194 



PLANT LIFE. 



the different stimuli have received different names, and those 

 names indicate the nature of the stimulus. A plant or an 

 organ is heliotropic when it reacts to the direction of the 

 rays of light falling upon it ; geotropic, when it reacts to the 

 force of gravity ; thermotropic, when it reacts to the presence 

 of a warm body ; hydrotropic, when it reacts to the presence 

 of a moist surface, etc. In each case the plants are said to 

 react positively when the movement is toward the source of 

 the stimulus ; negatively, when the movement is away from 

 the stimulus ; transversely, when it is transverse to the direc- 

 tion of the stimulus. These reactions are to a certain extent 



\ 



FIG. 190. Diagrams representing the transverse heliotropism of leaves of the garden 

 nasturtium (rrofxfolutii). Potted plants were subjected successively to light strik- 

 ing them in the direction shown by arrows. The petioles curved so as to place the 

 blades at right angles to the incident light. After Vochting. 



related to one another, and it will be convenient, therefore, to 

 consider the effect of each stimulus upon the two common 

 forms of plant organs namely, the radial (such as stems and 

 roots) and the dorsiventral (such as leaves). Organs are 

 sometimes physiologically dorsiventral, even though they 

 possess a radial structure ; for example, some sterns behave as 

 dorsiventral organs, although they are perfectly radial in 

 structure. 



285. (a) Heliotropism. Heliotropism is the state of a plant 

 or organ when it is irritable to the direction of light rays. 



