2] 



GEOTROPISM 



397 



b. Stems. The central fact that the upward growth of 

 stems is determined by gravity is established by the observa- 

 tions that on the klinostat no definitely 

 directed growth occurs, and that on the 

 centrifugal machine the stem turns in 

 the opposite sense to that of the cen- 

 trifugal pressure. The stem is nega- 

 tively geotropic. As with roots, so with 

 stems, a number of questions now arise : 

 Where is the sensitive region and where 

 the response? At what inclination of 

 the stem is the strongest geotropic cur- 

 vature called forth? What is the im- 

 mediate cause of the curvature? 



As Fig. 108 shows, the response of 

 the stem of a seedling is fundamentally 

 different from that of the radicle. In- 

 stead of the tropism beginning at one 

 point, and continuing there as in the 

 root (Fig. 106), it begins close below 

 the cotyledons of the seedling and passes 

 downwards towards the base as far as 

 growth is still occurring. Response, 

 consequently, takes place along the whole 

 "stretching" region. The sensitive re- 

 gion also, unlike that of the root, is not 

 confined to the tip, but extends along 

 the entire bending stem. 



The position in which the strongest 

 response is incited was believed by 

 SACHS ('79 a , p. 240) to be the horizontal 

 one, and BATESON and DARWIN ('88) 

 have confirmed this conclusion. Their 

 method depends upon the fact that a 

 stem placed horizontally and restrained 

 for several hours from taking the verti- 

 cal position will, upon being released, 

 suddenly spring upwards. In using this method it was found 

 that the stem springs through a greater arc after having been 



. Course of geotro- 

 pism in a plumule. The 

 successive figures 1 to 16 

 indicate successive stages 

 in the geotropic turning of 

 a seedling growing in half 

 darkness. Placed at first 

 horizontal as at 1, the plant 

 has become completely erect 

 at 16. The most rapid growth 

 is just behind the cotyle- 

 dons and diminishes toward 

 the base. The temporary 

 bending beyond the vertical 

 is to be noted. (FromSxRAS- 



BURGER, NOLL, SCHEXCK, 



and SCHIMPER, Textbook 

 of Botany, Macmillan.) 



