Significance of Tropisms for Psyciiuluuy 59 



These writers explain the geotroi)ie reactions of plants 

 by saying that in certain cells starch grains arc jjrcscnt which 

 serve the purpose of the otoliths in animals. These starch 

 grains are believed to press upon the sense-organs or nerve 

 endings in the plant cells concerned and the "pressure-sense" 

 of the plant is then supposed to give rise to the geotropic curva- 

 ture. I have no opposition to offer to the assumption that the 

 starch grains change their position with a change in the position 

 of the cells, and I am also willing to pass over for the present 

 the view that the starch grains form one of the two phases in 

 the cell. But I see no necessity for assuming besides this 

 the existence of intracellular sense-organs which perceive the 

 pressure of the starch grains. This is, in my opinion, an 

 unnecessary complication of simple relations. 



X 



The progress of natural science depends upon the dis- 

 covery of rationalistic elements or simple natural law>. We 

 find that there are two classes of investigators in biology, 

 grouped according to their attitude toward such simj)le laws 

 or rationalistic elements. One seems to aim at the denial of 

 the existence of such simple laws and every new case which 

 does not fall at once under such a law offers an ojjportunity 

 for them to point out the inadequacy of the latter. The other 

 group of investigators aims to discover antl not to disj)rove 

 laws. When such investigators have discovered a simi)le law 

 which is generally apphcable, they know that an apparent 

 exception does not necessarily overt lirow the law, but that 

 possibly an opportunity is offered for a n<>w discovery and an 

 extension of the old law. Mendel's laws have been brilliantly 

 confirmed in a number of cases. In some cases of ai)parent 

 deviations (from these laws), however, it has not always been 

 possible at once to recognize the cause. One group of investi- 

 gators has recognized that these deviations do not indicate the 

 incorrectness of Mendel's laws, l)ut that they are uwrvW the 



