Feb. 1902. j BOTANICAL SOCIETY OF EDINBURGH 181 



and when brought into light they recommence, but in 

 neither case is the manifestation of irritability immediate. 

 In the first instance, there is a period of katabolic, and in 

 the latter of anabolic inertia, and the time values of these 

 periods vary with the intensity of the light. 



Concentrated sunlight will inhibit the streaming of pro- 

 toplasm of Elodea after five to six minutes, while recovery 

 takes place in fifteeil minutes to an hour (Ewart, " Annals 

 of Botany," xii. p. 385). Here we have both phases, first 

 the katabolic and then the anabolic. 



When CO2 assimilation in Eloclca has been inhibited 

 by prolonged darkness, the recovery on exposure to light 

 requires from twenty minutes to half an hour (Darwin, 

 " Proc. C'amb. Phil. Soc," ix. 338). This is the expression of 

 the katabolic inertia. 



In what Wiesner calls photo-mechanical induction of 

 heliotropic heterauxesis, we have interesting cases where 

 the functional inertia amounts to a jj/i//S2o/o///m/ insuscepii- 

 hility. Thus, in order to induce the maximum heliotropic 

 curvature a definite time exposure is necessary, and no 

 additional exposure will cause the organ to curve farther 

 or faster. For the cress and vetch, Wiesner (Vines' 

 " Physiology," p. 435) found the period of photo-mechanical 

 induction to be about one-third of the reaction period, i.e. 

 something over eight minutes for the former and under 

 twelve minutes for the latter. Following this came the 

 period of anabolic inertia lasting from twenty-five to 

 thirty-five minutes, after which curvature began. 



A ]iarallel case is presented in Lewes' ^researches 

 ("Annals of Botany," vol. xii. p. 420) on the movements 

 of the chloroplast of Mesocarpus. Lewes found that a 

 definite time exposure — one and a half minute in 

 sunlight and two minutes in diffuse light — was necessary 

 to bring about the greatest possible movement of the 

 chloroplast, viz. rotation through 90°. Further, a definite 

 time was necessary for the performance of the movement, 

 viz. half an hour in the first instance, and twenty minutes 

 in the latter. This supplies a very lucid demonstration 

 that the protoplnsmic molecules have their own rate and 

 extent ot swing, and are not to be induced to pass it. It 

 is the case of the free wheel bicycle pedal, which no amount 



