GROWTH 319 
The grand period itself is not quite uniform, as the rates 
of growth in the active region may and do vary with changes 
in external conditions, and with differences in activity in 
the protoplasm from time to time. This can be observed 
very favourably in the case of a growing stem, which shows 
considerable differences in its rate of growth during twenty- 
four hours. The growth is greatest during the night and 
least during the day, and the variations in the rate are 
fairly regular, the total growth during successive periods 
of twenty-four hours being, on the whole, uniform. This 
regular variation of the rate constitutes what is known 
as the daily period of growth in length. 
An instrument by which the progress of growth of such 
a structure as a stem can be 
ascertained and registered is 
known as an auxanometer. 
A very convenient form, 
which registers the gradual 
increase in length automati- 
cally, has been constructed 
by Pfeffer, and is repre- 
sented in fig. 188. A thread 
attached to the plant passes 
over the small wheel az, 
which is cemented on the 
large wheel r, and accurately 
centred about the same axis. 
A thin lever z is attached 
to another thread which is 
passed over the large wheel, 
and is made to write upon 
the smoked surface of a Fig. 138.—Prerrer’s AUTOMATICALLY 
REGISTERING AUXANOMETER. (After 
paper fastened round the Debiner| 
cylindrical drum t. The 
string is kept tight by the counterbalancing weight g. The 
drum is caused to rotate slowly upon its axis by clock- 
work, so that the indicator traces a line along its surface, 
