230 PLANT PHYSIOLOGY 



The power exerted in geotropic response, together with the 

 very obvious lengthening which accompanies the bending of 

 roots and stems, suggests that the process must be ultimately 

 connected with Growth, and the problem arises: 



Are ordinary geotropic responses dependent upon growth? 



This may be determined in either of two ways: first, by arrang- 

 ing all conditions for a geotropic response, but withholding some essen- 

 tial condition of growth (e.g., oxygen, of favorable temperature), and, 

 second, by observing whether the place of response corresponds with 

 the place where growth is occurring. The first the student may 

 readily plan for himself (with suggestions from DETMER, 448) and 

 the second may be arranged thus : 



EXPERIMENT. Place a strong-growing seed (e.g., Horse Bean) 

 in a wire-and-sphagnum cage and, when the root is 1-2 cm. long, 



FIG. 66. CONVENIENT ARRANGEMENT FOR SACHS' GEOTROPIC FORCE EXPERI- 

 MENT; X i- 



mark it along one side by the roller space-marker (page 197); then 

 place it horizontal, marks upward, in a suitable moist-chamber, and 

 note the relation between the growing and the responding parts. 



Select a potted plant with slender-growing upright stem, mark it 

 with the roller space-marker, place it with stem horizontal, marks 

 underneath, and note effect of the response upon position of the marks. 



The localization of response here shown suggests an inquiry 

 as to whether this is confined to growing tips and may not occur 

 elsewhere, a question which can partly be answered thus: 



SUGGESTED EXPERIMENT. Select a growing grass-stem, or the creeping 

 greenhouse Wandering Jew, and cut out a piece some 10 cm. long with a 

 node in its middle. Place the lower end in water, and lay it horizontally, 

 with the node and upper end free, which can be accomplished by thrusting 

 it into a vial filled with wet sphagnum, or else, and better, by the arrange- 

 ment of figure 67, with the pins on the left of the node omitted. Give favor- 

 able conditions for growth, preferably in a moist-chamber, and observe the 

 resultant effects. 



Observation of this experiment is likely to suggest a question 

 as to the result in cases where all conditions are favorable for a 



