Page:Life Movements in Plants.djvu/278

248 effective in excitinsr the S3nsltiv3 Mimom and in retardin< the rate of growth.

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.

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. Thus the same excitatory reaction finds