256 LIFE MOVEMENTS IN PLANTS 



not only reveal very slight variation induced in the rate 

 of growth, but also the latent period and time-relations of 

 the change. For this purpose I at first devised the Optical 

 Method of Balance* which was considered at the time to 

 be extremely sensitive. The spot of light from the Optical 

 Lever (which magnified the rate of growth) was made to 

 fall upon a mirror to which a compensating movement 

 was imparted so that the light-spot after double reflection 

 remained stationary. Any change of rate of growth — 

 acceleration or retardation — was at once detected by the 

 movement of the hitherto stationary spot of light in one 

 direction or the other. 



A very careful manipulation was required for the 

 adjustment of the Optical Balance ; the record moreover 

 was not automatic. For these reasons I have been 

 engaged for several years past in perfecting a new appa- 

 ratus by which, (1) the balance could be directly obtained 

 with the utmost exactitude, (2) where an attached scale 

 would indicate the exact rate of growth, and (3) in which 

 the upsetting of the balance by external stimulus would 

 be automatically recorded, the curve giving the lime rela- 

 tions of the change. 



PRINCIPLE OF THE METHOD OF BALANCE. 



I shall take a concrete example in explanation of the 

 method of balance. Taking the rate of growth per second 

 of a plant to be ^—^ inch or 0*5 fx per second (equal 

 to the wave length of sodium light), the tip of the plant 

 will be maintained at the same point in space if we suc- 

 ceeded in making the plant-holder subside exactly at the 

 same rate. The growth-elongation of the plant will then 

 be exactly balanced by a compensating movement down- 

 wards. The state of exact balance is indicated when the 



* "Plant Response"— p. 413. 



