General Discussion 213 



of plants affected by copper. Analyses of water cultures of corn, 

 peas, and wheat showing slight toxic effects gave the following 

 ratios of copper in tops, root systems, and root tips : 



Water cultures Cu in p. p.m. 



showing slight , '^ ^ 



toxic effects Tops Roots exclusive of tips Root tips 



Corn 27.00 91.00 545.00 



Peas 17.00 1400.00 3428.00 



1680.00 



Wheat 130.00 297.00 2811.00 



The root tips in this material, by means of caustic potash 

 (the biuret reaction) and potassium ferrocyanide, show the char- 

 acteristic purple and dark-red reactions due to copper. In the 

 former case not only copper, but copper in combination wiih 

 proteids, is indicated — the purple color being due to the biuret 

 test, which identifies both copper and proteids simultaneously. 

 In roots grown in water culture, and then subjected to the 

 action of dilute copper solutions, the location of copper in a 

 poisoned root system can be seen under a low power with con- 

 siderable exactness. The purple of the biuret test begins very 

 definitely with the growing point of the root tip and fades 

 out gradually in comet-like fashion usually within one or two 

 millimeters distance of the tip. New growing points in process 

 of pushing their way through the epidermis along the sides of 

 the roots likewise give a strong but very local biuret reaction. 

 This combination of copper (in the form of oxide) and proteids 

 is one used for the precipitation of albuminoid nitrogen in chem- 

 ical analysis of feeding stuffs.-" The amount of copper entering 

 into the combination varies with proteids from various sources. 

 As a rule, animal proteids combine with much less copper than 

 vegetable proteids — averaging about 2.4 per cent of copper for 

 egg albumin. Vegetable proteids combine with from 11.60 to 

 16.97 per cent of copper oxide and average 11.7 per cent cop- 

 per.-^ Ordinarily, therefore, a vegetable proteid would be sat- 

 urated with about one-ninth of its Aveight of copper; but its 

 physiological activities are disarranged and the root killed by 

 much less than the amount required to saturate the proteid. 



20 See Bibliography, p. 237, reference 48. 



21 Mann, Chemistry of the proteids, p. 305. 



