TREE roots: their action and development. II 



than 12-13 years old. The nodules may be sessile or stalked, 

 and are frequently found detached from the root-system of the 

 tree, and lying embedded in the surrounding soil. These little 

 balls have root-tips protruding from their surface, and their 

 general appearance suggests the truffle. 



The fungus, so far as it can be observed, appears first as an 

 external web on the fine absorbing rootlets. These absorbing 

 rootlets with their enveloping fungus, branch dichotomously. 

 The dichotomous branching is repeated indefinitely, until a 

 small clump is formed. Each absorbing rootlet and each 

 branch of the dichotomous system receives some check to its 

 growth and is short and stout. The final result is a compact 

 ball, which in cross-section is seen to be composed of rootlets 

 and mycelium, the compactness being due to the mycelium of 

 neighbouring rootlets of the dichotomous system growing 

 together, the root structures themselves then showing up as 

 islands in a sea of mycelium (Fig. 4). 



The mycelium of the mycorhiza not only typically forms a 

 compact layer to the outside of the rootlet, but proceeds 

 inwards between the cells of the cortex towards the endoderm, 

 its progress being arrested about one cell layer distant from it. 

 Penetration of the cortical cell walls has been observed in 

 isolated cases. The mycorhiza may be said to be purely 

 Ectotrophic. 



On one and the same mycorhiza body two types of 

 mycelium may occur: — 



1. To the inside of the enveloping layer, and between the 



cells of the cortex, the mycelium is slightly or 

 sparingly septate, swollen, vesicular and irregular, 

 with frequent dark staining circular bodies embedded 

 in the interwoven mass and terminal to hyphae. 



2. To the outside of the enveloping layer, the mycelium 



may be regular, septate, with clamp connections. 



Strong evidence, as specified for Sitka spruce, exists that the 

 mycorhiza body is gradually destroyed by the fungus. 



Mycorhiza are not found developed on all Mountain pine 

 roots in peat, but whether developed or not, an endophytic 

 fungus is usually found in association with the older parts of 

 the root-system. It is similar in all respects in its penetration 

 and appearance to that present on the spruce. No vesicles 



