Power to Adjust Parts to Surroundings 247 



angles as to give to these plants their characteristic outlines. 

 Furthermore it also explains how stems can so readily recover 

 their natural positions when the plants are over-turned, whether 

 by accident, or by intention in experiment. 



We must next turn attention to this crucial matter of the 

 up-and-down line. Now there is in this world only a single 

 determinant thereof, and that is the attraction of gravitation, 

 which forever is drawing all objects towards the center of the 

 earth. Gravitation, therefore, would seem to be the stimulus 

 used by the plant in assuming the positions we are considering. 

 In other words, the parts of the plant are geotropic; — and all evi- 

 dence confirms this conclusion. 



The wide use of gravitation as a stimulus raises at once the 

 question as to the physiological value of gravitation to the plant. 

 In itself, however, it has no value, so far as anyone has been able 

 to discover. The plant has no object at all in sending roots 

 downward and shoots upward merely to have them down and 

 up; but it happens that down is the direction of moisture and 

 minerals, which roots need, and up is the direction of light, which 

 shoots need. No doubt those parts could be guided in the need- 

 ful directions by their hydrotropism and phototropism respect- 

 ively, but gravitation has this advantage over moisture and 

 light as a stimulus, that, while happening to act in the suitable 

 direction, it is present unvaryingly at all times, whereas light 

 and moisture are most variable in quantity, and sometimes- 

 absent altogether. This is especially true of light, which is missing 

 at night when gro^\i;h is most active and the guiding stimulus 

 most needed. Gravitation, therefore, is neither a direct, nor a 

 foster stimulus, like those we have already considered, but a 

 substitute stimulus, adopted by the plant in place of other 

 stimuli because it acts better than they. The use of the compass 

 has just the same advantage over observation of the sun and the 

 stars, which would also take the sailor to his port ; for the compass 

 is constant in its action, while the sun and the stars not only 



