The Electrical Effects of Light upon Green Leaves. 



131 



I thereafter took note of the external temperature, and tested the 

 leaves in a warm box with satisfactory results. 



A few days later (May 21st), Iris leaves even in the warm box were 

 notably inert. Two leaves were tested with negative results, a third 

 leaf gave a response of O'OOS volt, but its resistance was obstinately high 

 (nearly 3 megohms), a fourth leaf gave a response of 0'004 volt (plates 

 1770 and 1771). On May 23rd I was unable to find a satisfactory 

 leaf ; most of the plants were fully grown and in flower. I therefore 

 abandoned Iris and sought for other satisfactory leaves, in which it 

 might be possible to obtain evident differences of reaction in correla- 

 tion with evident differences of state. 



To sum up the effect of temperature upon the response of Iris the 

 normal response at 15 to 20 is diminished or abolished at low 

 temperature (10), augmented at high temperature (30), diminished 

 at higher temperature (50), and abolished by boiling. 



FlG. 2a. Normal Eesponse of 

 Nerve to Excitation. 



Light. 



o min. 10 so 30 40 



FIG. 26. Normal Eesponse of Iris Leaf to 

 Illumination. (1752.) 



Light. 



Voit 



OOI 

 0-O2 



fJ1//7S.O 5 10 IS 20 25 



FIG. 3. Failure of response in an inert leaf of Iris. 



Time of day. Leaves of Iris appear to give more marked response 

 at or about mid-day, than at or about 6 P.M. 



Young leaves of young plants act well. Old leaves of old plants do 

 not act at all. The older leaves of young plants act better than the 

 younger leaves of old plants. 



