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. Consequently, one side will grow faster than the other, and thence the geotropic bending. 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 different 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 correction 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 orientation occurred because by this means the nerve endings in the ears were injured. We 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 regulation 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
- Chapter Tropismen in "Vorlesimgen über die Dynamik der Lebenserscbeinungen."