SELF-RECORDING RADIOGRAPH I97 



The diurnal movements of plants are generally due to 

 the recurrent changes of light and darkness and to variation 

 of temperature, the resulting movement being due to the 

 algebraical summation of their individual effects. In 

 regard to these two factors, the effect induced by the rise 

 of temperature is often antagonistic to that of increasing 

 intensity of light ; a rise of temperature enhances the rate 

 of growth up to an optimum, whereas light acts as a 

 stimulus, retarding the normal rate of growth. Variation 

 of temperature, moreover, affects the organ as a whole, 

 whereas light may act unilaterally, depressing the rate of 

 growth of the particular side subjected to light. 



For a full analysis of the diurnal movements of plants, 

 it thus becomes necessary to obtain a continuous record 

 throughout every hour of the day and night of : 



1. The movement of the plant-organ ; 



2. The variation of temperature ; and 



3. The change in the intensity of light. 



In the next chapter I describe in detail the Auto- 

 matic Recorder for the diurnal movements of plants, the 

 variation of the temperature being also recorded by the 

 Thermograph on the same recording plate. The far more 

 difficult problem of the automatic record of variation of 

 light will be dealt with in the present chapter. 



The Self-Recordixg Radiograph 



The method adopted for obtaining a record of the 

 variation of intensity of light depends on the characteristic 

 property of the selenium cell, which exhibits a diminution 

 of its electric resistance under illumination. When a sele- 

 nium cell is placed in the dark, in series with a battery of 

 voltaic cells, it gives a small deflection of the galvanometer 

 in circuit ; illumination causes an increase of this deflection, 

 according to the intensity of hght. Several difficulties are, 

 however, encountered in the practical application of this 



