256 BOTANY 



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 geotro- 

 pism, renders possible the winding of stem-climbers. 



Negative Geotropism. — All vertically upward growing organs, 

 whether stems, leaves (Liliiflorae), flower-stalks, parts of flowers, or 

 roots (such as the respiratory roots of Avicennas, 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, geotropic curvature rksults 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 135° from its position of geotropic equilibrium ; 

 the more nearly the zone capable of curvature approaches this position, the 

 stronger is the motory stimulus ; (4) and, also, upon the fact that after a stimulus 

 lias ceased to act upon « 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. 



These considerations determine the actual course of the directive movement of 

 geotropism, which, as will be seen from the adjoining figure (Fig. 202), does not 

 consist merely of a simple, continuous curvature. The numbers 1-16 show, dia- 

 grammatieally, different stages in the geotropic erection of a seedling grown in semi- 

 darkness and placed in a horizontal position (No. 1). The growth in the stem of 

 the seedling is strongest just below the cotyledons, and gradually decreases towards 

 the base. The curvature begins accordingly close to the cotyledons, and proceeds 

 gradually down the stem imtil it reaches the lower, no longer elongating, portions. 

 Through the downward movement of the curvature, and partly also through the 

 after-effect of the original stimulus, the apical extremity becomes bent out of the 

 perpendicular (No. 7), and in this way a curvature in the opposite direction takes 

 place. Thus, under the influence of the stimulus, the stem bends backwards and 

 forwards, until, finally, the whole growing portion becomes erect and no longer 

 subject to the one-sided action of the geotropic stimulus. (A good example of ex- 

 cessive curvature beyond the vertical is afforded by vigorously growing aerial 

 shoots of Rippuris vulgaris. ) Analogous phenomena to those here described are 

 exhibited in the case of all paratonic curvatures of growth. In case of a different 

 distribution and rapidity of growth, or of the unequal sensitiveness, rigidity, or 

 thickness of the organ, as well as in the case of a difference in its position at the 

 commencement of the curvature, the process, as indicated in the figure, is corre- 

 spondingly modified. 



