A Self-Recording Porometer and Potometer. 355 
allows of free vertical movement of the base and at the same time 
maintains lateral rigidity. The base is raised or lowered by a cord 
passing over a pulley wheel P. To this base is affixed an electro¬ 
magnet M connected to terminals T, T '. The recording pen R is 
carried at one end of a long arm of sheet aluminium, the other end of 
which is fixed in a slit in the cork C. This cork works on a glass-tube 
bearing G" similar to those in the base BD' and is capable of 
movement horizontally as well as vertically, sliding up and down 
with the base. Also attached to the cork C is a piece of soft iron 
F which functions as an armature to the electro-magnet and is kept 
at a suitable distance from it by a small spiral spring S and a stop H 
(shown in Fig. 2). 
Thus, on passing a current through the electro-magnet, the cork 
revolves and moves forward the recording pen ; when the current 
ceases the pen is drawn back to its original position by the spring 
S. An auxanometer drum, placed so that it is touched by the pen 
when in the forward position, will therefore record the periods 
during which a current passes. 
2. The Modified Porometer (Fig. 3). The chief part of this 
apparatus consists of a glass tube some 30 cm. long and of such 
internal diameter that a mercury index shows no tendency to break 
up (2-3 mm. lias been found satisfactory). At a distance of 10 cm. 
from each end a pair of platinum terminals are sealed through the 
glass (tt and t't'). The wires should project as little as possible on 
the inside of the tube to avoid impeding the movement of the 
mercury index. Towards each end the tube expands to form a 
slight enlargement (b, h '), the purpose of which will be described 
later. Having introduced an index of clean mercury about 2 cm. 
long, the ends of the tube are plugged with cotton wool. One end is 
attached by rubber tubing r to the surface of the leaf l on which 
the experiment is being conducted, either by the simple attachment 
device used by Darwin, or more conveniently by the improved form 
used by Balls. The rubber tubing must be long enough to allow 
the movement of the tube hh' described below. The electrical 
connections are evident from the diagram: the pair of terminals 
t't' are connected to the pair tt, and these to a cell and the recorder. 
The method of working is as follows. Assuming the tube bb‘ 
is in the position shown and the mercury at the end b ', the tube is 
turned through 180°—until it is again vertical but with the 
end b' uppermost. The air in the tube becomes compressed owing 
to the weight of the mercury index and if the stomata are not 
completely closed, the index begins to move steadily down the tube 
