"Many plant organs, especially foliage and floral leaves take up, towards evening, positions other than those they occupy by day. Petals and perianth leaves, for example, bend outwards by day so as to open the flower, and inwards at night so as to close it.... Many foliage leaves also may be said to exhibit opening and closing movements, not merely when they open and close in the bud but also when arranged in pairs on an axis, they exhibit movements towards and away from each other. In other cases, speaking generally, we may employ the terms _night position_ and _day position_ for the closed and open conditions respectively. The night position may also be described as the _sleep position_." After reviewing the various theories proposed, he proceeds to say "that a completely satisfactory theory of nyctitropic pulvinus movements is not yet forthcoming. Such a theory can only be established after new and exhaustive experimental research." [39] Jost--_Ibid_, p. 500. The difficulties of the experimental reinvestigation here called for towards clearing up and explanation of the subject are sufficiently great; they are further increased by the fact that these diurnal movements may be brought about by different agencies independent of each other. Thus in _Crocus_ and in _Tulip_, the movement of opening during rise of temperature has been shown by Pfeffer to be due to differential growth in the inner and outer halves of the perianth. I shall in this connection show that a precisely opposite movement of closing is induced in _Nymphæa_ under similar rise of temperature. I shall for convenience distinguish the differential growth under temperature variation as _Thermonasty_ proper. Again certain leaflets open in light, and close in darkness in the so-called sleep position. Intense light, however, produces the 'midday sleep'--an effect which is apparently similar to that of darkness. The determining factor of these movements is the variation of light. There are other instances of diurnal movement, far more numerous, which cannot be explained from considerations given above. It has therefore been suggested that the "Day and night positions may arise by the combined action of geotropism and heliotropism. Thus Vochting (1888) observed in the case of _Malva verticillatta_, that the leaves, when illuminated from below, turned their laminæ downwards during the day, but during the night became erect geotropically. The sleep movements in leaves and flowers, referred to above, cannot however be explained by assuming such a combination of heliotropism and geotropism."[40] [40] For further information on the subject of Nyctitropism, _cf._-- Pfeffer--_Ibid_, Vol. II (1903), p. 112; Jost--_Ibid_, pp. 500, 507; Vines--Physiology of Plants (1886), pp. 406, 543.