EFFECT OF ALTERNATION OF LIGHT AND DARKNESS /)4S 



light, lies in the fact that the flexure of the diurnal curve 

 takes place in the former at about 2 p.m. when tempera- 

 ture undergoes change from ascent to descent ; in the case 

 of light, the change in the intensity of light begins to be 

 marked about three hours later. In the diurnal curve of 

 Des/nodiuin the record shows little change at 2 p.m., showing 

 that the leaflet is not afllected to any great extent by the 

 variation of temperature ; it is, however, strongly affected 

 by change in light as seen in the rapid closure movement 

 aboiTi 5 p.m. The leaflet remains tightly closed throughout 

 the night anil begins to open and spread out early in the 

 morning at about 5 a.m. This up-movement is also very 

 rapid and the leaflet assumes the fullest outspread position 

 by 7 a.m. It remains in this position till the afternoon, 

 after which the cycle becomes repeated. As the leaflet is 

 very sensitive to light, the position of equilibrium of the 

 leaflet is liable to be disturbed by the slightest variation 

 of light and the fluctuation of light from the sky 

 often gives rise to a wavy outline in the record. The 

 leaflet, moreover, has a tendency to exhibit rhythmic 

 pulsations. 



In the leaflets of Cassia and Desmodiuui, the daily 

 movement is thus brought about by the predominant action 

 of recurrent light and <larkness. 



MIDDAY SLEEP. 



I shall here briefly recapitulate the results given in 

 greater detail in an earlier paper (p. 352). I have shown that 

 the midday closure of leaflets is brought about by the 

 excitatory action of strong sunlight. The responsive 

 movement of motile pulvinus under diffuse stimulus is 

 determined by the greater contraction of the more excit- 

 able half of the organ. Under the action of the midday 

 sun the leaflets of Mimosa undergo a folding upwards, 



