TRANSPIRATION 99 
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. 63). - 
According to the varia- 
tions in the external condi- 
tions of the plant, including 
all the features already 
alluded to, the amount of 
watery vapour transpired 
is continually changing. 
The most favourable con- 
ditions being afforded in 
summer, it is not to be 
wondered at that  tran- 
spiration attains an annual 
maximum during that sea- 
son. 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 pe- 
riodicity, differing in this Fro. 69,—Apranarus To snow THE 
respect conspicuously from Now Uafior beimes) 
root-pressure. It is, how- 
ever, very sensitive to only slight changes in the environ- 
ment. 
It was mentioned in an earlier part of this chapter that 
