Life Movements in Plants, Volume I

by Jagadis Chandra Bose

Sir J. C. Bose.

Several phenomena of daily periodicity are known, but the relations between the recurrent external changes and the resulting periodic variations are more or less obscure. As an example of this may be cited the periodic variation of growth. Here the daily periodicity exhibited by a plant is not only different in varying seasons, but it also differs in diverse species of plants. The complexity of the problem is very great, for not only are the direct effects of the changing environment to be taken into consideration but also their unknown after-effects. Even in the case of direct effect, different factors, such as light, temperature, turgor, and so on, are undergoing independent variations; it may thus happen that their reactions may sometimes be concordant and at other times discordant. The nyctitropic movement of plants affords another example of daily periodicity. The fanciful name of ‘sleep’ is often given to the closure of the leaflets of certain plants at night. The question whether plants sleep or not may be put in the form of the definite inquiry: Is the plant equally excitable throughout day and night? If not, is there any definite period at which it practically loses its ex­cit­abil­ity? Is there, again, another period at which the plant wakes up, as it were, to a condition of maximum ex­cit­abil­ity?

In the course of my in­ves­ti­ga­tions on the irritability of Mimosa pudica, I became aware of the existence of such a daily periodicity; that is to say, the moto-ex­cit­abil­ity of the pulvinus was found to be markedly diminished or even completely abolished at a certain definite period of the day; at another equally definite period, the ex­cit­abil­ity was observed to have attained its climax. The observations on the periodic variation of ex­cit­abil­ity appeared at first to be extremely puzzling. It might be thought, for example, that light would prove to be favourable for moto-ex­cit­abil­ity; in actual experiment the results apparently contradicted such a supposition: for the ex­cit­abil­ity of the plant was found much higher in the evening than in the morning. Favourable temperature, again, might be regarded as an important factor for the enhancement of the moto-ex­cit­abil­ity; it was, nevertheless, found that though the excitatory response was only moderate at that period of night when the temperature was at its minimum, yet the ex­cit­abil­ity was altogether abolished at another period when the temperature was several degrees higher. The obscurities which surrounded the subject were only removed as a result of protracted in­ves­ti­ga­tion and comparison of continuous automatic records made by the plant itself during several months, beginning with winter and ending in summer.

The question whether a plant like Mimosa exhibits diurnal variation of ex­cit­abil­ity can be experimentally investigated by subjecting the plant at every hour of the day and night to a test-stimulus of uniform intensity, and obtaining the corresponding mechanical responses. Under these circumstances the amplitude of response at any time will serve as a measure of the ex­cit­abil­ity of the plant at the particular time. Any periodic fluctuation of response will then demonstrate the periodic character of variation of ex­cit­abil­ity.

The in­ves­ti­ga­tion thus resolves itself into:—

The successful construction of a Response Recorder which will automatically record the response of the plant to uniform periodic stimulation at all hours of day or night;

the study of the effects of various external conditions on ex­cit­abil­ity;

the diurnal variation of ex­cit­abil­ity and its relation to the changes of external conditions.

I will first give a diagrammatic view of the different parts of the apparatus which I devised for this in­ves­ti­ga­tion.[G] The leaf of Mimosa is attached to one arm of a light aluminium lever, L, by means of thread. At right angles to the lever is the writing index W, which traces on a smoked glass plate allowed to fall at a definite rate by clockwork the responsive movement of the leaf. Under a definite stimulus of electric shock the leaf falls down, pulling the lever L, and moving the writer towards the left. (Fig. 14.) The amplitude of the response-curve measures the intensity of excitation. The leaf re-erects itself after a time, the corresponding record exhibit­ing recovery. A second stimulus is applied after a definite interval, say an hour, and the corresponding response shows whether the ex­cit­abil­ity of the plant has remained constant or undergone any variation.