68 BOTANY. [CHAP. n. 



shoots depends on the presence of a proper amount of 

 water in the cells ; consequently, when the leaves exhale 

 more moisture than is taken up by the roots the plant 

 withers or droops. The drooping of plants on a very 

 hot day is due to the above cause. During the night, 

 when darkness and a lower temperature retard transpi- 

 ration, the plants revive, as the root continues to 

 take in water, which enters the cells and restores 

 their turgidity. 



Gravitation exercises a powerful influence on the 

 growing parts of plants, the upward direction of the 

 stem and the downward direction of the root being 

 influenced by this force. The action of gravitation on 

 growing parts of plants is called Geotropism. If a 

 seedling plant be placed horizontally the stem curves 

 upwards, and the root downwards ; the former is nega- 

 tively geotropic, the latter positively geotropic. The 

 branches and leaves of plants are also affected by geo- 

 tropism. The cause of geotropism is the unequal 

 growth of the cells on opposite sides of the stem or root, 

 as in the case of heliotropism ; hence we get similar 

 results brought about by distinct forces, and in a state 

 of nature the habit of every plant ia the result of the 

 influence of the various forces to which it is exposed; 

 but as there is a distinct individuality in the life of each 

 kind of plant, so we find that the various forces act in 

 different proportions on different species, the result 

 being a difference of habit as illustrated by the erect 

 branches of the poplar and the spreading branches of the 

 oak. That the above explanation as to habit being the 

 result of the balance set up between the life of a given 

 species and surrounding forces is in the main correct, is 



