INJURIES INDUCED BY PLANTS 



81 



partly shrivelled up and partly cast off, there remains but one 

 path of entrance into the interior of the root. After the fine 

 lateral roots that pierce the corky covering have been killed by 

 the parasite, openings or breaches are left at certain points, 

 through which the parasite 

 effects an entrance, and this 

 it accomplishes in a pecu- 

 liar manner (Fig. 28). At 

 such places frequently both 

 above and below the base 

 of the dead root fine white 

 mycelial outgrowths are first 

 formed. These develop into 

 fleshy tubers, ultimately pos- 

 sessing a dark brown cover- 

 ing, which send several fleshy 

 processes into the tissues of 

 the oak root (Fig. 28, c, cT). 



The adjoining cortical tis- 

 sues are killed and become 

 brown (Fig. 28, e). Should 

 dry or cold weather inter- 

 vene, the host-plant gains 

 time to form a new layer of 

 cork in the neighbourhood 

 of the infecting tubers along 

 the line that marks the limit 

 of living tissue. In this way 



the plant may be, for the 

 time, saved. Should the con- 

 ditions of growth remain 

 favourable for the fungus, 

 however, the fleshy pro- 

 tuberance pushes out a fine 



FIG. 28. Point of infection by R. quercina, 

 magnified twenty times. The delicate 

 lateral rootlet a, which has been killed by 

 the filamentous mycelium, displays fleshy 

 infection-tubercles, b c, at the place 

 where it has ruptured the periderm 

 of the tap-root. These tubercles send 

 processes, d, into the internal tissues. 

 The adjoining cellular tissue is brown, e, 

 but free from mycelium. A Rhizoctonia- 

 strand, f, has been produced by the 

 upper tubercle, which has consequently 

 parted with a portion of its nutritive 

 substance. 



filamentous mycelium, which 



spreads through all the tissues of the root and kills it. 



In the sclerotia the parasite possesses a means of existing 

 from one year to another, and of resisting the periods of drought 

 during summer which kill all filamentous mycelia as well as 



G 



