84 Kraemer : Color in plants 



observation that the intensity of color is in direct ratio to the 

 amount of sunlight and coolness of temperature, that is, of course, 

 within certain limits. On the other hand, high temperature and 

 diminution of light tend to decrease the intensity of color. The 

 combined effect of sunlight and coolness of temperature is also 

 seen in the foliage of the plants of alpine regions and higher lati- 

 tudes, as well as in the spring and autumnal foliage of plants in 

 the temperate regions. 



The supposition that certain constituents of the soil perceptibly 

 modify the color of plants probably dates from the experiments of 

 Risse, who found that the so-called u zinc-violet, ' ' a blue-flowered 

 form of Viola lutea (one of the alpine violets of Europe), grows in 

 a soil containing 20 per cent, of zinc oxide, and that the ash of the 

 plant contains as much as 1 per cent, of zinc oxide. The fact that 

 this "zinc-violet" maintains the blue color when transferred to a 

 soil which does not contain zinc, together with the fact that the 

 color varies when grown in a soil containing zinc, led Hoffmann 

 to conclude that the presence of zinc had no influence on the color. 

 On the other hand, it is commonly supposed that the blue color 

 of hydrangea is due to the addition to, or presence in, the soil of 

 certain chemicals, as alum or iron filings. It is also a common 

 practice among rose growers to treat the soil with a solution of 

 copperas or ferrous sulphate (approximately 1 part of salt to 1,000 

 parts of water) in order to intensify the color of red roses. But 

 curiously, the same treatment appears to decrease the intensity of 

 color in the pale-red roses. 



The fact that plant color-substances are modified to a greater 

 or less extent by certain chemicals, as already indicated, led to the 

 idea that it might be possible to influence the color-substances in 

 plants by feeding the plants with chemical substances. The cre- 

 ation of color-principles is quite another problem. 



During the past year I have been experimenting with roses, 

 carnations and pansies, using the following chemicals : Acetic 

 acid, citric acid, malic acid, phosphoric and other acids ; various 

 iron salts, as the acetate, citrate, chloride, sulphate, etc. ; certain 

 aluminum salts, as sulphate, phosphate, and the double salts of 

 aluminum and potassium sulphate and aluminum and ammonium 

 sulphate; ammonia water, potassium hydrate, ammonium nitrate, 



