_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 | Galvanometer | Galvanometer | | probe inside | deflection: | deflection: | | the organ. | A _above_. | 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_. TABLE XLIII.--ELECTRIC EXPLORATION OF DIFFERENT LAYERS ON THE LOWER SIDE OF THE ORGAN (_Nymphæa_). +-----------------------------+ |Position of the|Galvanometer | | probe. | deflection. | +---------------+-------------+ | Surface | 2 divisions.| | 0·2 mm. | 4 " | | 0·4 " | 8 " | | 0·6 " |16 " | | 0·8 " |20 " | | 1·0 mm. |32 " | | 1·2 |16 " | | 1·4 " |12 " | | 1·6 " | 4 " | | 1·8 " | 0 " | +-----------------------------+