524 LIFE MOVEMENTS IN PLANTS 



merely when they open and close in the bad but also 

 when arranged in pairs on an axis, they exhibit movements 

 towards and away from each other. In other cases, speak- 

 ing generally, we may employ the terms night position and 

 day position for the closed and open conditions respec- 

 tively. The night position may also be described as the 

 sleep position^ After reviewing the various theories pro- 

 posed, he proceeds to say " that a completely satisfactory 

 theory of nyctitropic pulvinus movements is not yet forth- 

 coming. Such a theory can only be established after new 

 and exhaustive experimental research." 



The difficulties of the experimental reinvestigation here 

 called for towards clearing up and explanation of the 

 subject are sufficiently great ; they are further increased by 

 the fact that these diurnal movements may be brought 

 about by different agencies independent of each other. 

 Thus in Crocus and in TiUip, the movement of opening 

 during rise of temperature has been shown by Pfeffer to be 

 due to differential growth in the inner and outer halves 

 of the perianth. I shall in this connection show that a 

 precisely opposite movement of closing is iniluced in 

 Nymphma under similar rise of temperature. I shall for 

 convenience distinguish the differential growth under tem- 

 perature variation as Thennonasty proper. Again certain 

 leaflets open in light, and close in darkness in the so- 

 called sleep position. Intense light, however, produces the 

 ' midday sleep ' — an effect which is apparently similar to 

 that of darkness. The determiniag factor of these move- 

 ments is the variation of light. 



There are other instances of diurnal movement, far more 

 numerous, which cannot be explained from considerations 

 given above. It has therefore been suggested that the 

 "Day and night positions may arise by the combined 

 action of geotropism and heliotropism. Thus Vochting 

 (1888) observed in the case of Malva verticillatta, that 



