30 GENERAL BOTANY 



stimuli discussed above. Thus, the arching of the epicotyl of 

 the pea seedling, already referred to, is not due to external 

 stimuli, but to internal conditions which bring about unequal 

 growth of the organ on two opposite sides, resulting in an 

 adaptive curvature. Some of the adaptive movements of 

 flowers and fruits are apparently due to similar internal con- 

 ditions which appear to affect the organism like external stimuli. 

 These latter are, however, special cases applying to particular 

 plant groups or species and do not affect the general principle 

 that plant movements are usually effected by unequal growth 

 in response to external stimuli. 



In designating the kinds of stimuli and the nature of the 

 response of plant organs to them botanists have adopted certain 

 terms which are useful in designating stimulus and response as 

 applied to any given organ of a plant. Thus, organs may be 

 said to be protropic when they grow toward the source of the 

 stimulating force, and apotropic when they grow away from 

 such a source. Protropic and apotropic organs will evidently 

 have their main axes parallel with the direction from which 

 a given stimulus acts. Diatropic organs, on the contrary, do 

 not grow toward the source of a directive force, but place their 

 main axes at some angle to the direction of a stimulating agent. 

 If we apply these definitions to the pea seedling (Fig. 13), the 

 main root would be protropic and the stem apotropic with 

 reference to gravity. The secondary roots would, however, be 

 diatropic with reference to gravity, and the leaves diatropic 

 with reference to light. If it is desired to combine the name 

 of the stimulus with the nature of the response, other terms 

 .may be combined with protropic, apotropic, and diatropic. 

 Thus, organs stimulated by gravity are said to be geotropic; 

 if stimulated by light, they are designated as phototropic. 

 Progeotropic, apogeotropic, and diageotropic may therefore be 

 used to indicate the responses to a gravity stimulus, on the 

 one hand, and prophototropic, apophototropic, and diaphototropic to 

 indicate the responses to a light stimulus, on the other. Similar 

 combined terms are used in connection with other stimuli, but 

 these need not be considered in our brief account. 



