1318 
X. General remarks. The idea of using a tracing-pencil periodi- 
cally moving backwards | got from the selfrecording anemometers 
used in meteorology. There the distance travelled is automatically 
differentiated as to time by the backward movement of the stiletto 
every 5 minutes, so the velocity is recorded. In the case of my 
apparatus the differentiation does not take place as usual according 
to time, but the velocity is taken as “the time needed to travel a 
certain distance”. 
In this connection I want to point out, that the recording appa- 
ratus by itself, therefore apart from the porometer, may be used 
for all kinds of other physiological inquiries, in which velocities have 
to be recorded. I hope soon to provide the ‘“arc-indicator of growth” 
with an apparatus, enabling us to record the growth in the way 
mentioned above. Then the alterations in rate of growth, which 
are really wanted, may be directly read, while with the usual 
method, they must be derived from the inclination of the curve. 
It may not be undesirable to consider the advantages of recording- 
instruments in general, apart from the special method followed here. 
In the first place we have the uninterrupted observation, next a 
great saving of time, for, when the construction is sufficiently sure, 
an apparatus as the one under discussion can work for 24 hours, 
without our having to look at it. Finally the accuracy with which 
everything can be regulated. For instance it is impossible for an 
experimentator to observe in the way followed here: an observation, 
during which he must patiently wait, then waiting for exactly 20 
minutes, another observation, another 20 minutes waiting — that 
is slow work and not very encouraging which cannot be kept up 
very long. Moreover with slow movement of the meniscus it cannot 
be precisely stated, when the exact point is reached. The mercury- 
contact tells us this with minute accuracy. Indeed working with 
such slight differences in pressure (i.e. in circumstances so slightly 
unfavourable for the plant) as is possible here, is almost impractic- 
able for an ordinary observer. 
Let us finally consider what advantages the apparatus discussed 
here offers above the existing recording porometers and we find as 
general advantage: the possibility of introducing periods of rest 
with renewal of air between the observations. 
Let us next consider separately the real porometer and the 
recording apparatus. The porometer itself may be built almost equally 
compactly as Baus’ stomatograph, and is therefore equally fit for 
use in hothouses or outside among the plants. The mechanism, which 
has to be in the immediate vicinity of the plant however is simpler 
