GEOWTH 



819 



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 canjue 

 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.) 



