198 



THE LIFE OF THE PLANT 



and read what division the index points to from time 

 to time. But we can do something better still : we can 

 turn this apparatus into an automatic one, and make the 

 plant itself record the progress of its own growth during 

 the different hours of the day. For this purpose let 

 us apply the point of the index to a cylinder, the 

 axis of which is set in motion by clock-work, so that it 

 makes a complete rotation in twenty-four hours in the 



direction indicated by 

 the arrow. In order that 

 the index of the appara- 

 tus should trace a visible 

 line it is convenient to 

 ^ blacken the surface of 

 the cylinder with a layer 

 ^c;"' of soot. Naturally, if 



J , the cylinder moves very 



rapidly in comparison 

 with the movement of 

 the index, the line traced 

 on it will be almost hori- 

 zontal (as is a ... w in fig. 56, II). On the other hand, 

 if the arrow moves very rapidly in comparison with the 

 rotation of the cylinder, it will trace an almost vertical 

 line (a . . . m). If the arrow moves at a moderate rate 

 the line will be oblique : the steeper its inclination the 

 quicker must the movement of the index have been ; 

 the more the line slants, the slower the movement of 

 the index. A glance at the line a, b, c, d, e, f shows 

 that the stem grew rapidly from a to b, slowly from b to c, 

 and so on. Knowing how long the cylinder was revolv- 

 ing, we can tell also to what hours of the day the rapid 

 growth and the slow growth respectively correspond, 

 and are able to trace the influences controlling such 

 acceleration and retardation. The plant, so to speak, 

 writes down its own impressions, as we have already 

 said. 



Fig. 56. 



