1319 
and, on account of the mercury-contacts, works more accurately. As 
a complication however it should be added, that the apparatus must 
be connected with an air-pump by means of a tube. The porometer 
of Lamraw and KNiGutT is in consequence of the necessity of placing 
a bath of a constant temperature around the jar of Mariotte, only 
practicable for laboratory-experiments of no long duration and is 
consequently inferior to the apparatus of Barrs and of myself, in 
spite of the pressure being almost constant. Moreover the forming 
and getting loose of a drop of water seems to me a less accurate 
measuring of time, than the change of level in a manometer. All 
methods mentioned however are to be preferred to that of Jonmrs, in which 
the recording-apparatus forms one inseparable whole with the porometer. 
The advantage of the electric connection between porometer and 
recording-apparatus, rendering an arbitrary distance between the two 
possible, cannot be valued too high. The plant can be examined in 
the most different circumstances, while the recording occurs in a fixed 
place and the apparatus suffers no injury from uncommon temperature 
or moisture. 
The special adventages of the recording-method used have already 
been mentioned: 1°. The fact that the readings can be taken much 
more accurately, also of very short periods, without the unities along 
the time-axis having to be particularly large; 2°. writing a directly 
practicable graphic of the course of the phenomenon to be studied ; 
3°. the fact that it can be used for recording other physiological pheno- 
mena respecting velocities and gives a better survey of those, than has 
been done hitherto. These properties of the recording-apparatus make 
up for the fairly bigh costs of procuring, and make it an apparatus 
that can be regularly used in the laboratory. 
The observations with a recording-apparatus that may be left alone 
for a whole day, require however, that the circumstances in which 
the plant finds itself, are recorded uninterruptedly. Otherwise it cannotbe 
controlled what the occurring phenoma are due to. It would be ideal 
to record temperature, moisture ete. on one recording-drum with the 
vital phenomenon to be examined. Until this has been achieved, we 
must be satisfied with the existing apparatus for this purpose, the 
only inconvenience of which is, that their drums revolving in one 
week are too small for us to read the time accurately even for a 
period of 5 minutes. Through the kindness of Prof. E. van EvERDINGEN, 
Chief-Director of the Royal Dutch Meteorological Institute, | had some 
apparatus in loan during my experiments. At present the Laboratory 
of Plant Physiology possesses a thermograph, a hygrograph and a 
sunshine-autograph. 
