NUTRITIVE FUNCTIONS OF SALTS 109 



remains to be done. Allison and Hunter (1929) reported that 

 zinc was especially needed by peanuts on certain soils of Florida 

 at the beginning of their growth period, but towards the later 

 stages of their development, copper was more essential. These 

 two elements thus seemed to complement each other. In the 

 nutrition of the fungi, zinc plays a noticeable role in promoting 

 vegetative growth and hindering the formation of spores. 



Copper. — Copper has been reported a necessary element for the 

 heather in Holland, and Felix (1927) found that copper improved 

 certain types of muck land in western New York, where lettuce 

 and onions will not grow normally in its absence. Lipman and 

 Mackinney (1931) state that barley will not head properly unless 

 1 part of copper in 8-16 millions of the culture solution is present. 

 At present there seems to be a distinct effort to prove that this 

 also is an " essential" element; but the work has not been entirely 

 convincing. Sommer (1931), for example, reported that copper in 

 small quantities was stimulating and essential to sunflowers, flax, 

 and tomatoes. In these experiments, the plants in the culture 

 solutions lacking copper gave much poorer growth than those with 

 copper. But to the culture solutions were also added aluminum, 

 arsenic, barium, boron, chlorine, cobalt, fluorine, iodine, lithium, 

 lead, manganese, mercury, nickel, rubidium, silicon, sodium, and 

 tin! It seems hard to believe that the plants grew poorly because 

 of the absence of copper. The wonder is that any grew at all! 

 In such experiments it is extremely easy to confuse a balancing 

 with a nutritive function. 



Also part of the difficulty here may arise from the confusion be- 

 tween a stimulating and an essential element. As many of us know, 

 not all things which stimulate are essential. An essential element 

 may be defined as one whose absence from a culture solution suffi- 

 ciently long results in the death of the plant and which cannot be 

 replaced by any other element. - Most of the essential elements 

 are among the twenty most common substances of the earth's 

 crust. Some elements commonly found in plants, as mentioned 

 before, such as nickel, cobalt, and titanium, all of which occur in 

 most plants in small quantities, are not essential as far as we know; 

 but no essential element is as rare as copper. It would be rather 

 surprising if plants, in general, were unable to develop properly 

 without their ration of this element. For the present, therefore, 

 copper must be considered a luxury rather than a necessity. 



