104 VEGETABLE PHYSIOLOGY 
its position can be removed by causing it to collect at the 
upper portion of the straight tube of the potometer. To 
take an observation of the rate of transpiration of the 
branch, a bubble of air must be admitted into the capil- 
lary tube by momentarily removing the vessel into which 
it dips, and replacing it ag soon as the transpiration has 
caused the air to enter. The bubble of air must be of 
uniform size in successive readings, to ensure that the latter 
shall be strictly comparable with each other. The bubble 
will rise in the tube, and finally make its way to the upper 
part of the straight limb of the instrument, the rate 
at which it travels serving as an index of the rate of the 
transpiration. The -capillary tube should be marked 
by a transverse line a few millimetres from its lower end, 
and by means of a stop-watch the time taken by the 
bubble to rise from this mark to the free end of the tube 
should be observed. The branch may be covered by a 
bell-jar, so that the variations of temperature, moisture, 
&c., of the air surrounding it can be controlled during a 
series of observations. Less accurate observations can be 
made by substituting for the capillary tube a tube of wider 
bore bent at right angles a little below the orifice of the 
potometer, and affixing to it a scale by means of which 
the rate of passage of the column of water in the tube can 
be observed (fig. 68). 
According to the variations in the external conditions of 
the plant, including all the features already alluded to, the 
amount of watery vapour transpired is continually changing. 
The most favourable conditions being afforded in summer, 
it is not to be wondered at that transpiration attains an 
annual maximum during that season. It does not, however, 
entirely cease during the winter, though it is reduced to a 
minimum, especially in the case of such trees as shed their 
leaves in the autumn. 
Apart from such changes in the external conditions, 
transpiration appears to show no independent periodicity, 
differing in this respect conspicuously from root-pressure. 
