TEE PERCEPTION OF TEE FORCE OF GRAVITY. 535 



for the pressure theory, and can not help us to decide between the two 

 points of view. 



Are there any general considerations which can help us to decide 

 for or against the statolith theory? I think there are — namely, (1) 

 analogy with the graviperceptive organs of animals; (2) the speciali- 

 zation and distribution of the falling bodies in plants. 



Berthold (to whom the credit is due of having first suggested that 

 Dehnecke's falling starch-grains might function as originators of geo- 

 tropic reaction) is perhaps somewhat bold in saying that 'the primary 

 effect of gravity ' as regards stimulation must depend on the passive 

 sinking of the heavier parts. Noll, too, says that Knight's experiment 

 depends on weight, and not the weight of complete parts of the plant- 

 body, but of weight within the irritable structure. I can not see that 

 these downright statements are justified on direct evidence, and I ac- 

 cordingly lay some stress on the support of zoological evidence. It has 

 been conclusively proved by Kreidl's beautiful experiment that in the 

 Crustacean Palcemon the sense of verticality depends on the pressure 

 of heavy bodies on the inside of cavities now known as statocysts, and 

 formerly believed to be organs of hearing. The point of the experi- 

 ment is that when the normal particles are replaced by fragments of 

 iron the Palcemon reacts towards the attraction of a magnet precisely 

 as it formerly reached towards gravity. 



It is unfortunate that Noll's arguments in favor of the existence 

 of a similar mechanism in plants were not at once followed by the 

 demonstration of those easily visible falling bodies, which, in imitation 

 more flattering than accurate, are called statoliths, after the bodies in 

 the statocysts of animals. Personally I was convinced by Kreidl, as 

 quoted by Noll, that here was the key to graviperception in plants. 

 But it was not until the simultaneous appearance of Haberlandt's and 

 Nemec's papers that my belief became active, and this, I think, was the 

 case with others. The whole incident is an instance of what my father 

 says somewhere about the difficulty of analyzing the act of belief. I 

 find it impossible to help believing in the statolith theory, though I 

 own to not being able to give a good account of the faith that is in me. 

 It is a fair question whether the analogy drawn from animals gives 

 any support to the theory for plants. The study of sense-organs in 

 plants dates, I think, in its modern development, at least, from my 

 father's work on root-tips, and on the light-perceiving apices of certain 

 seedlings. And the work on the subject is all part of the wave of 

 investigation into adaptations which followed the publication of the 

 ' Origin of Species.' It is very appropriate that one of the two authors 

 to whom we owe the practical working out of the statolith theory should 

 also be one of the greatest living authorities on adaptation in plants. 

 Haberlandt's work on sense-organs, especially on the apparatus for the 



