402 EFFECT OF GRAVITY UPON GROWTH [Ch. XV 



the end of the latent period the geotropism will occur in the 

 normal manner. The impulse once transmitted, the geotropic 

 function of the root tip is finished. Again, Sachs ('74") laid 

 a stem in a horizontal position for an hour or two, until up- 

 curving had commenced near the tip. The tip was now held 

 horizontal, and for from one to three hours, during which 

 stretching took place, the horizontal part grew horizontally, 

 while the new tip turned zenithwards. The tip in turning de- 

 termines the direction of the incipient stretching zone, and the 

 stretching continues in this direction without regard to subse- 

 quent changes in position of the stem. The stretching is inde- 

 pendent of anj^ control on the part of the tip. 



Other conditions of geotropic after-effect have been deter- 

 mined by WoRTMAKi^ ('84). He treated a stem according to 

 Sachs' method, and placed it in a chamber filled with hydro- 

 gen. Growth ceased. After oxygen was readmitted the tip 

 turned up at once, but no after-effect appeared. This had 

 been annihilated by the absence of oxygen. If, however, the 

 stimulated sprout was placed vertically in de-aerated water, 

 there was an after-effect, but no geotropic response. So we 

 may conclude that, in the absence of oxygen, the geotropic 

 response will not occur : and that an after-effect may occur if 

 the stretching tissue is bathed with water for imbibition, but 

 not under other conditions. 



Summary 



The rate of growth seems to be little affected by the absence 

 of gravity or the abnormal condition of its action, except in 

 some of the higher fungi, where the removal of gravity hastens 

 growth, and its abnormal direction retards. Geotropism is 

 found only in sessile organisms. In roots the turning occurs 

 in the region of greatest growth, and the tip alone is sensitive 

 to gravity's action. The response is preceded by a latent 

 period, and is strongest after the root has been placed hori- 

 zontal ,■ it is due, in the case of both roots and stems, to an 

 acceleration of growth on one side and its retardation on the 

 other. In plumules the turning begins near the tip and pro- 

 ceeds towards its base ; the whole bending region is responsive 



