422 CHAP. XXXVII. GENERAL REVIEW 



depleted, so that the growing organ has become subtonic, 

 growth comes to a standstill. Stimulation is found to 

 revive the activity of the arrested growth (p. 86). 



Effect of variation of temperature. — Sudden variation of 

 temperature acts as a stimulus and retards growth ; in 

 order to prevent this, experimental thermal change should 

 be gradual. In a number of tropical plants the minimum 

 temperature for arrest of growth is about 22°, the optimum 

 being 34° C. A continuous record of change of growth 

 during uniform rise of temperature gives the Thermo- 

 CRESCENT Curve from which the absolute rate of growth 

 at any temperature can be obtained (p. 39). 



Effect of Anaesthetics on Growth 



The effect on growing organs of exposure to COg, to 

 ether vapour, or to chloroform, is at first expansion and 

 increase in the rate of growth, followed by retardation, 

 even active contraction (pp. 44, 46, 293). Growth in 

 a state of standstill is revived by ether and chloroform, 

 w^hich offers an explanation of the action of anaesthetics 

 in forcing growth of dormant buds in winter. The accelera- 

 tion of growth by CO2 in the first stage of its action gives 

 rise to an enhancement of geotropic response, but continued 

 action causes contraction and brings about a reversal of 

 geotropic response, from an up to a down curvature. 



Response to Diverse Modes of Stimulation 



It has been generally assumed that the effects of diverse 

 modes of stimulation are specifically different. In reality 

 there is no such difference. Perception of stimulus and 

 the consequent reaction arise in all cases from excitation 

 of the sensitive protoplasm. Though in certain cases ana- 

 tomical structures, such as tactile hairs and pits, and others, 

 facilitate the perception of a particular form of stimulation 

 by causing deformation of the ectoplasm in an effective 

 manner, the normal results of stimulation are essentially the 

 same whatever the stimulus employed. The first effect on 



