766 
Bose.— On Diurnal Variation of 
Effect of sudden darkness a7id its continuation. In the next record 
(Fig- 5) i s shown the immediate and continued action of darkness. The 
Fig. 4. Effect of cloud. Dotted up-curve indicates responsive fall, and continuous down-line 
exhibits slow recovery. First four responses normal ; next three show depression due to diminution 
of light brought on by cloud, the duration of which is indicated by horizontal line below. Last 
three records show restoration of excitability brought on by clearing of sky. All records read from 
left to right. 
Fig. 5. Effect of sudden darkness. Plant subjected to sudden darkness beyond horizontal line 
seen below. First two responses normal. Note sudden depression of excitability, revival and final 
depression under continued darkness. 
first two are the normal uniform responses in light. By means of screens, 
the plant was next subjected to sudden darkness ; this brought about a 
marked depression of excitability. Subjection to sudden darkness thus acts 
as a stimulus inducing a marked but transient fall of excitability. Under 
the continuous action of darkness, however, the excitability is at first 
restored and then undergoes a persistent depression. 
