'94 



PLANT r.IFE. 



the different stimuli have received different names, and those 

 names indicate the nature of the stinudus. A idant or an 

 organ is helioinipic when it reacts to the dire<tion of tlie 

 ra\'s of light falling upon it ; geo/ropic, when it reacts to the 

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

 of a warm body ; hvdro/ropic, when it reacts to the presence 

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

 react posiiivelv when the movement is toward tlie source of 

 the stiumlus ; ?iegaliveh\ when the movement is away from 

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

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



Fir.. 190. — Diagrams representing tlie transverse lieliutropism of leaves of the garden 

 nasturtium { I'roptFoium). Potted plants were subjected successively to light strik- 

 ing them in the direction shown by arrows. 'J^lie petioles curved so as to place the 

 blades at right angles to tlie incident light. — .^fter Viichting. 



related to one another, and it will be con\enient, therelore, 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). ( )rgans are 

 sometimes physiologically dorsiventral, even thoiigli they 

 p)0ssess a radial structure ; for example, some stems licha\-(* 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 dii-ection of light rays. 



