IRRITABILITY. 



397 



elongated and their leaves remain small, and Rauwenhofif has 

 found that the structure of internodes grown in light of low 

 refrangibility is similar to that of etiolated internodes. On the 

 other hand, plants grown exposed to the rays of high refrangi- 

 bility (violet, indigo, blue) resemble those which have grown 

 in white light. It appears, therefore, that it is the rays of 

 high refrangibility which exercise the retarding influence upon 

 the rate of growth. 



The relative effect of exposure to the rays of high and of low refrangi- 

 bility is clearly shewn in the following tables of observations made by 

 me upon the sporangiferous hyphse of Phy corny ces nitens. The figures 

 are ^ s of a millimetre ; the time of exposure to light was an hour in 

 each case. 



A. Blue light. 



* Exposed to light. 



B. Yellow light. 



Exposed to yellow light. 



Exposed to daylight. 



