i 9 4 



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 ; geolropic, 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 



n n u 



Fig. 190. — Diagrams representing the transverse heliotropism of leaves of the garden 

 nasturtium {'fropceolum). 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 stems 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. 



