206 



DISEASES OF PLANTS 



foliage, especially when it is young. On the other hand, 

 sudden increase of humidity may cause such a change in 

 the cells and tissues that eruptions known as '' emer- 

 gences " or '' intumescences " may develop. These emer- 

 gences are outbreaks, through the epidermis or bark, of 

 soft underlying tissues in which the pressures rise over- 

 rapidly by reason of a sudden check to evaporation. 



Fkj. 52. — The KIToct of Sulphur Dioxide Fumes from a smelter 

 upon the forest two miles away 



Heat and Cold, Light and Shade. — Prolonged ex- 

 posure to unfavorable temperatures may produce disease, 

 showing itself mainly in changes in color of the foliage, 

 though extreme cases may exhibit injury as serious as 

 untimely leaf- fall. Insufficient or excessive light will also 

 show itself in abnormal color of the foliage; and very in- 

 sufficient light, as is well known, will be followed by fall of 

 the leaves. 



INTERNAL CAUSES 



It must be admitted, at least for the present, that the 

 so-called internal, or physiological, causes of disease are 

 acceptefl at all only because other and external cause has 

 not been recognized. Thus the malady known as " little 

 peach, " and another called " sour sap, " are undoubtedly 



