gations where it is required to make an electrolytic contact with the pulvinus without inducing any change in its motile excitability.
I shall now proceed to describe the effects of various gases and vapours on the excitability of Mimosa. The plant is enclosed in a small glass chamber, the different gases being made to stream in and out through entrance and exit tubes. The various effects induced may be classified as (1) stimulating, (2) depressing, and (3) toxic. The exaltation of excitability induced by stimulating agents is exhibited by the enhancement of amplitude of response. The effect of depressing agents is seen in the diminution of amplitude of response; in this class may be included agents which have slight narcotic action. In all these cases the removal of the gas is attended by the restoration of normal excitability of the plant. A curious fact noticeable in this connection is the phenomenon of accommodation. Under the action of a slightly depressing agent, there is induced a diminution of excitability. But the plant may accommodate itself to the change, in consequence of which the excitability is more or less restored to the original condition. It should also be borne in mind that the character of the reaction is modified to a certain extent by the tonic condition of the plant, a plant in a vigorous condition being better able to withstand unfavourable circumstances than one in a weak condition.
Lastly, there are gaseous agents which are toxic in their action; their application is attended by rapid loss of excitability and death of the plant. I will now describe in detail the effects of various gases, beginning with those which stimulate and ending with others which cause the death of the plant.
Ozone
The stimulating effect of this gas is clearly seen in fig. 46. The particular leaf, before the application of
