In the _negative_ type, rise of temperature induces a movement of closure. Here the outer half of the organ is the more excitable. The flower of _Nymphæa_ belongs to this type. The effect of stimulus is antagonistic to that of rise of temperature. In positive thermonastic organs stimulus induces a movement of closure; in the negative type it induces a movement of opening. XXIX.--ON PHOTOTROPISM _By_ SIR J. C. BOSE. In different organs of plants the stimulus of light induces movements of an extremely varied character. Radial organs exhibit tropic movements in which the position of equilibrium is definitely related to the direction of incident stimulus. Nastic movements under the action of light are, on the other hand, regarded as curvatures of the organ which show "no relation to the stimulus but is determined by the activity of the plant itself".[9] There are thus two classes of response to light which seem to be unrelated to each other. Returning to the directive action of light, radial stems often bend towards the light, while certain roots bend away from it. It may be thought that this difference is due to specific difference of irritability between shoot and root, the irritability of the former being of a positive, and of the latter, of a negative character. But there are numerous exceptions to this generalisation. Certain roots bend towards the light, while a stem, under different circumstances, moves towards light or away from it. Again an identical organ may exhibit a positive or a negative curvature. Thus the leaflets of _Mimosa pudica_ acted on by light from above fold upwards, the phototropic effect being _positive_. But the same leaflets acted on by light from below exhibit a folding upwards, the phototropic effect being now _negative_. Effects precisely the opposite are found with the leaflets of _Biophytum_ and _Averrhoa_. They fold downwards whether light acts from above or below. Finally, a radial organ in found to exhibit under light of increasing intensity or duration, a positive, a dia-phototropic, or a negative phototropic curvature. [9] Jost--_Ibid_, p. 428. In these circumstances the theory of specific positive and negative irritabilities is untenable; in any case, it throws no light on the phenomenon of movement. The difficulties of the problem are thus clearly stated by Pfeffer: "When we say that an organ curves towards a source of illumination, because of its heliotropic irritability and we are simply expressing an ascertained fact in a conveniently abbreviated form, without explaining why such curvature is possible or how it is produced.... Many observers have unfortunately devoted their attention to artificially classifying the phenomenon observed, and have entirely neglected the explanation of causes underlying them."[10] [10] Pfeffer--_Ibid_, Vol. II, p. 74. COMPLEXITY OF PROBLEM OF PHOTOTROPIC REACTION.