GROWTH 313 
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 succes- 
sive 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 z, 
which is cemented on the 
large wheel 7, 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 
paper fastened round the 
cylindrical drum ¢. The Fie. 188—Prerver’s avromaticaLLy 
string is kept tight by the ieee AUXANOMETER, (After 
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. 
