402 Evolution and Adaptation 



which the plants grow, and in this way insure the lodgement 

 of the seeds. Here we have an excellent example showing 

 that the negative heliotropism of the flower stalk could 

 scarcely have been acquired by slight changes in the final 

 direction, for only the complete change is useful to the plant. 

 Intermediate steps would have no special value. 



As has been pointed out in the case of the seedling plant, 

 the main stem responds positively and the roots negatively 

 to gravity. In addition to this, the lateral position taken by 

 the lateral roots and branches and by underground stems are 

 also, in part, due to a geotropic response. In this case also 

 the effect is produced by the increased growth on the upper 

 side when the response is positive, and on the lower when it 

 is negative. Leaves also assume a transverse position in 

 response to the action of gravity, or at least they make a 

 definite angle with the direction of its action. 



The most striking case of geotropic response is seen in 

 plants that climb up the stems of other plants. The twining 

 around the support is the result of a geotropic response of 

 the sides of the stem. The young seedling plant stands at 

 first erect. As its end grows it begins to curve to one side 

 in an oblique position, and this is due to an increase in growth 

 on one side of the apex of the shoot. As a result the stem 

 bends toward the other side. Not only does the end " sweep 

 round in a circle like the hands of a watch," but it rotates on 

 its long axis as it revolves. As a result of this rotation "the 

 part of the stem subjected to the action of the lateral geo- 

 tropism is constantly changing ; and the revolving movement 

 once begun, must continue, as no position of equilibrium can 

 be attained." This movement will carry the end around any 

 support, not too thick, that the stem touches. 



Most climbers turn to the left, i.e. against the hands of a 

 watch, others are dextral, and a few climb either way. 1 



1 These cases recall the spiral growth of the shell of the snail, but the spiral 

 in the latter is due to some other factor. 



