866 



Mr. F. Darwin. 



[Mar. 6, 



jected to vibration for about 8 — 20 minutes. The control and experi- 

 mental boxes are then placed on a klinostat to avoid further gravitational 

 stimulus, and the angular curvature estimated after a few hours. The 

 general result is clear, viz., that the plants subjected to vibration are 

 more strongly curved. In other words, that vibration increases the 

 gravitational stimulus. 



Precautions and Sources of Error. 



In performing the experiment it is necessary to take scrupulous 

 care that the control and experimental plants receive similar treat- 

 ment. In the case of both it is necessary to cement the plants 

 into their boxes without allowing any possibility of geotropism being 

 induced before the boxes are fixed to the tuning fork. In all the 

 later experiments the plants were cemented into their places vertic- 

 ally and received no gravitational stimulus until horizontal on the 

 fork, or on the place prepared for the control plants, as the case 

 might be. Any error arising for the want of this precaution in the 

 earlier experiments was equally divided between the experimental 

 and control plants. 



The most serious source of doubt and error is the tendency in 

 the grass seedlings to nutate in various directions ; all that can be 

 said is that the error in question is equally applicable to the experi- 

 mental and control plants. Sorghum saccharatum is especially liable to 

 nutation, and had to be discarded as material. 



A theoretical source of error arises from the fact that the movement 

 of a vibrating rod being part of a curve there must be a generation of 

 centrifugal force parallel to the rod and towards its free end. If the 

 force were sufficient it would cause a displacement of the starch in the 

 cotyledons of the grass seedlings, and this would affect those seedlings 

 whose apices were directed towards the free end of the vibrating rod, 

 precisely as if they were in an oblique position, the apices being down- 

 ward. Now this position is known* to be more favourable to geo- 

 tropism than horizontality, therefore the plants on the vibrating rod 

 could not be fairly compared with the motionless specimens. But we 

 were unable to see any displacement of starch in seedlings exposed for 

 short periods to the vibration of the tuning fork. By a rough method 

 I estimated the centrifugal force in my experiments as about 0'2 g. 

 This is a force quite sufficient to affect the starch grains, if enough 

 time is allowed ; but it is hard to believe that for periods of 8 — 20 

 minutes it could have any serious effect. However this may be, I 

 avoided, in the later experiments, all possible favouring of the experi- 

 mental plants, by placing them with their apices towards the base of 



* Czapek, ' Priugslieim's Jalirbiicher,' vol. 27, 1895, p. 283. 



