[Illustration: FIG. 139.--Diagrammatic representation of the effects of direct and indirect stimulus on the response of _Setaria_. Direct stimulation, represented by thick arrow gives rise to antagonistic concavities of opposite sides of responding hypocotyl, resulting in neutralisation. Indirect stimulus represented by dotted arrow gives rise to two impulses, the quick positive impulse represented by a circle, and the slower negative impulse represented by crescent (concave).] The results given above enable us to draw the following generalisations:-- 1. In an organ, the tip of which is highly excitable, the balanced state of neutralisation, induced by direct stimulation of the responding region, is upset in two different ways by two impulses generated in consequence of indirect stimulation at the tip. Hence arises two types of resultant response:-- Type A.--If the intervening tissue be semi-conducting, the positive impulse alone will reach the growing region and induce convexity of the same side of the organ giving rise to a _negative_ curvature. Type B.--If the intervening tissue be conducting the transmission of the excitatory impulse will finally give rise to a _positive_ curvature. Type B is exemplified by the seedling of _Setaria_ where the transmission of excitatory impulse from the tip upsets the neutral balance and induces the final positive curvature. Example of type A is found in the negative phototropism of the root of _Sinapis_. _Negative phototropism of root of_ Sinapis: _Experiment 140._--For investigation of the negative phototropism of the root of _Sinapis nigra_ I took record of its movement under unilateral action of light by means of a Recording Microscope, devised for the purpose.[22] When the root-tip alone was stimulated by unilateral light, the root moved away from the source of light. This was due to the longitudinal transmission of positive impulse to the growing region at some distance from the tip. The intervening distance between the tip and the growing region is practically non-conducting, hence the excitatory impulse could not be conducted from the tip. After a period of rest in darkness, I next took record of its movement under direct unilateral illumination of the growing region; the result was at first a positive movement; but this, on account of transverse conduction of excitation under continued stimulation, underwent a neutralisation and slight reversal. In taking a third record, in which both the tip and growing region were simultaneously subjected to unilateral stimulation of light, I found that a resultant responsive movement was induced which was away from light. [22] "Plant Response"--p. 604. Thus in the root of _Sinapis_, the expansive effect of indirect stimulation of the tip is superposed on that of direct stimulation of the growing region (neutral or slightly negative). The final result is thus a movement away from light or a _negative_ phototropic curvature. SUMMARY.