THE TROPISM THEORY OF JACQUES LOEB 345 



though the necessary conditions are present, the tropisms occur 

 in a way that openly contradicts the theory. 



2. In the second place, it will be shown that even in cases 

 which are apparently in agreement with the theory, the theory 

 cannot furnish an adequate explanation of the tropisms. 



3. Finally we shall set forth certain arguments which must 

 be brought against the theory on biological grounds. 



The examples that are used in the following arguments deal 

 exclusively with heliotropism and geotropism, for to these be- 

 long the most numerous and the most exact phenomena of the 

 whole subject. Whether chemotropism and especially thermo- 

 tropism belong at all in this category is doubtful, in my opinion; 

 galvanotropism certainly is a pure laboratory product, which for 

 the biologist is of no interest, in so far as he wishes to study 

 the animal, and not merely the chemical properties of protoplasm. 



CASES OF TRUE TROPISMS IN WHICH THE CONDITIONS 

 PREMISED BY THE THEORY ARE ABSENT 



Tropisms without rays of energy. — I repeat again that by 

 tropism is understood an involuntary movement definitely di- 

 rected with respect to a source of energy. Such a phenomenon, 

 notably, is the geotropism of certain aquatic invertebrates. It 

 is either positive, making the animal creep perpendicularly 

 downward, or it causes a horizontal movement, (diatropism) 

 at right angles to the axis of gravity, the result of which for 

 the organism is the so-called maintenance of equilibrium. In 

 the majority of these cases, and to these we shall confine our- 

 selves, the geotropism is bound up with definite sense organs, 

 the so-called statocysts, with whose structure I shall assume the 

 reader to be acquainted. It has been shown that their function 

 depends directly upon the fact that the heavy statolith exerts 

 a mechanical stimulus upon the sensory epithelium of the stato- 

 cyst. The whole action of the apparatus, which is to bring the 

 animal into a definite position with respect to gravity, then 

 takes place because this mechanical stimulus acts in a direction 

 determined by gravity, in consequence of the moveability of the 

 statolith, which always seeks the lowest point in the wall of the 

 statocyst cup. The theory of statolith pressure is demonstrated 

 by Kreidl's famous experiments with iron statoliths in Crustacea, 

 by which he showed with a magnet that the movements of the 



