532 LIFE MOVEMENTS IN PLANTS 



the two periods, early in the morning when the light 

 appears, and in the evening when it disappears. In the 

 tropics there is little twilight : in Calcutta, the sun rises 

 in summer at about 5-30 a.m., and sets at 6-30 p.m. In 

 winter the sun rises an hour later, and the sunset is an 

 hour earlier. The average dawn may therefore be taken 

 approximately at (3 a.m., and the average sunset at 6 p.m. 

 Unlike the diurnal variation of temperature which is gradual, 

 the change from light to darkness or from darkness to 

 light is very abrupt. If we succeed next in obtaining 

 a continuous curve of the diurnal movement of the 

 plant, the phototropic action would be evidenced by some 

 flexures of the curve in the morning and towards evening. 



The other two types of daily movement depend on 

 the diurnal variation of temperature, and there is some 

 difficulty in distinguishing the effect of variation of light 

 from that of temperature, since both are connected with 

 the appearance and disappearance of the sun. 



Diurnal variation of light and of temperature. — There 

 are certain differences, however, which enable us to dis- 

 tinguish the two variations. Light appears in the morning, 

 say at G a.m., becomes most intense at noon ; after 4 p.m. the 

 light wanes, and darkness sets in quickly after 5 p.m. 

 and remains persistent till next morning. The coarse 

 of variation of temperature is somewhat different. The 

 minimum temperature is attained in my green house at 

 about 5 a.m. in summer, and at about 7 a.m. in winter. 

 The maximum temperature is reached at about 3 p.m. 

 in summer, and about 1 p.m. in winter. The range of 

 daily variation in summer may be taken to be from 

 about 23' C. to 34° C. ; in winter it is from 16- C. to about 

 29^0. The above gives the normal variation and not the 

 sudden fluctuations tha*". occur during uacertaiii w^i^ther 

 conditions. 



