232 PLANT PHYSIOLOGY 



made to grow into bent tubes of such construction that the tip and elon- 

 gating zones may be bent at right angles to one another so that one at a time 

 may be exposed to one-sided action of gravitation. Unfortunately this 

 beautiful experiment is difficult, but directions are accessible in Darwin 

 and Acton, 174. Compare also Pfeffer in Annals of Botany, 8, 1894, 

 317, and in Czapek's own papers in Jahrbucher fur wissenschaftliche 

 Botanik, 27, 1895, 243, and 35, 19°°, 3*3- 



In connection with this experiment the student should in- 

 form himself as to our knowledge of the method of conduction 

 of stimuli from cell to cell, including the possible existence of 

 conducting structures. 



The facts that plant parts grow away from and at various 

 angles to gravitation as readily as towards it, that, as shown by 

 the just-preceding experiment, the responding zone need not 

 be the part which gravitation affects in causing the response, 

 and that a response may occur long after the stimulus has ceased 

 to act, all combine to prove that gravitation in its relation to 

 the geotropic response does not act directly, that is, through 

 weight of the parts. Nor are the phenomena by any means 

 those which are characteristic of a tonic relation (page 70). 

 There remains only one possibility, that gravitation is acting 

 simply as a guide or signal of direction, that is, as a signal stimu- 

 lus, and all the facts combine to sustain this conclusion. This 

 leads to a further problem, precisely how, that is, by what phys- 

 ical method, the gravitation stimulus is perceived, or impresses 

 itself, upon the sensitive protoplasm of the plant. As a first 

 step toward solving this we ask whether there is any way by 

 which gravitation can be replaced by some other force of equal 

 power whose effects we can observe. Fortunately we have such 

 a force in centrifugal force, and hence this problem: 



What effect is produced upon the directions of young growing 

 parts if gravitation is replaced by centrifugal force? 



Experiment. To the margin of the disc of a centrifuge attach 

 strong germinating seeds {e.g., Corn, Horse Beans); arrange to keep 

 them revolving rapidly in a vertical plane in a suitable moist-cham- 

 ber, and observe the resultant positions of the young parts. 



Centrifuges. These must be of such construction as to revolve rapidly 

 enough to give the seeds a centrifugal throw of some grams, and yet not 



