TREE ROOTS I THEIR ACTION AND DEVELOPMENT. 9 



This destruction of the cortical tissue is especially evident in 

 the dense vesicular type of mycelium, which is also intercellular. 

 The destruction is assumed to take place for the following 

 reasons : — 



1. The outline of the root-section is not regular and in 



places cells appear to be missing (Fig. 2). 



2. In the dense web itself, cells are found far removed from 



their neighbours. These isolated cells have escaped 

 destruction for some reason. 



3. In places the outer wall of the outermost layer of 



cortical cells has disappeared, and the mycelium fills 



the cell cavity. 

 In the root-tip region where Ectotrophic mycelium exists, it 

 is rather uncommon to find intracellular mycelium penetrating 

 from the web towards the endoderm. Behind this region, 

 however, and in older parts of the root-system, where cork is 

 not strongly developed, fungal filaments are almost invariably 

 found to be present inside the cortical cells. This fungus has 

 no features in common with Endotrophic mycorhiza. For 

 instance, it is regular, septate, as contrasted with the swollen, 

 crowded, coiled mycelium of typical Endotrophic mycorhiza, 

 and its general trend is in a radial direction, whereas in Endo- 

 trophic mycorhiza the trend is more or less in a longitudinal 

 direction. Root specimens of Sitka spruce from peat at Borgie 

 (Sutherlandshire), however, show one or two interesting features. 

 First, the roots show the usual internal mycelium mentioned 

 above, but this mycelium often ends in a vesicle similar in size 

 and appearance to the vesicles of the Ectotrophic mycelium. 

 Some of these vesicles are empty, others have granular contents. 

 The vesicles are not cut off by a septum, and resemble very 

 much superficially the reproductive bodies of members of the 

 Chytridiaceae, e.g. Olphidium, but here the mycelium is septate, 

 whereas in the Chytridiaceae it is non-septate. Second, a much 

 stouter swollen mycelium occurs both in the intercellular spaces 

 of the cortex and in the cortical cells. This stout mycelium has 

 also terminal vesicles, but these bodies are never empty and 

 contain granular contents and oil globules. The stout mycelium 

 and the ordinary mycelium might be regarded as quite distinct, 

 were it not observed that the stout mycelium in places suddenly 

 merges into the thin septate mycelium typical of the other. 

 Although not conclusive evidence, if taken in conjunction 



