XIII] STARCH-PHOTOGRAPHS 137 



yellowish leaf is then placed in an alcoholic solution of 

 iodine for a few minutes, and finally washed in distilled 

 water. The result is shown by all those parts of the 

 lamina which contain starch being coloured deep violet- 

 blue, and, if the conditions of the experiment are properly 

 observed, these parts will be those exposed to the light 

 e.g. the letter or figure cut in the stencil-plate. In the 

 summer, with strong sunlight, it is even possible to obtain 

 prints from coarse photographic negatives in this way. 



A modification of this procedure has been employed 

 by Sachs to determine the quantities of starch formed 

 per hour per square metre of leaf-surface, and although 

 these quantitative results can only be approximate the 

 experiments are as instructive as they are ingenious. 



