Test of microscopic examination.—I next made sections of Bryophyllum and Nymphæa and on examining them under the microscope discovered certain striking changes. A fortnight ago the group of large starch grains stained with iodine were the most striking feature of the starch sheath. But now these starch grains could not be found in any of the numerous specimens examined. The presence of the starch grains thus appears to be associated with the sensitiveness of the perceptive layer.
REACTION AT LOWER SIDE OF THE ORGAN.
There remains now the important question of the physiological change induced on the lower side of the horizontally laid shoot. The physiological reaction of two sides of the organ must be different, since the upper side exhibits contraction and the lower side expansion. It may be urged that the effect of one of the two sides might result from the passive yielding to the definite reaction induced on the opposite side. Investigation by the electric method enables us, however, to discriminate the two reactions from each other, since the electric response characteristic of the induced physiological change takes place in the organ, even under condition of restraint by which movement is prevented. We shall therefore investigate the geo-electrical reaction on the lower side of the securely held organ, and find out whether the induced electric change undergoes any variation in different layers from below upwards. There are two different ways in which the electric explorations of the lower side of the organ may be carried out. In the first method, the probe is introduced from below, and successive readings for geo-electric response taken as the probe enters the organ by successive steps. It is understood that the true geotropic effect is found from difference of galvanometer readings in vertical and horizontal positions. In the second method, the probe is introduced from above, and successive readings for the response taken for different positions of the probe as it enters the organ from the upper side and comes out ultimately at the lower side. This I shall call the METHOD OF TRANSVERSE PERFORATION. The intrusion of the probe on the upper side gives, as we have seen, increasing negative deflection of the galvanometer which reaches a maximum at the perceptive layer. Passage of the probe to still greater depths give deflections which decline to zero. But when the probe comes within the influence of the perceptive layer of the under side, the electric indication, as we shall presently find, undergoes a reversal.
ELECTRIC EXPLORATION OF THE LOWER SIDE OF THE ORGAN.
I shall first describe the results obtained from the first method, the probe entering the organ from the lower side.
Experiment 189.—The investigation was carried out with the stem of Bryophyllum, and the flower stalk of Nymphæa. The probe was made to enter the organ through 0·4 mm. and the geo-electric effect found, on rotation of the flower stalk of Nymphæa from the vertical to the horizontal, was a deflection of +6 divisions of the galvanometer. The change induced at the lower side by geotropic stimulus is thus galvanometric positivity, indicative of enhancement of turgor, and, of expansion. Intrusion of the probe through 0·6 mm. gave rise to an increased positive geo-electric response. That the sign of electric response depended on the relation of the side of the organ to the vertical lines of gravity was demonstrated by alternate rotation of the plant through +90° and -90°, the probe remaining at a definite position. Rotation through +90° brought A above, and rotation through -90° brought A below. When the probe was in the up position the geo-electric response was negative, but when rotation through -90° brought it below, the response became positive. Thus with an identical contact in the plant, the electric response underwent reversal from negative to positive. This will be understood from the following table.
| Position of the probe inside the organ. | Galvanometer deflection: A above. | Galvanometer deflection: A below. |
| 0·4 mm. | - 8 divisions. | + 5 divisions. |
| 0·6 mm. | -16 " | +10 " |
It will thus be seen that physiological change induced at any point is modified by its relation to vertical lines of gravity. When the point is above, the induced change is negative, when below, the induced change is positive.
I shall next describe the variation of effect at different layers of the under side of the organ.
Experiment 190.—A complete set of readings of the geo-electric reaction at different layers of the organ was taken, as the probe entered the lower side by successive steps of 0·2 mm. The following table gives the results obtained with a specimen of Nymphæa.