CHAPTER XXVII 



RESPONSE TO STIMULUS OF LIGHT 



Heliotropic plant movements reducible to fundamental reaction of contraction 

 or expansion Various mechanical effects of light in pulvinated and growing 

 organs Electrical response induced by light not specific, but concomitant to 

 excitatory effects Electrical response of plant to light not determined by 

 presence or absence of chloroplasts Effect of unilateral application of stimu- 

 lus on transversely distal point Positive response due to indirect effect and 

 negative to transmission of true excitation Mechanical response of leaf of 

 Mimosa to light applied on upper half of pulvinus Mechanical response 

 consists of erection or positive movement, followed by fall or negative move- 

 ment Electrical response of leaf of Mimosa to light applied on upper halt 

 of pulvinus; induction in lower half of pulvinus of positivity followed by 

 negativity Longitudinal transmission of excitatory effect, with concomitant 

 galvanometric negativity Direct effect of light and positive after-effect 

 Circumstances which are effective in reversing normal response Plants in 

 slightly sub-tonic condition give positive followed by negative response 

 Exemplified by (a) electrical and (b) growth response Examples of positive 

 response to light Periodic variation of excitability Multiple mechanical 

 response under light Direct and after effect Multiple electrical response 

 under light, with phasic alternations of (- + +) or (+ + ) After 

 effects ; unmasking of antagonistic elements, either phis or minus Three 

 types of after-effects. 



THE first important point that arises, in connection with the 

 response of living tissues to light, is the question whether 

 such response is peculiar in its character, or fundamentally 

 similar to that which is evoked by other forms of stimulus. 

 The mechanical movements of plants under light are 

 apparently so diverse that it would at first sight appear 

 almost impossible to derive them all from any common 

 fundamental reaction. Thus, some plant organs are found 

 to turn towards the light, others away from it, and 

 others again to remain perpendicular to it. Thus three 

 different typical effects positive, negative, and dia-helio- 



