DISEASES DUE TO SOIL-INFLUENCE 281 



In this experiment the Scotch pine proved least sensitive, a 

 result which was possibly due to its deeper roots. 



The injurious effects of urine on plants are generally well 

 known, and may be sufficiently explained from its saline 

 contents. 



Many acids and leys act as true poisons, and are sometimes 

 conveyed to the soil in large quantities in the impure water that 

 flows from factories. As experience proves, they are highly 

 injurious, but this is not the place to discuss the many poisons 

 that may occur in such contaminated water. 



A certain amount of interest also attaches to the injurious 

 influences exerted on vegetation by continuous exhalations of 

 carbonic acid gas from the soil. At the baths of Cudowa in 

 Silesia many springs of water containing carbonic acid are 

 distributed throughout the park. At such places one finds 

 only grass, shrubs being unable to grow. This is probably due 

 to the soil being so permeated by free carbonic acid that the 

 respiratory processes of the roots are rendered impossible. Grass, 

 however, is enabled to grow because the circulation of air close 

 beneath the surface of the ground is sufficient to maintain the 

 roots alive. 



It has been proved that the roots of trees are injured by 

 coal gas when it escapes from pipes into the soil in large 

 quantity. The unhealthy condition or death of trees that line 

 the streets of towns is, however, not altogether to be attributed to 

 this form of injury. The cause is rather to be found in the close 

 paving of the streets and footpaths, which precludes the entrance 

 of water and even air, so that the tree-roots suffer both from 

 want of moisture and of air. 



It may be shortly mentioned here that coal gas also interferes 

 considerably with the cultivation of flowers in rooms. This is the 

 case even when but little gas is burned, for small quantities are 

 always escaping from the pipes. Camellias, azaleas, and ivy are 

 very sensitive to gas, the least sensitive plants being palms and 

 Dracaena* 



* [In many cases, at least, these injuries are due to the sulphurous anhy- 

 dride of which traces are frequently present. ED.] 



