202 LIFE AND WORK OF SIR JAGADIS C. BOSE 



curvature is thus produced. Rise of temperature reduces 

 the geotropic effect and flattens this curvature, while fall 

 of temperature accentuates it. Hence under the daily 

 variation of temperature, all branches of trees and their 

 leaves exhibit a periodic up and down movement. This 

 is clearly seen in the records given in Fig. 25 of the diurnal 

 movement of the palm tree, that of the procumbent stem 

 of Tropaeolum, and of the leaf of the palm. In the tropics 

 the thermal noon or the period of highest temperature 

 is about 3 P.M., while the thermal dawn or temperature 

 minimum is about 6 A.M. The different plant-organs 

 are seen to move continually upwards from the thermal 

 noon to the thermal dawn. The reverse movement takes 

 place after 6 A.M., and the maximum fall is attained at 

 the thermal noon at 3 P.M. Several hundreds of records 

 obtained with different plants show that their daily 

 movements hitherto unexplained are brought about by 

 therm o-geotropic action. 



An animal experiences a daily cycle of change passing 

 through the stages of what we know as sleeping and waking. 

 The fanciful name of sleep has been given to the closure 

 of the leaflets of certain plants at night. Bose has shown 

 how these opening and closure movements are brought 

 about, these being in no way related to true sleep. The 

 question as to whether plants sleep or not can be put in 

 the form of a definite inquiry : Is the plant equally 

 excitable throughout the day and night ? If not, is there 

 any period at which it practically loses its sensibility ? 

 Is there again another period at which it wakes up, as it 

 were, to a condition of maximum excitability ? 



This problem was solved by Bose by means of a 

 specially invented apparatus which delivers a questioning . 

 shock to the Mimosa plant every hour of the day and 

 night, and records automatically the answering response 

 of the plant. The size of the answering twitch gives a 

 measure of the ' wakefulness ' of the plant during twenty- 



