1917] Forbes: Irrigation Effects of Copper Compounds Upon Crops 465 



For instance, in samples of wheat and pea roots grown in water 

 culture, it was found by means of nitrogen and copper deter- 

 minations, using the factor 11.7 per cent copper for saturation 

 of albuminoids, that in wheat roots 4.96 per cent of the copper 

 required for saturation was present and in pea roots 7.99 per cent. 



It appears, therefore, first, that copper attacks plant proteids 

 at the most delicate and vulnerable points in the whole plant 

 organization — the growing points of the root systems ; and, sec- 

 ond, that a small proportion of the copper required for complete 

 reaction is sufficient to kill the protoplasm at these points. 

 Again, it is to be observed that, especially in the seedling stages 

 of growth, the number of growing points is small so that only 

 extremely minute amounts of copper are required to arrest the 

 growth of root tips, the spread of root systems and the nutrition 

 of the plant. 



Inasmuch, also, as plants vary greatly in the physical struc- 

 ture and the physiological activity of their root systems, includ- 

 ing the number, delicacy and absorptiveness of their growing 

 points, it is not unlikely that the varying sensitiveness to copper 

 salts of different plants, and of the same plant at different ages, 

 may be explained by these observations. Corn, for instance, the 

 most sensitive plant worked with, is characterized in its seedling 

 stages by a small number of vigorously absorptive growing 

 points. 



B}" means of the more delicate dark-red potassium ferro- 

 cyanide test, copper may usually be traced through the vessels 

 of the root systems for considerable distances, showing that it 

 is through these channels that small amounts of the metal finally 

 reach the stems and leaves. Here the maximum amounts of 

 copper are found in the outer and upper portions of the plant, 

 where evaporation is most active, and where the greatest residuum 

 of copper therefore occurs. The potassium xanthate (yellow) 

 and h^'drogen sulphide (brown) tests also reveal copper in root 

 structures but are not so satisfactory for this purpose as potas- 

 sium ferrocyanide. (See plate 9.) 



The above described reactions, which are so conspicuous in 

 water-culture material killed by copper, are very obscure or 

 imperceptible in roots grown in soils containing copper. The 



