l82 DISEASE IN PLANTS. 



acid, etc., in the neighbourhood of chemical works, 

 or from the escape of coal-gas in streets, etc., 

 points of importance in connection with the use of 

 fungicides and insecticides. 



Yellowness is the prevailing symptom in many- 

 cases of fungus attack of the roots or collar of the 

 plant, the resulting stoppage of transpiration being 

 also sometimes supplemented by rotting of the 

 roots, and the consequent deprival of oxygen and 

 accumulation of foul gases. In other cases Fungi, 

 and even Bacteria, have been found to have made 

 their way into the principal vessels, the lumina of 

 which they stop up, thus reducing the transpiration 

 current. 



Certain insects may also induce a general 

 yellowing and wilting of plants by entering or 

 destroymg the tissues concerned in the transpira- 

 tion e.^. Osdnt's, the Frit Fly, and Cecidoniya, 

 the Hessian Fly, which attack young winter wheat 

 within the sheaths and cause the plants to turn 

 yellow and wilt. 



Albinism and Variegation are apparently due to 

 causes totally different from any yet mentioned. 

 Church's analyses have shown that albino leaves 

 contain more water and less organic matter than 

 green ones of the same plants, but not necessarily 

 less ash constitutents. The composition of the 

 ash points to there being more potash and less 

 lime in the white organs than in the green ones, and, 

 speaking generally, the former are related to the 

 latter much as young leaves are related to mature 

 ones. 



