184 DISEASES AND INSECT ENEMIES. 



Where plants are mulched and the soil kept 

 constantly wet the trouble is likely to follow, 

 especially if heavy shading is adopted. Where 

 the plant is grown for some time under these 

 conditions all of its tissues get into a dropsical 

 or oedemic state, and it needs only an injury of 

 some kind to cause the formation of the wart-like 

 growths. If the conditions are very favorable, in- 

 juries are not necessary for the wart-like growths, 

 as they will be produced by the plant without 

 intervention of anything of this kind. This is 

 due to the fact that the plant is really making an 

 abnormal effort at growth and the warts are 

 nothing more than excessive growth of the cells 

 at particular points. This trouble seldom occurs 

 in houses properly lighted and ventilated. In 

 underground pits and in outdoor frames where 

 heavy shading is used it is apt to occur, especially 

 if the practice of mulching is adopted. 



By paying proper attention to light, ventila- 

 tion, and watering, little or no difficulty is experi- 

 enced from the trouble. In case it is seen that 

 plants are becoming oedemic it will be necessary 

 to at once modify the surroundings to such an 

 extent as to admit more light to the leaves and 

 more air to the soil. It will not do, however, to 

 bring about these changes too rapidly. Light 

 should be gradually given and the amount of 

 water added to the soil should be slowly dimin- 

 ished. If the change is too abrupt serious 



