531 LIFE MOVEMENTS IN PLANTS 



In the diurnal movement of plants the most important 

 factors are the effects of light and darkness, of variation 

 of temperature on differential growth, and of thermal 

 variation on geotropic curvature. 



These three classes of phenomena may be discriminated 

 from each other by the following tests. The effects of light 

 and darkness are most pronounced in the morning when 

 light appears, and in the evening when light disappears. 

 A pronounced flexure in the diurnal curve at these periods 

 indicates the dominant character of the phototropic action. 

 The effect of light can also be distinguished from that 

 of temperature from the fact that the period of 

 maximum intensity of light, or light-noun, is about two 

 hours earlier than the thermal-noon, at which the 

 temperature is maximum. 



A flexure of the diurnal curve about thermal noon, 

 at which an inversion takes place from rise to fall of 

 temperature, indicates the effect of temperature. The addi- 

 tional test of the effect of temperature is furnished 

 by the close resemblance of the diurnal curve of 

 the plant with the thermographic record for 24 hours. 



Two different classes of phenomena arise from 

 variation of temperature — Thermonasty and Thermo-geotro- 

 pism. In the former the movement is autonomous and 

 determined by the differential growth-activity of the 

 two sides of an anisotropic organ. In the latter the 

 movement is not in relation to the plant but directed 

 by the external stimulus of gravity. 



