498 The Physiology of Plants Book hi 



meister showed the same thing as early as 1867 in the 

 case of Linaria when the fruit is formed. 



In 1874 Sachs showed that a change in the mode of response 

 to geotropic stimulation takes place when the main root 

 is cut out of a root system. One of the lateral branches 

 which has been growing at an angle of about 6o° changes 

 its direction and grows vertically downwards. 



The latent period or induction period elapsing between 

 the reception of a stimulus and the consequent response 

 received little attention till towards the end of the century. 

 In 1875 Pfeffer found that the heliotropic curvature of the 

 pulvini of Lourea vespertilionis commences one minute after 

 they have been illuminated. Darwin investigated many 

 cases of the phenomenon up to 1880. A more detailed 

 examination was made by Wiesner in 1878. He found 

 that an hour's exposure to unilateral light produced no 

 effect of curvature on the epicotyls of Phaseolus, but if 

 they were then transferred to darkness a well-marked 

 bending was observable after a further interval of two 

 hours. This delay of the response, followed by its appear- 

 ance after the removal of the stimulus, Wiesner called 

 photomechanical induction. The length of exposure neces- 

 sary to bring it into evidence is about one-third the time 

 usually elapsing between the reception of the stimulus and 

 the appropriate response when conditions remain constant. 

 Wiesner made some observations on the length of this 

 normal interval, which he determined to be twenty-five 

 minutes in the case of cress, and thirty-five minutes in that 

 of vetch seedlings. The induction period varies greatly in 

 length. In 1897 Ewart showed it to be one or two seconds 

 after strong sunlight in the case of the leaflets of Mimosa ; 

 Oltmanns found the sporangiophores of Phycomyces react in 

 from one to three minutes ; Czapek, a year later, determined 

 that geotropic reaction requires twenty to thirty minutes. 



We have seen that Wiesner showed that when stimulation 

 is once received it produces its effect even if the stimulation 



