THE METABOLISM OF PLANTS. 263 



retarded. On the other hand it appears that the process is 

 retarded when the light is very intense, for it has been fre- 

 quently observed that the leaves of etiolated plants become 

 green more rapidly in diffuse daylight than in sunshine. 



It appears from the researches of Wiesner and of Mikosch 

 that the formation of chlorophyll in an etiolated plant does 

 not commence directly the plant is exposed to light, but that 

 a longer or shorter time must elapse before any perceptible 

 amount of chlorophyll is produced. They found also, in har- 

 mony with the foregoing, that the effect of exposure to light 

 does not cease directly a plant is placed in darkness, but 

 that the formation of chlorophyll is continued for a time in 

 the dark. This mode of action of light they term photo- 

 chemical induction, an expression suggested by Bunsen and 

 Roscoe. 



With regard to the relative efficacy of the different rays 

 of the spectrum in the formation of chlorophyll, it appears, 

 from Wiesner's researches, that all the rays between Frauen- 

 hofer's lines B and H promote it to a greater or a less extent. 

 Both Gardner and Guillemin found that seedlings turned 

 green more rapidly in the yellow than in any other part of 

 the spectrum, and this result has been confirmed by Wiesner. 

 Wiesner has observed, namely, that, in diffuse daylight, 

 chlorophyll is formed first in plants exposed to white light, 

 then in those in yellow light, then in those in green light, 

 then in those in red light, and finally in those in blue light. 

 But these results hold only for light of low intensity, for he 

 observed that when the light is very intense the formation 

 of chlorophyll takes place earlier in blue than in yellow light. 

 The reason of this is that in intense light chlorophyll under- 

 goes decomposition, and this decomposition goes on most 

 actively in yellow light. We shall recur to this point sub- 

 sequently. 



Guillemin states that the invisible rays of the spectrum, both the 

 ultra-red and the ultra-violet, induce the formation of chlorophyll. 

 Wiesner, who has re-investigated this point, fails to confirm Guillemin's 

 statement as regards the ultra-red rays, and leaves it still uncertain 

 whether or not it holds with reference to the ultra-violet ravs. 



