So 



DISEASES OF TREES 



seedlings a few months old, a mycelium is very soon developed 

 from the black tubers which we may call resting-mycelia 

 (Sclerotia) which breaks through the bark at different places 

 and forms a dense whitish-grey mildew-like tissue, and also 

 spreads rapidly over the surface of the ground (Fig. 27). This 

 mycelium consists of septate hyphae, which are at first 

 colourless but afterwards turn brown. These, after a time, 



arrange themselves side by 

 side, grow together laterally 

 at places, and form the 

 fine strands called Rhizoctonice, 

 which consist of numerous in- 

 dividual hyphae very loosely 

 united to each other. Should 

 such a mycelium whether in 

 the form of isolated hyphae or 

 of Rhizoctonice come into con- 

 tact with the sound roots of a 

 neighbouring plant, it embraces 

 them in its meshes, and bores 

 directly into such of the corti- 

 cal cells as are still alive. 

 These are found in the delicate 

 lateral rootlets and near the 

 apex of the tap-root. The my- 

 celium penetrates as far as the 

 medulla, should such be pre- 

 sent, and in a short time kills 

 the root. In the living cortical 

 parenchyma of the tap-root 

 which is only to be found on 



the lowest and youngest parts the cells become plugged up 

 with a luxuriant growth of pseudo-parenchymatous tissue, 

 which, owing to the occurrence of numerous oil-globules, 

 may be recognized as a resting-mycelium. These bodies, which 

 germinate under favourable conditions, may be designated cham- 

 bered sclcrotia. On account of the formation of a periderm layer 

 in its cortex, the older parts of the tap-root are protected against 

 the direct attack of the parasite. The outer cortical cells being 



FIG. 27. Oak root enveloped by the 

 mycelium of R. quercina, a, on which 

 perithecia have developed at b. 



