2G4 LIFE MOVEMENTS IN PLANTS 



As it is easy to detect a drift of 1 cm, per miuiite a 



0"53 

 variation of , ^ , __ ' _„ . , or 1 part in 27,000 may thus be 

 lo"177 X 7^0 



detecred by the Method of Balance. The spectroscopic 

 method enabled us, as we saw, to detect change of wave 

 length 1 part in a thousand. The sensibility of the 

 Balanced Crescograph is thus seen to rival, if not surpass 

 that of the spectroscope. 



For obtaining a general idea of the sensitiveness, the 

 absolute of growth in the instance given above was 0'0001<S 

 inch per second, and the Balanced Crescograph was shown 

 capable of discriminating a variation of I part in 27,000 ; 

 hence it is possible to detect by this means a variation 



of millionth of an inch per second. 



1,500 ^ 



This method of unprecedente;l delicacy opens out a new 

 field of investigation o;i the effect of changes of environ- 

 ment in modification of growth ; instances of this will be 

 found in subsequent chapters. I give below accounts of 

 certain demonstrations which will no doubt appear as very 

 striking. 



After obtaining the exact balance a match was struck 

 in the neighbourhood of the plant. This produced a marked 

 movement of the hitherto quiescent spot of light, thus indi- 

 cating the perception of such an extremely feeble stimulus 

 by the plant. 



Breathing on the plant causes an enhaucemeni of growth 

 due to the joint eft-ects of warmth and carbonic acid gas. 

 A more striking experiment is to fill a small jar with 

 carbonic acid and empty it over the plant. A violent move- 

 ment of the spot of light to the right demonstrates the sti- 

 mulating effect of this gas on growth. 



The method described above is excessively sensitive ; for 

 general purposes and for the method of direct record, a ies& 



