I shall give below a general classification of different stimuli which cause excitation in vegetable tissues.
Electric Stimulus.—Induction shock, condenser discharge, the make of kathode and the break of anode.
Mechanical Stimulus.—Mechanical blow, friction, prick or cut.
Chemical Stimulus.—Effect of certain acids, and of other chemical substances.
Thermal Stimulus.—Sudden variation of temperature; application of heated wire.
Radiation Stimulus.—Luminous radiation of the more refrangible portion of the spectrum; ultra-violet rays; thermal radiation in the infra-red region.
All these different forms of stimulus induce an excitatory contraction, a diminution of turgor, and a negative mechanical response or fall of a motile leaf.
SUMMARY.
A radial organ responds to stimulus by contraction in length; as all its flanks are equally excitable there is no lateral movement under diffuse stimulus.
Physiological anisotrophy is induced in an organ, originally radial and isotropic, by the unequal action of the environment on its different sides. Diffuse stimulus induces a greater contraction of the more excitable side.
In a curved tendril the concave side is less excitable than the convex. Diffuse stimulus tends to straighten the curved tendril.
