472 The Physiology of Plants Book hi 



phenomena of stimulation. By means of it has been made 

 possible the equal distribution of either the geotropic 

 or heliotropic stimulus over a cylindrical receptive organ 

 — a result which has the same effect as the elimination of 

 the reception of stimulation. 



Sachs' results were confirmed and greatly extended by 

 Charles Darwin in 1880. In the same year Elfving added 

 considerably to the instances of diageotropism then known. 

 On the basis of these researches Darwin suggested in 1880 

 that plants possess a sense of direction or a faculty of what 

 has since become known as gravi-perception, a suggestion 

 endorsed by Pfeffer the next year; it was supported 

 by Vochting's observations in 1882, that the geotropic 

 movements of the peduncle of the poppy vary during its 

 life, and show a definite purposefulness in its adaptation 

 to the different needs which accompany it at different times 

 in its life. 



The recognition that we are concerned with stimulation 

 and response has led in this case, as in that of heliotropism, 

 to a differentiation of the sensory from the motor mechan- 

 ism. Ciesielski showed, in 1872, that the geotropic curva- 

 ture of a root can take place only so long as it has an 

 uninjured growing-point. Darwin, following the same line 

 of research, determined in 1880 that the tip is especially 

 the sensitive region of the root, or what we may call the 

 region of perception as distinguished from the part which 

 moves, which we may term the motor area. He originally 

 relied on amputation of the tip, taking off a piece about 

 2 mm. long. Roots so treated remained uncurved when 

 placed horizontally. When he allowed uninjured roots to 

 remain in that position for some time, and then removed 

 the tip, he found they curved, whatever position they were 

 placed in. He claimed therefore that geotropic induction 

 is localized in the root-tip. 



Darwin's views were immediately vehemently opposed 



