THE REQUIREMENTS OF PLANTS 



47 



traced in part to the presence of acids, which affect the leaves as well 

 as the roots. Wieler i found that assimilation of carbon dioxide was 

 profoundly modified by sulphur dioxide, most injury being done in 

 moist weather when the stomata were more widely opened and the 

 gas could readily enter the leaf tissues. Crowther and Ruston (72) 

 obtained the following yields from pots of Timothy, showing that acid 

 water gradually kills the plant : — 



Table XXII. — Effect of Acid Rain-water on the Growth of Timothy Grass. 

 Crowther and Ruston (72). 



Weight of dry matter obtained when plants were regularly watered with : — 



Metallic Salts. — Complaints are sometimes made by farmers in 

 mining districts that their crops suffer damage from the waste products 

 — generally metallic salts — turned into the streams from the works, 

 especially where the water is wanted for irrigation, or where, as in 

 Japan, rice is grown in the marshes. A vast number of experiments 

 have shown that copper salts are extraordinarily toxic in water cultures 

 or where they actually come into contact with the plant, even the 

 minute trace sometimes present in distilled water being harmful. 

 This property finds useful application in removing algae from water 

 and in killing weeds. For example, a 3 per cent, solution of copper 

 sulphate is sprayed over cornfields in early spring at the rate of fifty 

 gallons per acre to destroy charlock (Brassica sinapis), one of the most 

 troublesome weeds on light soils. The solution adheres to the rough 

 horizontal leaves of the charlock and kills the plant, but runs off the 

 smooth vertical leaves of the wheat without doing much damage. 

 Even the insoluble complex copper salt present in Bordeaux mixture, 

 and sprayed on to fruit trees to kill fungoid pests, was found by Amos 

 (2) to retard assimilation of carbon dioxide by the leaf. 



Copper salts do not appear to be anything like so toxic in the soil 

 as in water culture. 



It is often asserted that any toxic substance must, at proper dilu- 



1 Bied. Zentr., igo8, xxxvii., 572. 



