THE SIGNIFICANCE OF TROPISMS 121 



enlargement of the latter increases the mass of reacting molecules. A 

 shifting of the contact surfaces would act in the same manner. Finally, 

 a third possibility remains which could perhaps be realized in plant 

 roots and stems. If in the geotropically sensitive elements two masses 

 of different specific gravity are present, only one of which reacts to the 

 flowing sap in the center or the periphery of the stem, the cells of the 

 upper side of a stem which is laid horizontally will acquire a different 

 rate of reaction from those of the lower side, because in the former the 

 specifically heavier substances are directed toward the center of the stem, 

 while in the latter the specifically lighter ones are directed toward the 

 center. Consequent!}', one side will grow faster than the other, and 

 thence the geotropic bending. 20 In the frog's egg, we can actually 

 directly demonstrate the existence of two substances of different specific 

 gravity and can study their behavior, since in this case they are of dif- 

 ferent color. 



In animals it has been observed that orientation toward the center 

 of gravity of the earth often becomes less compulsory when the inner ear 

 has been removed. Mach first pointed out the possibility that the 

 otoliths are responsible for this. They might press upon the end-organs 

 of the sensory nerves and every change of pressure might cause a cor- 

 rection of the position of the animal. It is generally assumed that this 

 view has been verified by experiment. I cannot, however, agree with 

 this, although I once described experiments which seemed to support 

 Mach's otolith theory. I had found that when the otoliths of the 

 inner ear of the shark are scraped out with a sharp spoon the normal 

 orientation of the animal suffers ; but if the otoliths are simply washed 

 out from the internal ear by a mild current of seawater the orientation 

 of the animal does not suffer so easily. 



In the latter case, the doubt is present as to whether all the otolith 

 powder has been removed from the ear. The matter was decided by 

 experiments on flounders, which have only a single large otolith which 

 can easily be removed from the ear. E. P. Lyon carried out these 

 experiments, which showed that no disturbance of the orientation 

 resulted from this operation. We may conclude, therefore, that in my 

 experiments of scraping out the otoliths a disturbance of the orienta- 

 tion occurred because by this means the nerve endings in the ears were 

 injured. AVe have, therefore, no right to say that the orientation of 

 animals in relation to the center of gravity of the earth is regulated by 

 the pressure of the otoliths upon the nerve endings, but that this regula- 

 tion takes place in the nerve endings themselves, and probably, indeed, 

 as a result of the existence there of two different phases of different 

 specific gravity which react upon one another. Through the change of 



20 Chapter Tropismen in " Vorlesimgen iiber die Dynamik der Lebens- 

 erscbeinuiigen. ' ' 



VOL.LXXIX. — 9. 



