SECT, ii PHYSIOLOGY 279 



to nullify the constantly operative influence of gravity upon plants 

 and so directly prove its influence, he submitted them to the action 

 of centrifugal force an accelerative force operating like gravity 

 upon the masses of bodies, which had, in addition, the advantage 

 that it could be increased or diminished at will. KNIGHT made use 

 of rapidly rotating, vertical wheels, upon which he fastened plants 

 and germinating seeds in various positions. The result of his 

 experiments was that the stems all turned towards the centre of the 

 wheel and the roots directly away from it. On wheels rotating in a 

 horizontal plane, where, in addition to the centrifugal force, the one- 

 sided action of gravitation was also still operative, the shoots and 

 roots took up an intermediate position depending on the combined 

 influence of the two forces. In this way it was positively ascertained 

 that terrestrial gravitation determines the positions of plant organs 

 in respect to the earth ( 84 ). 



Later, it was also shown that not only the perpendicular direction 

 of stems and primary roots, but also the oblique or horizontal direction 

 taken by lateral branches, lateral roots, and rhizomes, is due to a 

 peculiar reaction towards the force of gravitation. 



The property of plants to assume a definite position with respect 

 to the direction of gravitation is termed GEOTROPISM. It is customary 

 also, as in the case of heliotropism, to speak of positive and negative 

 geotropism, diageotropism,> and transverse geotropism, according to 

 the position assumed by the plant or organ with respect to the centre 

 of the earth. Still another form of geotropic irritability, lateral 

 geotropism, renders possible the twining of stem-climbers. 



Negative Geotropism. All vertically upward growing organs, whether stems, 

 leaves (Liliiflorae), flower-stalks, parts of Howers, or roots (such as the respiratory 

 roots of Avicennia (Fig. 213), Palms, etc.), are negatively geotropic. In case 

 such negatively geotropic organs are forced out of their upright position, they 

 assume it again if still capable of growth. As in heliotropism, GEOTKOPIC CURVA- 

 TURE RESULTS FROM THE INCREASED GROWTH OF ONE SIDE AND THE RETARDED 



GROWTH OF THE OPPOSITE SIDE, and the region of greatest growth is, in general, 

 also that of the greatest curvature. In negatively geotropic organs, growth is 

 accelerated on the side towards the earth ; on the upper side it is retarded. In 

 consequence of the unequal growth thus induced, the erection of the free-growing 

 extremity is effected. After the upright position is again attained, the one-sided 

 growth ceases and the organ continues to grow in an upward direction. 



The process of negative geotropic movement is dependent : (1) upon the vigour 

 of the existing growth ; (2) upon the sensibility of the organ ; (3) upon the fact 

 that the stimulus of gravity works most energetically when the apex of the ortho- 

 tropic organ is removed about 90' from the vertical ; the more nearly the zone 

 capable of curvature approaches this position, the stronger is the motory stimulus ; 

 deviations from this optimal position of stimulation do not result in such striking 

 differences in reaction as are caused by even very small (|) deviations from the 

 vertical ; (4) and also upon the fact that after a stimulus has ceased to act upon a 

 plant, the induced stimulation continues to produce so-called AFTER EFFECTS, just 

 as by a momentary stimulus of light an after-perception persists in the eye. 



