Nature of Plant Diseases 95 



most expedient to use is air-slaked lime which has 

 been slaked dry by sprinkling lightly with water. 

 This is diluted in water and applied as a spray. If 

 new lime is used it will be more difficult to wash 

 off later. Moreover, it seems that air-slaked lime 

 sticks a good while, but rubs off easily. It is far 

 more desirable to use shading material that must be 

 applied twice in the summer than something that 

 will stick hard and remain during the fall and win- 

 ter season. 



Smoke injury. As a rule large greenhouse estab- 

 lishments are situated near large cities which are 

 centers of industrial production and manufacture. 

 Greenhouse plants are often injured from the effect 

 of smoke or gases which escape from the furnaces 

 into the air. 



The sources of smoke may be classified into three 

 divisions: (i) Smoke from large buildings or from 

 manufacturing plants; (2) Smoke from locomo- 

 tives; (3) Smoke from chimneys of dwelling houses. 

 Smoke is generally produced because of improper 

 furnace construction, such as improper draft, over- 

 loaded boiler, insufficient air space, insufficient air 

 supply to boiler room, and also by carelessness qf 

 operation. 



Smoke contains large quantities of carbon dioxide, 

 steam and sulphur dioxide, besides its characteristic 

 soot. The latter consists of carbon, tar, and mineral 

 matter mixed with small quantities of sulphur, 

 arsenic and nitrogen compounds which are of an 

 acid nature. Soot adheres to plants, especially to 



