!44 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



The disease varies considerably in its degree of virulence; 

 in many cases only twenty trees per acre escape, in others a 

 large percentage of the original crop may survive, but it is very 

 rare to find that a full crop reaches maturity without active artificial 

 measures of amelioration, by the interpolation of other species. 



The circular expansion of the disease proves without doubt 

 that it is spreading by root contact, and that Polyporus annosus 

 is the immediate cause of death. All visible signs, moreover, 

 point to this fungus as the original cause of the disease, but as 

 all direct infection experiments on growing roots failed, Dr Albert 

 refused to accept this view without further inquiries and direct 

 proof. No instance had been observed of the fungus attacking 

 trees growing on forest soil, although mycelia and spores of 

 Polyporus and other similar fungi are found everywhere in the 

 immediate vicinity on dead and rotting wood. This fact 

 suggested that, whether Polyporus was the primary cause of the 

 disease, or was merely a secondary infection, the predisposing 

 cause must be looked for in the condition of the soil on farm 

 lands, and this was where Dr Albert looked for it. 



In the first instance, he verified by personal observation the 

 fact that Scots pine and red pine planted on farm land, whether 

 the original soil was rich or poor, were invariably attacked by 

 root disease. Second, he found that where the first crop had 

 failed, and the area was restocked with the same species, the 

 disease greatly increased in virulency. 



Dr Albert next made, in numerous localities, careful and 

 searching analyses, physical ^ as well as chemical, of soils on 



^ The chemical constituents of the soil can be fully utilised only if the 

 physical ones are favourable. We obtain the most reliable picture of these 

 conditions by measuring the porosity of the soil, recording the result as a 

 percentage. Thus a porosity of 40 per cent, would mean that 100 parts of 

 soil contain 40 per cent, of hollow spaces and 60 per cent, of solids. This 

 gives us a direct measure of the density of the soil, and shows us at a glance 

 how air and w ater circulation are affected. Numerous and reliable observa- 

 tions can only be made on the spot, but nothing is easier. All that is required 

 is a specially constructed steel receptacle of known dimensions and weight 

 (Ramann's), by means of which equal portions of the soil can be taken up 

 in their natural position, a very accurate scale, and a number of air-tight tubes 

 of known weights. The exact weight of the soil taken out by means of the 

 steel receptacle is ascertained on the spot and recorded, and specimens of the 

 same soil are put into the test-tubes. Those specimens are carefully tested 

 in the laboratory as regards water and specific weight. This done, the 

 porosity percentage can be ascertained by arithmetic. 



