PSATHYRELLA DISSEMINATA Z7 



mycelium. A fungus which produces an ozonium has its myceUum 

 differentiated into two kinds. The first kind is the ordinary vege- 

 tative mycelium. This is composed of thin-walled colourless hyphae 

 which, by means of enzymes which they excrete, actively attack 

 the substratum, such as wood or bark. Thus the solid contents of 

 the substratum are made available for the growth of the mycelium 

 as a whole and, ultimately, for the production of fruit-bodies. The 

 second kind of mycelium is the ozonium. This is developed at the 

 expense of the ordinary vegetative mycelium and grows on the 

 surface of, and away from, the nutritive substratum. Its hyphae 

 are thick-walled and red, and are combined to form anastomosing 

 cord-like strands. There is no evidence that it derives food- 

 substances from the medium in which it grows, although it is 

 doubtless able to absorb water. Its functions are two : (1) it 

 serves to spread the fungus locally in a rapid manner, and (2) it 

 enables the fruit-bodies to arise in more suitable locations than 

 would often be possible, were these organs always obliged to 

 arise directly upon the ordinary vegetative mycelium at the surface 

 of the substratum. 



One can easily conceive of the manner in which the ozonium of 



Psathyrella disseminata effects the local spread of the fungus. Let 



us suppose that one root of a stump has become infected with the 



mycelium. In a little while an ozonium is produced at the surface 



of the root and grows outwards through the soil. It is likely to 



meet with other roots of the same stump, give rise to ordinary 



hyphae which may penetrate them, and so cause their infection. 



One can further imagine that the fungus, by means of its ozonium, 



might even pass from one dead stump to another. Of course, in 



order to establish the correctness of these views, the support of 



direct experiments is required ; and, unfortunately, up to the 



present, such experiments have not been undertaken. But the 



rhizomorpha subferranea of Armillaria mellea affords a parallel 



instance. Robert Hartig has shown that this structure, which 



is produced at the expense of the mycelium infecting the 



wood of the host-plant, can not only give rise to fruit-bodies, 



but can creep through the ten-centimetre-deep uppermost layer 



of the soil from one tree to another, penetrate the bark of new 



