growth occurs on that side, causing the stem 

 to bend away from the site of application; 

 but under similar treatment, the root bends 

 toward the side where the extra auxin has 

 been applied. 



Tropisms. In both plants and animals the 

 direction of many movements is determined 

 by the direction from which the stimulus 

 impinges upon the body. Stems generally 

 bend toward a steady source of light, but 

 roots turn away, and oppositely directly re- 

 actions are obtained when gravity is the 

 stimulus (Figs. 12-35 and 14-5). Regardless of 

 how these responses are effected, oriented 

 movements, in which the direction is deter- 

 mined by the orientation of the stimulus, are 

 called tropisms. 



The Responses of Higher Plants - 263 



due to an inhibiting action of auxin upon 

 the growth of the cells on this side of the 

 root. Just how an unequal distribution of 

 the growth substances occurs has not been 

 demonstrated, but without auxin no curva- 

 ture takes place. If the meristem of the grow- 

 ing point is removed by decapitating the 

 rootlet about 1 millimeter above the tip, the 

 responsiveness of the root to gravity is abol- 

 ished. 



The negative geotropism of the stem is 

 also effected on a similar basis. In the case of 

 the stem, however, auxin stimulates growth. 

 Greater concentrations of auxin are con- 

 veyed to the lower side of the stem, and 

 consequently the stem bends upward rather 

 than downward (Fig. 14-6). 



Fig. 14-5. Negative geotropism of a bean stem; 

 successive photographs made on same plate at inter- 

 vals of 45 minutes. 



Movements directed toward or away from 

 the source of stimulation are designated, re- 

 spectively, as positive or negative tropisms. 

 Accordingly, one speaks of the positive photo- 

 tropism of leaves and stems, the positive 

 geotropism and hydrotropism of roots, the 

 negative geotropism of stems, and the nega- 

 tive phototropism of roots. 



In plants most tropisms are growth move- 

 ments, in which the curvature results from 

 an unequal distribution of auxins to the 

 sides of the growing part. For example, when 

 a primary root is placed in a horizontal 

 position (Fig. 14-6), gradually a curvature 

 develops as the rootlet grows longer, and 

 finally the tip turns straight downward, to- 

 ward the gravitational center of the earth. In 

 this response it can be proved that more 

 auxin is conveyed to the cells on the lower 

 side of the root, and that the curvature is 



Fig. 14-6. These tropisms, both the positive geo- 

 tropism of the root and the negative geotropism of 

 the stem, are growth movements, determined by 

 auxin. Note, however, that a higher concentration on 

 the lower side of the roof retards local growth; whereas 

 the same condition in the stem accelerates growth. 



Likewise the positive phototropism of the 

 stem and the negative phototropism of the 

 root are explained by the effects of light 

 upon the auxin distribution in the plant. 

 The growing point sends more auxin to the 

 darker side of the stem or root, although 

 why this occurs has not been demonstrated. 

 In the stem, the stimulating effect of the 



