Page:Life Movements in Plants.djvu/279

Rh diverse concomitant manifestations, in diminution of turgor, in movement, in variation of growth, and in electrical change. The correspondence in the different phases of response in pulvinated, ordinary, and growing organs may be stated as follows : Excitation induces diminution of turgor, contraction and fall of the leaf of Mimosa ; it induces an incipient contraction or retardation of rate of growth in a growing organ ; it gives rise in all plant organs to an electric response of galvanometric negativity and of changed resistance. All these excitatory mani- festations will, for convenience, be designated as the negative response. There is a responsive reaction which is opposite to the excitatory change described above. In Mimosa the fall of leaf under excitation is due to a sudden diminution of turgor ; the erection of the leaf is brought about by natural or artificial rastoratiou of turgor. Rise of temperature induces an expansive reaction which is antagonistic to that induced by stimulus. Warmth also enhances the rate of growth and induces an electric change of galvanometric positivity.* The restoration of normal turgor or enhancement of turgor is associated with expan- sion, erection of the leaf of Mimosa, enhancement of rate of growth in a growing organ, electric response of galvano- metric positivity, and contrasted change of electric resistance. All these will be distinguished as j^ositive response.

There are thus several independent means of detecting the excitatory change or its opposite reaction in vegetable tissues. It will be seen that the employment of these ditferent methods has greatly extended our power of in- vestigation on the phenomenon of irritability of plants.

We have seen how essentially similar are the respons- ive reactions in pulvinated and in growing organs. It is therefore rational to seek for an explanation of a parti-


 * BosE — " Comparative Electro-physiology " — p. 75.