Fistulina hepatica a7td Stag-headed Oaks 213 



able to cope with the adv^ance of the myceHiim and become 

 destroyed quickly. Callus growth would be encouraged between 

 the healthy wood of the main roots and the diseased wood and 

 it is thought that the fibrous roots which develop in such cases 

 would arise in the first place from such a "callus area." Possibly 

 the accumulation of food from the sound, but now severed, 

 phloem cells causes the gout}^ appearance round the base of the 

 trunks and on the buttresses of the main roots, and may also 

 assist in giving rise to the abundance of adventitious roots 

 which ultimately grow from the upper surface of the main roots. 

 Part of the stem is thus no longer in direct connection with the 

 wood and bast elements of the lower portion of the main roots, 

 and the holes and rifts which frequently arise at such places 

 giving access to the hollow interior of the tree show that the 

 fungus makes a special onslaught on this portion of the stem. 



TREE MYCORRHIZA. 



(Chiefly Field Notes.) 



With Plates XI and XII. 

 By Robert Paulson. 



The close association of a fungus with the rootlet of a higher 

 plant produces the phenomenon known as mycorrhiza or fungus- 

 root, a condition that occurs abundantly, almost without 

 exception, on the roots of Qiiercus Robur, Fagus sylvatica (i) fig. i, 

 Carpimis Betulus(i) fig. 2, Betula alba, Castanea saliva fig. 3, 

 Pinus sylvestris and Taxus baccata in woodlands of the south- 

 eastern counties, especially in those located on a light soil. 



The south-eastern counties are mentioned for the reason that 

 the writer's observations on fungus-roots have been carried on 

 mostly in that part of the country. 



Mycorrhiza is developed to its fullest extent upon roots that 

 do not enter the soil but spread out horizontally throughout 

 masses of decaying leaves which accumulate in depressions of 

 the ground near tree trunks. Should there be carpets of moss on 

 the more level ground, such as those formed by Mnium undu- 

 latum and by the coalescent cushions of Lcucobrymn glaucum, 

 roots of trees grow upwards into the lower, decaying layers of 

 the moss and there develop the state of mycorrhiza. 



Figs I, 2 and 3 each represents a horizontal plane as seen by an 

 observer on bending forward to look upon the ground. All that 

 was necessary when preparing to expose the photographic plate 

 was lightly to remove, preferably with the fingers to avoid 

 disturbance of the roots, the topmost layer of the leaves to a 



