IRRITABLE RESPONSE 231 



response, but this is forcibly prevented. It may be tested as 

 follows : 



SUGGESTED EXPERIMENT. Prepare a piece of grass-stem as for the pre- 

 ceding experiment, but fasten it throughout horizontally in a way to keep 

 the lower end wet, and to leave the node free; this may be accomplished 

 by use of a block arranged as shown by figure 67, to which the stem is at- 

 tached by crossed pins. Leave it for a day or two under favorable condi- 



FIG. 67. ARRANGEMENT FOR STUDY OF GEOTROPISM OF NODES; X 



tions for growth, preferably in a moist-chamber, then carefully observe the 

 node. 



Select a potted plant with an actively growing vertical stem, and by 

 loops of thread tie this, quite to its tip, to a glass rod thrust into the earth. 

 Place it horizontally for a few hours, then quickly cut the threads, and note 

 the result. 



The foregoing studies help to explain the mechanics of geo- 

 tropic responses, upon which the student should extend his knowl- 

 edge by study of the literature. He should include here, also, 

 an inquiry into the effects produced upon the geotropic response 

 by external conditions. It remains now to consider the exact 

 way in which gravitation produces these results. Observing 

 that the responses are localized in somewhat definite areas, one 

 of the first questions to be settled is whether gravitation produces 

 its effect directly upon the responding parts, or whether it may 

 operate through other parts. The experimental testing of this 

 matter presents so much difficulty, that the student may be 

 obliged to turn for answer to the literature; but it can be 

 answered thus: 



SUGGESTED EXPERIMENT. Selecting the root as the part most practi- 

 cable for experiment, and noting that its tip contains two prominent areas, 

 the growing point and the elongating (which is the responding) zone, we 

 proceed to test the possibility that gravitation may affect the former rather 

 than the latter directly. This can be done by DARWIN'S method of cutting 

 away the tip, leaving the elongating zone, and noting whether this zone 

 can then respond geotropically; but as the method produces a profound 

 functional change, a much better way is CZAPEK'S. In this the roots are 



