466 The Physiology of Plants BOOKIII 



carried out by Francis Darwin in 1880. Early in the 

 course of these investigations Sachs emphasized the theory 

 that heliotropic and geotropic curvatures are evidence of 

 the possession of irritability, and of purposeful response 

 to stimulation on the part of the curving organs. 



Frank's hypothesis thus attained general acceptance and 

 was the subject of much critical examination during the 

 remaining years of the century. It received strong support 

 from the researches of Pfeffer in 1877, to which reference 

 has already been made ; it was aided by those of Wiesner 

 in 1878, and was emphasized by F. Darwin in 1880. Further 

 evidence was forthcoming when N. J. C. Miiller noted in 

 1872 that certain seedlings of cress are positively heliotropic 

 in weak light, and negatively heliotropic when a particular 

 intensity is exceeded. Stahl showed in 1880, and Berthold 

 in 1882, that certain Algae behave in the same manner. 

 Oltmanns, in 1892 and 1897, observed a similar reversal 

 of response to light in the case of Phycomyces, which is 

 positive in dim but negative in bright light. Oltmanns' 

 experiments showed that with light of a particular inter- 

 mediate intensity no response to the stimulus is manifested. 

 It is clear that in this case we must be dealing with 

 interaction between the protoplasm and the light, and not 

 with any mechanical interference with growth. Other 

 researches dealing with various aspects of the problem 

 were published by Vochting in 1888, and by Krabbe 

 in 1889. 



By this long series of researches the phenomena were 

 shown to be purposeful and not merely mechanical, and 

 it became possible to study them from the point of view 

 of the adaptation of organisms to their surroundings. The 

 conception that the heliotropic curvatures indicate percep- 

 tion and response was followed by the discovery that the 

 region of the plant by which the stimulus is received is 

 not identical with that by which the curvature is effected. 



