EXPERIMENTAL ARRANGEMENTS.
The diurnal record is often taken continuously for several days, and it is therefore necessary to take precautions against the disturbing effect of watering the plant. The record is also liable to be affected by the twist induced by light when it acts on one side of the organ.
Irrigation.—There is, as is well known, a periodic variation of turgor in the plant. This normal variation is, however, disturbed by watering the plant at irregular intervals. Precaution against this was taken by placing the three flower pots on a long trough filled with water (Fig. 190). The height of water in the trough is always maintained constant by a syphon.
Vertical illumination.—The direction of sunlight changes from morning to evening, and the leaves exhibit appropriate phototropic movements or torsions under changing directions of lateral light. In order to obviate this, a special chamber was constructed, which allowed light from the sky to fall vertically on the plant through a sheet of ground glass which covered the roof. The sides and the base of the chamber are impervious to light. A narrow slit covered with red glass allows inspection of the curve during the process of record.
The Ventilator.—A revolving ventilator, acted on by the wind, sucks the air away from the chamber, thus ensuring constant supply of fresh air, without causing any disturbances of the record.
The Recorder.—The Oscillating Recorder employed is of the quadruplex type carrying four recording plates (Fig. 190). The first lever records the daily variation of temperature. The other three are attached to three different specimens of the same plant, or to three different plants. In the former case, three records are obtained of the same species of plant, under identical external condition. If they agree in all essentials, the periodic curve may be taken as characteristic of the given plant. A very great saving of time is thus ensured, and it is thus possible to obtain characteristic curves of numbers of different species of plants within the short period of a season. The quadruplex recorder enables us also to obtain simultaneous records under identical external condition of leaves of different age of the same plant, or of leaves of three different species of plant. I have for the last five years taken records of numerous plants at all seasons of the year. The autograph of the plant is often so characteristic that it is possible to name it by mere inspection of its daily record.
Fig. 190.—The Nyctitropic Recorder with four writing levers. The flower pots are placed in a trough filled with water to a constant height. The first two levers are shown in the figure to record movements of leaves, the third to record movement of a horizontally laid shoot; the fourth lever attached to a differential thermometer, T, records diurnal variation of temperature.
Thermograph.—For obtaining a continuous record of diurnal variation of temperature, I use a compound strip, T, made of brass and steel. Variation of temperature induces a curvature of the compound strip which is recorded by means of the attached lever. The oscillation of the plate takes place once in fifteen minutes, and the successive dots thus produced give time records of the diurnal curve. The record thus consists of a series of dots. An additional device makes the plate oscillate three times in rapid succession at the end of each hour; the hourly dot is thus thicker than others. The movement of the plant, corresponding to the particular variation of temperature at any period, may thus be easily determined. I shall now give a typical example of diurnal movement induced by variation of light and darkness.