206 CHAP. XVIII. DIURNAL MOVEMENTS OF PLANTS 



of light by the Radiograph, and of the corresponding move- 

 ment of the plant-organs by the Plant-Recorder. 



The Radiograph gives a record of the diurnal variation 

 of light. On a clear day in January the intensity was 

 found to increase rapidly from 6 a.m. to 12 noon, when it 

 reached its maximum. Light began to decline slowly up 

 to 5 P.M., the decline being less rapid than the rise in the 

 forenoon. The fall of intensity was extremely rapid after 

 5 p.m. Any fluctuation of light, due even to a passage of 

 a cloud, is accurately recorded by the Radiograph. 



The individual effects of the main factors can be to some 

 extent discriminated from each other. The contrasted 

 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 predominant character of the 

 action of light. 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-noon, is about 2 hours 

 earlier than the thermal noon, at w^hich the temperature is 

 maximum. 



A flexure of the diurnal curve about thermal noon, at 

 which an inversion takes place from rise to fall of tempera- 

 ture, indicates the effect of temperature. The additional 

 test of the effect of temperature is furnished by the close 

 resemblance between the diurnal curve of the plant and the 

 thermographic record for 24 hours. 



