246 Charles Darwin. 



sesses the power of spontaneous movement, and 

 under nearly all circumstances actually exercises 

 that power. 



There are a great many distinct kinds of move- 

 ment, depending in all cases upon the special ad- 

 vantages thereby gained to the plant. The laws 

 under which these movements take place have re- 

 ceived from him an admirable terminology. Most 

 of them are conditioned either by light, by gravity, 

 by radiation, or by insect agency. 



We thus have of the first class : heliotropism, or 

 movement towards the light ; apheliotropism, or 

 movement from the light ; diaheliotropism, or move- 

 ment at right angles to the source of light ; and 

 paraheliotropism, embracing such movements as 

 screen the plant from excess of light. 



To the second class belong : geotropism, or move- 

 ment towards the earth or into the soil ; apogeo- 

 tropism, or movement contrary to the force of 

 gravity ; and diageotropism, or movement at right 

 angles to the force of gravity. 



The third class embraces the so-called nyctotropic 

 movements of plants by which they appear to sleep, 

 and which prove to be devices for the prevention of 

 excessive radiation of the plants' heat. 



Under the fourth class fall all those wonderful 

 movements which aid the plant in preventing self- 

 and securing cross-fertilisation, a subject of the most 

 absorbing interest, and of which you have already 

 listened to so able a presentation by Professor Riley 

 from the point of view of the entomologist. 



But Darwin's great service has been to show that 



