DISEASES DUE TO SOIL-INFLUENCE 



275 



danger of frost in the case of many plants, and conduces to 

 seedlings being thrown out by frost, &c., the subject need 

 not be further discussed. 



77 7/9 W 



FIG. 156. Transverse section of an oak which in consequence of much accelerated 

 growth has ruptured in two places, x and y, two years before being felled. At 

 the three places marked a b the cambium has occluded the surface of the wood 

 with new tissues, which are possessed of an independent cortex, d d. The loose 

 flaps of cortex have at e e formed new wood on their inner surface. This has 

 formed a kind of callus-cushion at c, which constitutes the edge of the wound. 

 The wood-ring formed underneath the cortex in 1876, the year in which the 

 cortex was ruptured, is a sort of double ring, and consists of two parts, f and g, 

 both of which contain a porous zone and a zone where vessels are comparatively 

 scarce. The porous zone of the inner of these two parts, namely/, was formed 

 in the spring before rupturing had taken place. 



CIRCULATION OF AIR IN SOIL 1 



The metabolic processes in the roots demand an abundant 

 supply of oxygen. The roots die owing to asphyxia if 

 they are excluded from a constant supply of this element. 

 Oxygen is necessary not only for growth but also for the 

 formation and solution of reserve materials, processes which 

 are specially active in roots. The air in the soil is impoverished 

 to an extent corresponding to the amount of oxygen thus 

 abstracted. Under normal conditions the loss is abundantly 

 compensated for, partly by the variations of temperature in the 



1 R. Hartig, Zersetzungserscheinungen, pp. T$et seq. 



T 2 



