1311 
but in the illustration (period 1) is just kept closed by a fork. Is the 
switch changed over at the end of period 1, VI is interrupted and 
the frame 21 is pushed upward by the elasticity of the rubbertube 
itself so that 19 is opened and the pressure inside and outside the 
porometer becomes equal. When the period of rest is finished, 19 
must be closed, otherwise the pump cannot reduce the pressure. In 
order to attain this end, circuit VI has also been conducted past C,, so 
that 19 is closed off, as soon as 16 dips into the mercury. As T, 
(thus 16) is not released by T, before the switch has taken the 
position illustrated in fig. 1, consequently the current VI is also 
closed in C,, VI keeps going and the rubber tube 19 remains 
closed, also during the transition from period 2 to 1. 
The arrangement is such, that circuit III is entirely closed, when 
both in C, and in C, contact has been made, whereas the current 
already goes through VI, when in either of the contacts C, of C, the 
fork dips into the mercury. 
It had appeared in practice, that during the period of rest, in conse- 
quence of the transpiration of the leaf, the air in the glass-chamber was 
saturated with water-vapour, the result of which was, that the condition 
of the glue-rim did not remain trustworthy. Therefore it was necessary 
to renew the air during the rest and this could be attained, by giving 
the waste rubber-tube 4, with an open end at 7, (where during 
period 1 and the rest-period air is sucked in by the pump from 
outside,) a side-limb (17) passing clamp K,, next past 18 and ending 
at the bottom of the glass-chamber. If K, is open, a current of air 
enters at 19, and passes through 2, 1, 18, 17, 4 to the pump. 
Since 7 always remains open, the current of air through the chamber 
is not too strong. When the period of rest is over, 17 in K, is closed 
by the clamp at the same time as 19 and the circulation stops. 
With the apparatus thus modified 3 periods are to be observed: 
Period 1. Porometer-space closed. The air enters through stomata. 
Period of rest. Porometer-space connected both with atmosphere 
and pump. The air circulates. 
Period 2. Porometer-space only connected with pump. The air 
is rarified. 
Fig. 3 is a photograph of a diagram, showing the periods of rest 
between the observations. The laboratory possesses two recording- 
porometers, writing on one drum, enabling us to examine two plants 
simultaneously. 
In this case Ficus elastica and Peperomia maculosa were treated. 
It is striking, that in the case of Ficus, since the introduction of 
