122 PLANT PHYSIOLOGY. 



EXPERIMENT 49. This may be determined by so marking a 

 geotropically-responding root that the place where growth is most 

 active becomes evident ; this has already been done for another 

 purpose in Experiment 36, but it should be here repeated with the 

 marked root placed horizontally. Along with it should be placed 

 another similar root also fixed horizontally, but marked with rings. 

 Observe results after a few hours. 



A correlated question of much importance is this : If response is pre- 

 vented in one place, can it take place in another ? Experiment upon this 

 subject is not easy ; but I have had fair success by using horizontally- 

 placed roots with small glass caps made just long enough to cover the 

 tip and the usual growth zone (following the general method of Czapck 

 mentioned in the next section)- 



84. In what part of the root does the perception of the gravita- 

 tion-stimulus take place ? 



This may be determined by means of the extremely beauti- 

 ful and conclusive experiment devised by PfefTer and Czapek,"' 

 described by the former in the Annals of Botany, 8, 317. The 

 method is described also by Darwin and Acton, 174, but is 

 rather too difficult for use here. 



85. Are there cases in which the geotropic response is localized 

 in specially differentiated structures ? 



Answer by Experiment 50. 



EXPERIMENT 50. Select two fairly well-grown grass stems (or 

 stems of a greenhouse Tradescantia), and cut from each a piece 10 

 to 12 cm. long, containing at about its middle one of the swollen 

 nodes. Fill a small flat wide-mouthed bottle with wet loose sphag- 

 num. Push the lower end of one of the pieces of grass horizon- 

 tally into this (Fig. 33). Push the two ends of the other piece into 

 glass tubes brought close enough to one another to allow only the 

 node and the bottom of the stem to remain uncovered ; bind these 

 tubes with rubber bands to another piece of tubing so they will not 

 bend with the stem, and thrust the lower end into the sphagnum, 

 which will keep both pieces wet. Cover with a bell-jar and darken 

 (to eliminate possibility of light-responses), and observe results 

 particularly upon the form of the node. 

 Another somewhat different way of holding the node in position is 



* Czapek's original paper is in Jahrbucher fur wissenschaftliche Botanik, 27, 

 243 ; see also 35, 313. 



