498 The Physiology of Plants BOOK n 



meister showed the same thing as early as 1867 in th< 

 case of Linaria when the fruit is formed. 



In 1874 Sachs showed that a change in the mode of respons< 

 to geotropic stimulation takes place when the main roo 

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

 which has been growing at 'an angle of about 60 change 

 its direction and grows vertically downwards. 



The latent period or induction period elapsing betweei 

 the reception of a stimulus and the consequent respons' 

 received little attention till towards the end of the century 

 In 1875 Pfeffer found that the heliotropic curvature of tfo 

 pulvini of Lourea vespertilionis commences one minute afte 

 they have been illuminated. Darwin investigated man] 

 cases of the phenomenon up to 1880. A more detailec 

 examination was made by Wiesner in 1878. He foum 

 that an hour's exposure to unilateral light produced n< 

 effect of curvature on the epicotyls of Pkaseolus, but i 

 they were then transferred to darkness a well-marke( 

 bending was observable after a further interval of tw< 

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

 ance after the removal of the stimulus, Wiesner callec 

 photomechanical induction. The length of exposure neces 

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

 usually elapsing between the reception of the stimulus anc 

 the appropriate response when conditions remain constant 

 Wiesner made some observations on the length of thi: 

 normal interval, which he determined to be twenty-fiv< 

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

 of vetch seedlings. The induction period varies greatly ir 

 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 Phy corny ces react ir 

 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 



