8 TRANSACTIONS OF ROYAL SCOTTISH ARBORICULTURAL SOCIETY. 



Other cases, isolated strands from the intercellular mycelium 

 pierce the cortical cell walls and become intracellular. 



As regards the density of the fungal web, two extremes occur. 



1. The fungus may form an extremely thick, interwoven 



mass, equal sometimes in thickness to the cortex itself. 



2. The fungus may be represented by scattered strands of 



mycelium only. 

 Between these two extremes all degrees of density occur. In 

 the form and structure of the mycelium itself, extremely obvious 

 differences can be observed. 



1. There is the swollen irregular type of mycelium with 



swellings or vesicle-like structures, either terminal or 

 intercalary, with only occasional septa (Fig. 3 A). 



2. There is the regular parallel walled type of mycelium 



showing septa and clamp connections (Fig. 3 B). 



3. There is the intermediate type which is neither definitely 



swollen and irregular, nor definitely regular in outline. 



This type may show occasional vesicles and clamp 



connections (Fig. 3 C). 

 The form and structure of the mycelium may be correlated 

 with the density of the fungal cap. The diffuse scattered fungal 

 layer is usually composed of the regular septate mycelium, 

 while the vesicular irregular mycelium is typical of the dense 

 interwoven web. Intermediate forms again occur which provide 

 the following series : — 



1. Dense web with swollen, irregular, vesicular mycelium; 



2. Dense web with swollen, irregular mycelium to the in- 



side (nearest the cortex) and ordinary regular mycelium 

 to the outside ; 



3. Dense web or web of average thickness which has 



mycelium of the intermediate form ; 



4. Diffuse web with regular mycelium. 



Wherever the dense interwoven web occurs, root-hairs are 

 entirely absent. Root-hairs usually occur with the diffuse type, 

 and in several instances sections have been obtained showing 

 the passage of a fungal filament through a root-hair, and its 

 final branching in the cell of the piliferous layer. A point of 

 extreme interest and of possible value in estimating the re- 

 lationship of mycorhiza to the tree, is, that there appears to 

 be a gradual destruction of the cortical cells by the enveloping 

 fungus, so that the fungus is in reality a natural root pruner. 



