XXIV PLANT RESPONSE 



growth ; (2) by responsive movement of pulvinus; and (3) by growth- 

 curvature— Method of application of chemical reagent — Effect of alkali 

 — Effect of acid — Effect of copper sulphate— Action of sugar solution— 

 Chemo-tactic movements — Explanation of anomalous osmotic or plas- 

 molytic action — Excitatory versus plasmolytic reaction in pulvinus of 

 Mimosa : (1) Favourable tonic condition— (2) Ordinary tonic condition 

 — Polar effects of currents inducing growth-curvatures — Localised polar 

 effects on pulvinus — Anodic and kathodic effects on longitudinal growth 

 — Generalised law of polar excitation in plants — Galvano-tropic response 

 — The indirect effect of polar excitation— The effect on growth of 

 ' electrification ' of soil 546 



PART VIII.— HELIOTROPISM 

 CHAPTER XLI 



FUNDAMENTAL RESPONSIVE ACTION OF PLANT-TISSUES 

 TO STIMULUS OF LIGHT 



Diversity of movements induced by light — Differentiation of responsive 

 movements — Action of light on tissues in sub-tonic condition — Effect of 

 light on pulvinated organs — Effect of diffuse stimulation of light on 

 non-growing radial organs — Retarding effect of light on longitudinal 

 growth — Phenomenon of oscillation under long-continued stimulation — 

 Similarity of responsive reaction under light and under other forms of 

 stimulation ........... 565 



CHAPTER XLII 



POSITIVE HELIOTROPISM 



Introduction — Theory of de Candolle — Inadequacy of de Candolle's theory 

 — Definition of terms positive and negative — Darwin's theory of 

 modified circum nutation — Response of terminal leaflet of Desmodium — 

 Extreme sensitiveness of some plant-organs to light — Merging of 

 multiple in continuous response — Orientation induced by light — The 

 perceptive region in the terminal leaflet of Desmodium — Heliotropic 

 response in radial organ — Magnetically controlled recorder — Heliotropic 

 response of hypocotyl of Sinapis — Recovery and theory of recti-petality. 579 



CHAPTER XLIII 



NEGATIVE HELIOTROPISM 



Incomplete parallelism between actions of light and of gravitation — 

 Theoretical considerations — Recording microscope — Negative helio- 



