78 COMPARATIVE ELECTRO-PHYSIOLOGY 



which sub-tonicity is manifested by arrest of growth, we may 

 select a specimen in which, while the tissue is not fully tonic, 

 there is still, nevertheless, a feeble rate of growth. In such 

 a case we may expect the income from the absorption of 

 stimulus to prove greater than the expenditure in the form of 

 true excitatory response. Hence, if we subject such a tissue 

 to the constant action of an external stimulus, we shall in 

 the first stage obtain the predominant effect of the internal 

 factor, with its positive turgidity-variation and enhanced rate 

 of growth. 



But by the continued action of this accumulating income, 

 the tonic condition of the tissue will be raised to the normal, 

 with a concomitant increase of excitability. It will now 

 therefore be the excitatory component which becomes pre- 

 dominant, resulting in the negative turgidity-variation, con- 

 traction, and retardation of growth. 



In order to detect these variations of the normal rate of 

 growth, under the action of stimulus, it is necessary to have 

 at our disposal some very delicate means of record. This 

 need I have, however, been able to meet, by devising the 

 Balanced Crescograph, more fully described in my book 

 on ' Plant Response. 3 Here, the uniform rate of elongation 

 of a growing organ causes a rotation of the recording Optic 

 Lever. The spot of light from this lever falls upon a second 

 mirror, which is subject to a compensating movement. When 

 the balance is exact, the spot of light, reflected from the two 

 mirrors, remains quiescent. When, however, the normal rate 

 of growth, under the action of any agent, undergoes varia- 

 tion, the balance is upset. Thus, when growth is accelerated, 

 there is a movement of the recording spot of light in one 

 direction, say up, and when retarded, a movement in the 

 opposite, say down. 



In order to study the effect of external stimulus on a 

 tissue in a slightly sub-tonic condition, I took a flower-bud 

 of Crinum lily, and first obtained a balanced record, seen as a 

 horizontal line (fig. 52). Stimulus of light was now applied, 

 and it will be seen that after a short latent period the 



