, 
enabling the electro-magnet to connect the porometer with the pump, 
has not only been interrupted in contact C, (at the automatic switch), 
but moreover at C, (mercury-cups in ebonite block, fixed to the 
clockwork U,). Even if at the end of period 1 the connection in 
C, is brought about, yet the current will not pass through the 
magnet before C, has likewise been closed. This may be attained 
by a fork (16), fixed to the cog-wheel T,. In the position as illus- 
trated (during period 1), this cog-wheel is free from the clockwork 
and is kept in this position by a weight, that keeps the fork 16 
at a special distance from C,. At the end of period 1 T, goes down 
and its teeth catch into those of T,. Now this latter turns slowly 
back and after some time 16 reaches C,, the circuit III is 
entirely closed and the reduction of the pressure in the porometer 
may begin. As the rod, to which 16 has been fixed, insulated, can 
be turned round the shaft of T, with friction, the angle through 
which it has to be moved by the clockwork, may be taken arbi- 
trarily and in this way the duration of the period of rest can be 
determined. 
Our purpose, viz. to have atmospheric pressure in the porometer 
during the period of rest has not yet been attained, for at the beginning 
of the period of rest there is still a difference in pressure and it would 
take long to adjust this quite. 
The porometer-space therefore should be brought into direct connection 
with the atmosphere and this happens by means of the rubber tube 19 
(Fig. 1. bottom half), attached to the side-tube 2 of the glass chamber 
and which is not — as is the case with VAN SLOGTEREN’s porometer — 
closed off with a common clamp, but runs through an electromagnetic 
clamp. Tube 19 had been closed off between the frame 20 and 
the fixed block 21, during period 1, because the armature 22 was 
attracted by the electro-magnet K,. Circuit VI serves this purpose, 
which can be interrupted at the mercury-contact C, of the switch, 
