IRRITABILITY. 



395 



This table is rendered more intelligible by the following diagram, in 

 which the thick line is the course of growth, the thin line that of the 

 temperature, and the unshaded spaces the periods of exposure to light. 

 The figures on the left side represent ^s of a millimetre, those on the 

 right degrees of temperature, and those on the top hours. 



FIG. 43. Diagram illustrating the retarding influence of light upon the growth of 

 a sub-aerial hypha of Phycomyces. 



The growth of leaves also, when they are in a phototonic 

 condition, is retarded by exposure to light ; this is clear from 

 Prantl's observations and from my own. 



The following table illustrates my observations. The plant used was 

 Secale cereale. The plant was kept in darkness and exposed at intervals 

 to light. 



Exposed to light. 



