304 Studies in Forestry [CHAP. xin. 



vicinity of which the bark-tissue turns brown ; whilst small, 

 round, dark-brown or blackish pustules, about the size of a pin- 

 head, make their appearance here and there on the main root. 

 The drying-up process begins at the terminal leaves and con- 

 tinues downwards. The small round pustules on the tap-root 

 are fruits (sderotid) from which numerous brown thread-like 

 rhizomorphous filaments proceed, encircling the roots and 

 extending throughout the soil. By means of these sclerotia, 

 a perennial form of mycelium, the fungus can retain its 

 vitality from year to year, thus withstanding the effects of dry 

 summer periods which would be fatal to the ordinary myce- 

 lium, to the development of which damp warm weather is 

 essential. As the tap-root is protected against infection by 

 a corky epidermis, the mycelial filaments usually effect an 

 entrance by means of the tender side-rootlets, whence they 

 push into the main body of the seedling. The spread of the 

 disease can, however, also take place through the gonidia 

 developed by the mycelium above the soil, or by means of 

 the spores of the black, spherical peritheda produced on the 

 surface of the soil or in the stems of the diseased Oak-plants. 



This disease is best combated by the removal of all diseased 

 seedlings from the nursery-beds, and by isolating all parts 

 infected by means of narrow trenches about i foot deep. 



The Changes in Generation which not infrequently take 

 place have already been alluded to briefly ; the more important 

 of them may be here named, although space prohibits any 

 details being given concerning their life-history. They are 

 specially characteristic of the rusts ( Uredineae}, all of which are 

 parasitic. 



In forming spore-producers portions of the mycelium of 

 Agaricus melleus develop into Rhizomorpha fragilis^ of which 

 there are two varieties, viz. subterranea^ where the mycelial 

 strands remain under the ground, and subcorticalis, where they 

 are under the bark of trees. 



Aeddium pint becomes Coleosporium senedo on species of 



