72 Studies in Forestry [CHAP. iv. 



a genuine symbiosis bringing mutual advantages both to the 

 fungus and the tree. 



Oxygen (O), requisite for the process of respiration, is, 

 like carbonic acid, always present in sufficient quantity in the 

 atmosphere, which is composed of 20-93 % oxygen and 79-04 % 

 hydrogen by volume, or 23-28 % oxygen and 76-67 % hydrogen 

 by weight. Unless, however, the aeration of the soil is also 

 favourable, it may happen that the root-system has difficulty in 

 finding a sufficiency of oxygen for the supply of its requirements. 



Ebermayer found that under Beech the soil was always much 

 poorer in carbonic acid than under crops of Spruce and on fallow 

 land, a fact which he ascribed to the better aeration of the soil 

 by the numerous and deep-going roots of the Beech. Ramann 

 agrees with this view and adds 1 : 



'The "soil-improving" action of the Beech is probably mainly ascrib- 

 able to its thorough aeration of the soil. As a large percentage of 

 carbonic acid in soils rich in humus indicates a considerable reduction in 

 the amount of oxygen in the air circulating throughout the soil, there is 

 no reason why the former should be advantageous to plant life on the 

 contrary, the greater amount of carbonic acid may rather be taken to be 

 a sign of deficient aeration and of deterioration in the soil.' 



Prof. Ebermayer's own most recent utterance with regard to 

 the soil-improving qualities of the Beech an opinion that 

 must be accepted as of the highest authority is by no means 

 so direct and unconditional as the above. It is as follows 2 : 



' The shallow surface-roots of the Beech mostly ramify throughout 

 the loose upper layers of soil that are rich in humus. Under normal 

 circumstances the suction-roots are not here provided with hairs, but 

 are covered with a fungus (Mycorhiza^, which acts in the capacity of 

 a go-between with respect to water and food-supplies, and by means of 

 which the trees derive much better nutriment from the layers of soil 

 rich in humus, than is obtainable through the root-hairs from the 

 mineral soil (B. Frank). A very valuable attribute of the Beech is that, 

 judging from my numerous investigations as to the influence of woodland 

 growth on the percolation of soil-moisture and on the chemical compo- 



1 Op. cit., p. 266. 



2 Forstlich-natur-wissenschaftliche Zeitschrijt, 1893, p. 237. 



