386 



LIFE MOVEMENTS IN PLANTS 



the corresponding tropic reaction would therefore be away 

 from stimulus or negative curvature. The tonic condition 

 is, however, raised to the normal by the action of stimu- 

 lus itself, and the tropic curvature becomes positive. 



I give below a table which will show at a glance all 

 possible variations of phototropic reaction. 



TABLE XXXI. — MECHANICAL RESPONSE OF PULVINATED AND GROWING 

 ORGANS UNDER LIGHT. 



Description of 

 tissue. 



I Tissue sub- 

 ionic. 



II Normally 

 excitable 

 organ 

 under uni- 

 lateral 

 light. 

 A. Organ 

 radial. 



B. Dorsi- 

 ventral 

 organ. 



tll Rhythmic 

 tissue. 



Action. 



Stimulus causes increase of 

 internal energy. 



A 1. Moderate light, causing 

 excitatory contraction of 

 proximal and positive expan- 

 sion of distal. 



A 2. Strong light. Excitatory 

 effect transmitted to distal, 

 neutralising first. 



A .8. Intense and long-continu- 

 ed light. Fatigue o\ proximal 

 and excitatory contraction of 

 distal. 



B 1. Excitatory contraction of 

 proximal predominant, owing 

 either to greater excitability 

 of proximal or feeble transverse 

 conductivity of tissue. 



B 2. Transmission of excitation 

 through highly conducting 

 tissue to more excitable lower 

 or distal. Greater contraction 

 of distal. 



Considerable absorption of 

 energy, immediate or prior. 



Effect observed. 



Expansion or enhanced rate of 

 growth, e.g.^ Pileus of Coprinus 

 drooping in darkness, made 

 re-turgid by light. Renewed 

 growth of dark rigored plant 

 exposed to light. 



1. Curvature towards light, e.g., 

 llower bud of Crinnm. 



2. Neutralisations, e.p., seedling 

 of Setaria. 



3. Reversed or negative response, 

 e..(/., seedling of Zea ^lays . 



1. Positive response, e.g., up- 

 ward folding of leaflets in so- 

 called " diurnal sleep " of Ery- 

 thrina indica and Clitoria 

 ternatea. 



2. Negative response, e.g., down- 

 ward folding of leaflets in so- 

 called "diurnal sleep " of Bio- 

 phytum and Averrhoa. 



Initiation of multiple response 

 in Desmodinm gyrans previous- 

 ly at standstill ; multiple 

 response under continuous 

 action of light in Biophytum. 



SUMMARY. 



There is no line of demarcation between tropic and 

 nastic movements. 



