a. Radial thick organ, in which transverse conduction is absent. Curvature is positive, i.e., movement towards light. The result will be similar when light strikes in an opposite direction, i.e., from right to left.
b. Radial thin organ. There is here a possibility of transverse conduction. Sequence of curvature: positive, neutral, and negative. Reversal of direction of light gives rise to similar sequence of responses as before (e.g., seedling of Sinapis).
c. Anisotropic thick organ; transverse conduction possible. Thick line represents the more excitable distal side. Sequence of curvature: positive, neutral and pronounced negative. When light strikes from opposite direction on the more excitable side the curvature will remain positive, since the pronounced reaction of the more excitable side cannot be neutralised or reversed by transmitted excitation to the less excitable distal side (e.g., leaf of Mimosa).
In the absence of transverse conduction, the curvature remains positive (e.g., leaflet of Erythrina).
d. Anisotropic thin organ with high transverse conductivity. Sequence of curvature: transient positive, quickly masked by predominant negative. Light striking on the more excitable side will give rise only to positive. The response in relation to the plant, will apparently be in the same direction whether light strikes the organ on one side or the opposite (e.g., leaflets of Mimosa, Averrhoa and Biophytum).
I have shown that tissues in sub-tonic condition exhibit an acceleration of the rate of growth under stimulus (p. 224) 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 stimulus 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. | Action. | Effect observed. |
| I Tissue sub-tonic. | Stimulus causes increase of internal energy. | 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. |
| II Normally excitable organ under unilateral light. | A 1. Moderate light, causing excitatory contraction of proximal and positive expansion of distal. | 1. Curvature towards light, e.g., flower bud of Crinum. |
| A. Organ radial. | A 2. Strong light. Excitatory effect transmitted to distal, neutralising first. | 2. Neutralisations, e.g., seedling of Setaria. |
| A 3. Intense and long-continued light. Fatigue of proximal and excitatory contraction of distal. | 3. Reversed or negative response, e.g., seedling of Zea Mays. | |
| B. Dorsiventral organ. | B 1. Excitatory contraction of proximal predominant, owing either to greater excitability of proximal or feeble transverse conductivity of tissue. | 1. Positive response, e.g., upward folding of leaflets in so-called "diurnal sleep" of Erythrina indica and Clitoria ternatea. |
| B 2. Transmission of excitation through highly conducting tissue to more excitable lower or distal. Greater contraction of distal. | 2. Negative response, e.g., downward folding of leaflets in so-called "diurnal sleep" of Biophytum and Averrhoa. | |
| III Rhythmic tissue. | Considerable absorption of energy, immediate or prior. | Initiation of multiple response in Desmodium gyrans previously at standstill; multiple response under continuous action of light in Biophytum. |