2] HYDROTROPISM 357 



The fact of its existence being granted, the conditions of its 

 occurrence were carefully studied. Thus DARWIN ('80, Chap- 

 ter III) investigated the locus of the irritable protoplasm. 

 Some of the young bean-radicles were coated for the distance 

 of a millimetre or two from the apex with a mixture of olive 

 oil and lamp black in order to exclude the moist air. Such 

 showed almost no hydrotropic movements. Killing the tip by 

 caustic produced the same result. Thus the terminal two 

 millimetres or so include the irritable protoplasm. 



This conclusion was disputed by WIESNER ('81, p. 133) and 

 DETLEFSEN ('82) on the grounds that on the one hand coating 

 or killing the tip introduced abnormal conditions to which the 

 failure of hydrotropism might be ascribed, and, on the other 

 hand, after the tip of the root was cut off a curving might still 

 occur. However, a very careful review of the subject with 

 new experiments by MOLISCH ('84), a pupil of WIESNER, con- 

 firmed DARWIN'S conclusion. Thus MOLISCH covered all of 

 the radicle excepting the terminal 1 to 1.5 mm. with wet 

 paper. This upper part could then hardly be irritated by an 

 unequal distribution of moisture in the environment. Never- 

 theless, when a strip of moist filter-paper was placed opposite 

 the tip the hydrotropic response occurred. The response must 

 then have been due to a stimulus received exclusively at the 

 tip, and it may be concluded that the tip alone is stimulated 

 by moisture. 



Now, although only a millimetre or two of the tip is irritable, 

 the response of bending takes place some distance, 7 to 28 mm., 

 behind the tip, nearly in the region of maximum growth. 

 Thus sensitive and responsive regions do not coincide there 

 is a transmission of stimuli. The facts that the hydrotropic 

 response occurs in the region of rapid growth and that at the 

 minimum temperature of growth response no longer occurs, 

 indicate clearly that the hydrotropism of roots is not the result 

 of mechanical loss of turgescence on one side, but that it is on 

 the contrary a growth phenomenon a localized growth which 

 is a response to a stimulus. 



2. Rhizoides of Higher Cryptogams. While it is a priori 

 probable that the rhizoids of hepatics should react like the 

 roots of phanerogams, MOLISCH desired to demonstrate the fact 



