effective in excitinsr the S3nsltiv3 Mimom and in retardin<

the rate of growth.

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Fig. 92. — Effect ot infra-red rays on the pulvimia of .himosa. DIVERSE MODES OF RESPONSE TO STIMULUS.

In Mimosa excitation id followed by the striking mani- festation of the fall of the leaf. But in rigid tree^ con- traction under excitation cannot find expression in move- ments. I have shown elsewhere that even in the absence of realised movement, the stat3 of excitation can be detect- ed by the induced electro-motiv3 change. I have shown that not only every plant but every organ of every plant is sensitive and reacts to stimulus by electric response of galvanometric negativity.^[1]

There is an additional electric method by which the excitatory change may be recorded. I find that excitation induces a variation of the electrical resistance of a vegetable tissue.^[2] Thus the same excitatory reaction finds