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. 138. A thread 

 attached to the plant passes 

 over the small wheel x, 

 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 

 paper fastened round the 

 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. 



FIG. 138. PFEFFER'S AUTOMATICALLY 



REGISTERING AUXANOMETER. (After 



Detmer.) 



