SECT. 11 



PHYSIOLOGY 



309 



seen that there are not only orthotropous 

 organs which place themselves in the direc- 

 tion of gravity, and grow positively geotropi- 

 cally (downwards) or negatively geotropi- f^ 

 cally (upwards), but also plagiotropous 1 

 organs which take up a horizontal or oblique oV^ 

 position. The positions assumed by the — -^^ 

 lateral organs are also — though as a rule 

 not exclusively — determined by gravity. 



All vertically upward growing organs, •whether 

 stems, leaves (Liliiflorae), flower-stalks, parts of 

 flowers, or roots (such as the respiratory roots of 

 Airicennia (Fig. 193), Palms, etc.), are negatively 

 geotropic. AVhen such negatively geotropic organs 

 are forced out of their upright position, they assume 

 it again if still capable of growth. As in photo- 

 tropism, geotropic curvature 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-groMung extremity is 

 effected. After the upright position is again 

 attained, the one-sided growth ceases and the 

 organ continues to grow in an ujJward 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 is most 

 euergetic when the apex of the orthotropic organ is 

 removed about 90^ from the vertical ; the more 

 nearly the zone capable of curvature approaches 

 this jiosition, the stronger is the motory stimulus ; 

 deviations from this optimal position of stimula- 

 tion do not result in such striking differences in 

 reaction as are caused by even very small (^°) de- 

 viations 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 pro- 

 duce so-called after effects. 



These considerations determine the actual 

 course of the directive movement of geotropism, 

 which, as will be seen from the adjoining figure 

 (Fig. 230), does not consist merely of a simple, 

 continuous curvature. The numbers 1-16 show, 

 diagrammatically, diflerent stages in the geotropic 



Fig. 230. — Different stages in tlie pro- 

 cess of geotropic movement. The 

 ligures 1-16 indicate successive 

 stages in the geotropic curvature of 

 a seedling grown in semi-darkness : 

 at 1, placed horizontally ; at 16, 

 vertical. For description of inter- 

 mediate stages see text. (Dia- 

 grammatic.) 



