CHAP, ii.] CHEMISTRY, ETC., OF PLANT LIFE. 69 



shown by the variation of habit that can be induced in a 

 plant when intentionally placed under unusual conditions 

 relative to the forces that under normal conditions collec- 

 tively determine its habit. When seeds are caused to 

 germinate on a horizontally-placed rotating plate, the 

 plantlets are placed under the influence of centrifugal 

 force, a factor that exerts no influence on them in a 

 state of nature ; but under these artificial conditions 

 this force at once disturbs the previous balance arrived 

 at between the plant and its surrounding potent forces, 

 and we find that the 'rootlets follow the centrifugal force 

 and grow outwards, also downwards, due to the influence 

 of geotropism ; the stem, on the contrary, grows towards 

 the centre of the plate and also upwards. When a 

 number of plates are used that rotate at different speeds 

 different directions can be given to the root and stem 

 depending on the predominance given to centrifugal 

 force or gravitation, the two dominant external forces in 

 the experiment. Other external influences also possess 

 the power of neutralizing the influence of geotropism as 

 exercised under normal conditions. Thus, when seeds, 

 as those of the bean, are grown in a wire sieve filled 

 with damp sawdust, the main roots are at first positively 

 geotropic and consequently grow downwards, but after 

 passing through the bottom of the sieve into dry air 

 they bend upwards again and enter the sawdust, being 

 drawn by the water, which, under these exceptional 

 conditions, completely neutralizes the action of geo- 

 tropism. This attracting influence of water is termed 

 Hydrotropism. 



The influence exercised by the vegetable kingdom on 

 surroundings in performing those functions necessary for 



