TROPISMS 157 



(see Figure 48). He found that more auxin diffuses out of 

 the dark than out of the Hght side, and that the amount 

 coming from the dark half was more than the amount 

 coming from half an unilluminated tip. This shows that 

 the principal effect of unilateral illumination is to cause the 

 lateral transport of some auxin from the light to the dark 



side. 



On the basis of these experiments and concepts the gen- 

 eral theory of plant tropisms, sometimes called the Cholodny- 

 Went theory, may be formulated as follows (see e.g. Chol- 

 odny, 1929) : 



Growth curvatures, whether induced by internal or by external fac- 

 tors, are due to an unequal distribution of auxin between the two sides 

 of the curving organ. In the tropisms induced by light and gravity 

 the unequal auxin distribution is brought about by a transverse polar- 

 ization of the cells, which results in lateral transport of the auxin. 



''These researches, which are grouped around a central 

 idea, allow us to approach step by step to the solution of 

 one of the most interesting problems of physiology. From 

 the standpoint of the history of science, the present ^ state 

 of the problem is of the greatest interest. Gradually there 

 emerges from a chaos of facts the splendid form of a theory 

 which promises to unite and to coordinate, in the very near 

 future, the enormous mass of varied experimental data 

 into a single principle" (Cholodny, 1927). This prophetic 

 statement has to a large extent come true, and it will now 

 be necessary to consider the ''enormous mass of experi- 

 mental data" in more detail. In succession we shall treat 

 geotropism, phototropism, and other tropisms so far as they 

 are related to our subject. 



II. GEOTROPISM 

 C. Geotropism of Shoots 



An interesting forerunner of the Cholodny-Went theory 

 is Loeb's view of geotropism (1917). His experiments on 



» 1927. 



