CHAPTER IV
ELECTRIC RESPONSE IN PLANTS—BLOCK METHOD
- Method of block
- —Advantages of block method
- —Plant response a physiological phenomenon
- —Abolition of response by anæsthetics and poisons
- —Abolition of response when plant is killed by hot water.
I shall now proceed to describe another and independent method which I devised for obtaining plant response. It has the advantage of offering us a complementary means of verifying the results found by the method of negative variation. As it is also, in itself, for reasons which will be shown later, a more perfect mode of inquiry, it enables us to investigate problems which would otherwise have been difficult to attempt.
When electrolytic contacts are made on the uninjured surfaces of the stalk at A and B, the two points, being practically similar in every way, are iso-electric, and little or no current will flow in the galvanometer. If now the whole stalk be uniformly stimulated, and if both ends A and B be equally excited at the same moment, it is clear that there will still be no responsive current, owing to balancing action at the two ends. This difficulty as regards the obtaining of response was overcome in the method of negative variation, where the excitability of one end was depressed by chemical reagents or injury, or abolished by excessive temperature. On stimulating the stalk there was produced a greater excitation at A than at B, and a current of action was then observed to flow in the stalk from the more excited A to the less excited B ([fig. 6]).
But we can cause this differential action to become evident by another means. For example, if we produce a block, by clamping at C between A and B ([fig. 14], a), so that the disturbance made at A by tapping or vibration is prevented from reaching B, we shall then have A thrown into a relatively greater excitatory condition than B. It will now be found that a current of action flows in the stalk from A to B, that is to say, from the excited to the less excited. When the B end is stimulated, there will be a reverse current ([fig. 14], b).
Fig. 14.—The Method of Block
(a) The plant is clamped at C, between A and B.
(b) Responses obtained by alternately stimulating the two ends. Stimulation of A produces upward response; of B gives downward response.