464 LECTURE XVIII. 



geotropically when the angle made by their long axes with 

 the vertical lies between 90 and O in the upper quadrant, the 

 response being greatest when the angle is o. 



The question as to the validity of the assumption of 

 Diageotropism as a special form of geotropic irritability will 

 be discussed in connexion with the geotropic phenomena 

 presented by dorsiventral organs. 



Just as in the case of the action of light upon growing 

 organs, so also in that of the action of gravity, the response of 

 the organ to the action of the stimulus, as indicated by com- 

 mencing curvature, is not at once exhibited, but a longer or 

 shorter " latent period " precedes it. Sachs observed, for 

 instance, in the case of slender stems, that when they were 

 laid horizontal, the upward curvature could first be detected 

 at the end of J 2 hours. Similarly Darwin observed that 

 the stem of a young plant of Cytisus fragrans began to curve 

 upwards after having been in a horizontal position for three 

 quarters of an hour. Again, as in the case of the action 

 of light, so also in that of gravity, the induced effect per- 

 sists after the stimulus has ceased to act ; this is, in fact, an 

 induction of geotropism similar to the induction of helio- 

 tropism. Sachs observed that when shoots were kept in a 

 horizontal position until they began to exhibit a distinct up- 

 ward curvature, and were then either placed vertically or were 

 turned through 180 round their own axes so that their position 

 was reversed, the curvature continued to increase during the 

 next I 3 hours. In roots this persistent after-effect, this 

 induced geotropism, is less marked : Sachs was in fact unable 

 to detect it, but Frank and Ciesielaki state that they have 

 done so. 



Now that we know something as to the conditions of the 

 action of gravity upon orthotropic radial organs, we may pass 

 to the consideration of the manner in which these organs per- 

 form geotropic curvature. It is, like heliotropic curvature, 

 a phenomenon of induced heterauxesis. It is effected in a 

 rapidly growing organ by the greater elongation of the side of 

 the organ which becomes convex as compared with that of 

 the side which becomes concave ; when, however, the organ is 



