A Self-Recording Porometer and Potometer. 359 
of drum d), but first in one direction and then in the reverse hack 
to its original position. The axis of the drum d is conveniently 
constructed of a retort clip by which the tube bb' of Fig. 3 is held. 
Lastly, the axis h of the klinostat (which should be 2 mm. to 3 mm. 
in diameter) has wound round it a cord which, running over the 
pulley wheel P (Fig. 1) supports the base BB' of the recorder. 
Every hour, therefore, as the axis h of the klinostat makes a 
half revolution, the base BB' of the recorder is lowered about # 5 
cm. If the recording drum is revolving once an hour, this will 
mean that each record of the porometer will begin on a fresh line 
*5 cm. or so below the previous record. With the usual sized drum, 
continuous records for some thirty hours may be taken before it 
becomes necessary to renew the record-paper on the drum. 
The general arrangement of the component parts of the 
apparatus is illustrated diagrammatically in Fig. 6. 
II.—A Self-Recording Potometer (Fig. 7). 
Rate of water-loss, as distinct from transpiration proper, 1 is 
usually measured (1) directly, by weighing a plant at intervals, (2) 
indirectly, by means of the potometer. A self-recording instrument 
of the former type has been described recently. 3 The potometer is 
specially convenient when the loss of water that has to he measured 
1 Darwin, F. “ On a Method of Studying Transpiration.” Proc. Roy. 
Soc., Vol. 87 b, 1914, p. 209. 
2 Blackman, V. H., and Paine, S. G. 
Ann. of Bot., Vol. 28, 1914, p. 109. 
“ A Recording Transpirometer.” 
