162 THE AMERICAN BOTANIST 



secreted by the stigmas and styles. If the organ bends so as 

 to grow towards the source of the substance it is positively 

 chemotropic to that substance; and if it turns away it is neg- 

 atively chemotropic. In elongated organs, such as roots, the 

 movement is a matter of growth. The curvature is brought 

 about by the retardation on one side or the acceleration on the 

 other; or by both together. The side on which the retarda- 

 tion occurs becomes concave, while the other is convex. If 

 a root is positively chemotropic towards a substance, the side 

 coming in contact with most of the substance, will be retarded 

 in growth and become concave, as the substance diffuses thru 

 the medium in which the root is growing. If the root is neg- 

 ativelv chemotropic, acceleration of growth takes place with 

 an accompanying convexion of the root. Thus the root tip is 

 turned either toward or away from the source of the diffusing 

 substance and tlie bending continues until all sides are equally 

 stimulated. 



MINOR TROPISMS 



There are other movements in plants which, although of 

 equal importance with the aforementioned are generally con- 

 sidered modifications thereof. 



Aerotropism is a term applied to the power of responding 

 to gases, particulary oxygen, in the same way as chemotro- 

 pism. Illustration of this is found in water plants having 

 ftoating leaves. The length of the petioles is altered in accor- 

 dance with the depth of the water. The petioles continue to 

 grow until ihc loaves reach a supply of free oxygen, when they 

 stop. This case illustrates the fact that stimuli serve as sig- 

 nals to stop a process as well as to guide it and such are un- 

 doubtedly very important in controlling the various processes 

 of growth. 



