PLANT RESPONSES 



the reaction of the organism may be altered, by an ex- 

 ternal agent which is not the stimulus to response. The 

 rhizomes of Adoxa and Circcea, in normal surroundings 

 growing horizontally because of their diageotropism, be- 

 come positively geotropic when exposed to light, and 

 grow toward the earth. The rhizomes of some other 

 plants, in similar treatment, become negatively geotropic. 

 Experiments have shown that these changes of direction 

 in growth in these rhizomes are not heliotropic, but due 

 to a change in disposition toward gravitation. Light does 

 not cause the bending, but changes the physiological 

 state, and gravitation causes the bending. Another il- 

 lustration of a change in disposition due to external 

 agency is furnished by the behavior of seedling peas and 

 vetches which become more heliotropic when the atmos- 

 phere contain- small quantities of illuminating gas, or of 



cean copepods and other organisms, are rendered more 

 responsive to light by putting a weak solution of carbon 

 dioxide, acetic or hydrochloric acid, in the water in which 

 they are swimming. This author suggests that this influ- 

 ence on the physiological state- is due to the acid pre- 

 venting the formation in the organism of some anti-posi- 

 tive substance, normally generated. This hypothesis may 

 not be supported by future study, but there is some evi- 

 dence offered in its behalf, and it is a worthy attempt to 

 come a step nearer to the intimate processes. 



The "trial and error" hypothesis as applied by Jen- 

 nings to some phenomena of behavior in protozoa is de- 

 fined by him as "the selection through varied movements 

 of conditions not interfering with the physiological proc- 

 esses of the organism." 4 The term itself, "trial and 

 error," has lately been abandoned by Jennings, 6 but the 

 value of the hypothesis as describing behavior he still 

 maintains. It is plain to see that the so-called direction 



