GROWTH 



117 



is bent under the load of its fruit, but a certain stimulus to 

 which the plant reacts by a spontaneous adjustment of 

 its growing parts. In other words, 

 geotropism is an active and not a 

 passive function, and the plant will 

 even overcome considerable resist- 

 ance in response to it. If a sprouted 

 bean is laid on a dish of mercury 

 covered with a layer of water, as in 

 Figure 254, the root will force its way 

 downward into the liquid, although the mercury is fourteen 

 times heavier than an equal bulk of the bean root substance, 

 and the geotropism of the root must thus overcome a resist- 

 ance equal to at least fourteen times its own weight. 



162. Other Factors. — The direction of growth is influ- 

 enced by many other factors, such as light, heat, contact 



254. — Experiment bhow- 

 irg the root of a seedling 

 forcing its way downward 

 through mercury. 



255- 



-A piece of a haulm of millet that has been laid horizontally, righting itself 

 through the combined influence of contact and negative geotropism. 



with other bodies, and perhaps by electricity. The result 

 of all these forces is an endless variety in the forms and 



