12 LIGHT AND THE BEHAVIOR OF ORGANISMS 



as Knight had assumed. About the same time Dutrochet 

 appHed endosmose and exosmose to explain the movement 

 of plants mechanically. In 1833 De Candolle proved that 

 it is light which causes plants to grow toward a window and 

 not difference in temperature on opposite sides as Ray had 

 thought one hundred forty years earlier. De Candolle 

 discovered that light retards growth in plants and con- 

 cluded that they turn toward a source of light because 

 growth is retarded on the more highly illuminated side. 

 The reaction according to De Candolle is due to difference 

 in intensity of light on opposite sides. 



The turning toward the light was called heliotropism by 

 De Candolle (1835, Vol. 2, p. 609), who was, according to 

 Pfeffer (1906, pp. 154-155), the first to use this term. He 

 used it merely to indicate the exciting agency and not to 

 express the physiological response involved. Hofmeister 

 (1863, p. 86) added the terms positive and negative heli- 

 otropism; Frank (1870), invented the term geotropism; and 

 Darwin (1881), Rothert (1896) and Massart (1902) intro- 

 duced various special terms. While all these expressions 

 were at first very generally used to designate the relation 

 between the movement of the reacting organism and the 

 source of stimulation, they soon came to be used to desig- 

 nate also the processes underlying the reactions. De 

 Candolle's explanation of the reaction to light assumed a 

 direct effect of the external agent on the tissue involved in 

 the reaction; and the same was true with reference to 

 Knight's explanation of the reactions to gravity. The 

 cells in which the processes producing the curvatures took 

 place were supposed to be stimulated directly. The idea 

 of irritability, of transmission of stimuli, of a differentiation 

 between sensitive and reacting tissue, in plants had not 

 yet been promulgated. The term " tropism" then gradually 

 came to signify not merely turning, but turning due to the 

 direct effect of the stimulating agent on the tissue produc- 

 ing the movement, and this signification it has retained to 

 some extent to the present time. 



