Page:Life Movements in Plants.djvu/273

Rh abnormal acceleration into normal retardation of growth (p. 225).

Direct stimulus induces in Mimosa and other ' sensitive ' plants a negative response. There is a diminution of turgor and contraction in the motile organ, resulting in the fall of leaf. Indirect stimulus, on the other hand, gives rise to a 2^ositive or erectile response, indicative of increase of turgor and expansion (p. 138).

In growing organs Direct stimulus induces an incipient contraction and retardation of rate of growth ; the effect of Indirect stimulus is expansion and accelaration of the rate of growth (p. 216).

The opposite reactions to Direct and Indirect stimulus are also found in the electric response given by all plant organs. Thus while Direct stimulus induces an electromo- tive change of galvanometric negativity, Indirect stimulus induces the opposite change of galvanometric positivity (p. 211).

The autonomous activity of Desmodium gyrans exhibit- ed by the pulsation of its leaflets come to a stop under condition of sub-tonicity. The arrested movement is, how- ever, revived by the action of stimulus (p. 228). The depressed or arrested growth of a growing organ is simi- larly accelerated or revived by the action of stimulus (p. 230).

In vigorous specimens stimulus induces the opposite effect by retarding or arresting the pulsatory acrivity or growth.

Warmth induces an effect which is antagonistic to that of stimulus. The contractile effect of stimulus is seen in the pulsations of leaflet Desmodium by the reduction of their expansive or diastolic limit, and in growing organs by the retardation of the rate of growth. The expansive effect of warmth is seen in reduction of the systolic limit