898 



Sir J. G. Bose and Mr. G. Das. 



and arrest at a definite temperature. In Scirpus Kysoor the arrest takes 

 place at 22° C. Rise of temperature enhances the rate of growth to an 

 optimum, beyond which there is a decline. At 60° C. a violent contraction 

 takes place, which is the spasm of death. 



2. Heat induces two effects, which are diametrically opposite to each other. 

 The thermal effect of heat is to increase the rate of growth ; the radiation 

 effect is a retardation. 



3. The closest parallelism has been established between the response to 

 stimulation given by pulvinated and by growing organs respectively. 

 Conditions which give rise to negative mechanical or electric response of the 

 former also give rise to negative variation or retardation of growth. This is 

 also true of positive mechanical or electric response and positive variation 

 or enhancement of growth. The physiological machinery is the same in 

 pulvinated and non-pulvinated, in growing and non-growing organs. 



4. Every stimulus is shown to give rise to two distinct impulses : a hydro- 

 positive, independent of the conductivity of the tissue, and an excitatory 

 negative, which is dependent on the conducting power. The former is 

 transmitted quickly ; the latter, being a phenomenon of conduction of proto- 

 plasmic change, is propagated slowly. The hydro-positive impulse gives rise 

 to expansion, the excitatory negative to contraction. The transverse conduc- 

 tivity of an organ is very much less than its longitudinal conductivity. 



5. Investigation on the effect induced by all forms of stimuli has led to 

 the establishment of the following law : Direct application of stimulus 

 induces contraction ; indirect application gives rise to expansion. 



Direct stimulation of the responding region causes a contractile fall of the 

 motile leaf, or a retardation of growth in a growing organ. The transmitted 

 or indirect effect of stimulus applied at a distance is to induce an erection 

 of the leaf and an acceleration of the rate of growth. 



6. This latter fact may offer an explanation of the opposite effects of the 

 stimulus of gravity on root and shoot. In the root, the stimulus is received 

 at the tip, and transmitted to the distant region of growth. Stimulation here 

 is indirect. In the shoot, stimulation is direct, since the growing region of 

 the shoot is both sensitive and responsive. The opposite signs of response in 

 shoot and root may, therefore, be due to the fact that in one case the stimu- 

 lation is direct and in the other indirect. 



7. Tropic movements also are subject to the laws of direct and indirect 

 stimulation. The directly excited proximal side undergoes contraction, the 

 opposite side undergoes expansion ; both these factors conspire to induce a 

 positive curvature. This fundamental effect undergoes modification on 

 account of transverse conduction of excitation bringing about neutralisation 



