HEAR-ROT CAUSED BY OTHER FUNGI 139 



wood at any time. In this respect the spruce apparently 

 differs from the oak, in whicri Hartig noted that the mycelium 

 almost completely fills some of the tracheides and vessels. 

 I find it very difficult to distinguish microscopically 

 between old rotted wood of larch decomposed by P. sul- 

 phur eus and P. Schweinitzii. The oblique fissures in the 

 tracheide walls are very similar in the two cases, though 

 with each fungus the range of variation in this respect is 

 considerable. Transverse cracks crossing one or two 

 tracheides are, however, more frequently associated with 

 P. sulphur eus than P. Schweinitzii. It should be easier to 

 control this fungus than any of the root-rotting species. 

 Infection is sub-aerial, and fructifications when made are 

 easily seen. Destruction of fructifications as soon as they 

 are spore-bearing will do much to prevent the spread of 

 the fungus, but as they can grow again after being removed 

 early in the season, a constant watch has to be kept, and 

 it is better to cut down infected trees and, after utilizing 

 undamaged portions, to burn the remainder. Hartig found 

 that in the vessels and tracheides of oak wood a form of 

 conidium was often produced, apparently by the mycelium 

 of this fungus ; but its connexion with the fungus was not 

 proved by culture experiments, and the conidia may have 

 belonged to a saprophytic fungus which gained admission 

 to the wood subsequently to its destruction by P. sulphureus. 

 If it is found that these conidia belong to the parasite and 

 can carry infection, stringent measures would be necessary 

 to localize the disease in districts where the oak is attacked 

 by the fungus. For such oaks become hollow and often 

 open at the side, and the powdered decomposed wood can 

 be blown about by the wind, and with it the conidia. Con- 

 sequently all these hollowed old oaks, which are common 

 in parks, would be a source of infection for other trees, quite 

 apart from the production of visible fructifications. But, 

 as yet, the relation between these conidia and P. sulphureus 

 is conjectural. Schrenk found no trace of them in the 

 wood of the spruce, and Brefeld (1889), who grew pure 

 cultures of the fungus, states that no conidia were borne 



