HEART-ROT CAUSED BY OTHER FUNGI 139 
wood at any time. In this respect the spruce apparently 
differs from the oak, in which 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- 
phureus 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. sulphureus 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 
