202 



Bulletin 80 



The slow development of biuret tests for copper in such material 

 after sufficient exposure to copper solutions, indicates the presence 

 of protoplasm. 



It is possible that the same observations may apply to other 

 poisons, metallic or otherwise, brought into contact with absorp- 



Fig. 13. — Photomicrograph of root tip of corn grown in water culture 

 and poisoned by 1:200,000 of Cu in solution. The copper is shown as red 

 copper ferrocyanide, which appears black in the photomicrograph. The 

 irregular inner black line show^s the penetration of the copper and also 

 indicates sharply the differences in permeability of adjacent cells, some 

 of which are penetrated before others. ( X 80 diam.) (Photo by J. T. 

 Barrett.) 



