In order to remove the drawback connected with the supply of sufficient material, I commenced an investigation to find whether a detached leaf preparation could be made as effective for the study of irritability as the whole plant. Here we have at the central end of the leaf the pulvinus, which acts as the contractile organ; the conducting strand in the interior of the petiole, on the other hand, is the vehicle for transmission of excitation. The problem to be solved is the rendering of an isolated petiole-and-pulvinus of _Mimosa_ as efficient for researches on irritability as the nerve-and-muscle preparation of a frog. On the success of this attempt depended the practical opening out of an extended field of physiological investigation which would be unhampered by any scarcity of experimental material. In connection with this it is well to note the surprising difference in vegetative growth as exhibited by plants grown in soil and in pots. A pot-specimen of _Mimosa_ produces relatively few leaves, but one grown in the open ground is extremely luxuriant. As an instance in point, I may state that for the last five months I have taken from a plant grown in a field about 20 leaves a day for experiment, without making any impression on it. A large box containing soil would be practically as good as the open ground, and the slower rate of growth in a colder climate could be easily made up by planting half a dozen specimens. The protection of the plants from inclemencies of weather can be ensured by means of a glass cover with simple heat-regulation by electric lamps, in place of an expensive green-house. Returning to the question of the employment of an isolated leaf, which I shall designate as a petiole-pulvinus preparation, instead of the entire plant, the first attempts which I made proved unsuccessful. The cut leaf kept in water would sometimes exhibit very feeble response, at other times all signs of excitability appeared to be totally abolished. It was impossible to attempt an investigation on the effect of changing environment on excitability when the normal sensitiveness itself underwent so capricious a change These difficulties were ultimately overcome from knowledge derived through systematic investigation on the relative importance of the different parts of the motor apparatus, on the immediate and after-effect of section on the excitability of the leaf, and on the rate of decay of this excitability on isolation from the plant. The experience thus gained enabled me to secure long-continued and uniform sensibility under normal conditions. It was thus possible to study the physiological effects of changing external conditions by observing the responsive variation in the isolated petiole-pulvinus preparation. I propose to deal with the different aspects of the investigation in the following order:-- 1. The effect of wound or section in modification of normal excitability.