578 LIFE MOVEMENTS IN PLANTS 



Second phase. — There is a sudden fall of the leaf in the 

 evening which continues till 9 p.m. or thereabout. 



Third ^j/jase. — The leaf erects itself till thermal-dawn 

 at about 6 a.m. next morning. 



Fourth phase. — There is a fall of the leaf during the 

 rise of temperature from thermal-dawn to thermal-noon. 

 The uniformity of the fall is, however, interrupted by one 

 or more pulsations in the forenoon. These pulsations are 

 more frequent in summer than in winter. 



It will thus be seen that the difference between the 

 normal thermo-geotropic curve, and the curve of Mimosa is 

 not so great as appears at first sight. With the exception 

 of the spasmodic fall in the evening, the diurnal curve shows 

 an erectile movement during lowering of temperature, and 

 a movement of fall during rise of temperature. I shall 

 presently explain the reason of the sudden fall in the even- 

 ing, and of the multiple pulsations in the forenoon. 



I have, moreover, been able to trace a continuity in 

 Mimosa itself, between the standard thermo-geotropic 

 reactions and the modification of it by the action of light. 

 The young leaves which sprout out at the beginning of 

 spring take some time to become adjusted to the diurnal 

 variation. There are two intermediate stages through which 

 the leaves pass before they exhibit their characteristic 

 diurnal curve. Slow rhythmic pulsations are at first seen 

 to occur during day and night. At the next stage the 

 leaves exhibit the diurnal movement of fall from thermal- 

 dawn to thermal-noon, and movement of erection from 

 thermal-noon to thermal-dawn next morning, the record 

 being in every way similar to the standard thermo-geotropic 

 curve. It is only at the final stage that there is a spasmodic 

 fall in the evening which we shall find is the characteris- 

 tic after-effect of light. 



