274 



DISEASES OF TREES 



ruptured, when trees that have grown for a long time in a wood, 

 overcrowded owing to neglect in thinning, have been suddenly 

 isolated, or when trees that have been reared in restricted light 

 have been suddenly exposed by the removal of the standards. 



The augmented supply of 

 food-materials in the soil, 

 and the intensified action 

 of the light, resulted in 

 such an acceleration of 

 growth that fissures of 

 various sizes were formed 

 all over the stem. Fig. 

 156 represents the trans-- 

 verse section of such an 

 oak a hundred years 

 old, and exhibits the 

 interesting manner in 

 which new tissues are 

 produced as a result of 

 the formation of the fis- 

 sures. These wounds are 

 injurious, not only be- 

 cause the resulting cica- 

 trization and formation 

 of callus interferes with 

 the splitting of the wood, 

 but also because they 

 offer a means of ingress 

 to parasitic wood-de- 

 stroying fungi. They 

 may almost always be 

 avoided by strongly thin- 

 ning the plantation some 



FIG. 155. Hornbeam whose cortex has been 

 ruptured, a, a crack which does not extend 

 to the wood ; 6, a fissure reaching to the wood, 

 but which has been occluded by the formation 

 of callus (see Fig. 154, b] ; c, a crack which 

 extends to the wood only in the upper portion. 

 One half natural size. 



years before it is in- 

 tended to " lighten " it. 



It being taken for granted that roots rot and the whole plant 

 dies when excess of stagnant water in the soil prevents the 

 entrance of air to the roots, and, further, that the same cause 

 induces the formation of injurious humic acids, increases the 



