418 FIRST FORMS OF VEGETATION. 
mycelium forms a dense membrane or branched 
creeping strings, while the pores of the hymenium 
are long, slender, and minute. So virulent is the 
nature of dry-rot that it extends from the wood- 
work of a house even to the walls themselves, and 
by penetrating their interstices, crumbles them 
into pieces. We have every reason to believe that 
the leprosy of houses, so graphically described in 
Leviticus, was a dry-rot caused by some species of 
Merulius or Polyporus ; the materials and sanitary 
condition of Eastern houses being peculiarly favour- 
able for the development and spread of fungous 
growth. ‘I knew,’ says Professor Burnett, ‘a 
house into which the rot gained admittance, and 
which, during the four years we rented it, had the 
parlours twice wainscoted, and a new flight of 
stairs, the dry-rot having rendered it unsafe to go 
from the ground floor to the bed-rooms. Every 
precaution was taken to remove the decaying 
timbers when the new work was done; yet the 
dry-rot so rapidly gained strength that the house 
was ultimately pulled down. Some of my books 
which suffered least, and which I still retain, bear 
mournful impressions of its ruthless hand ; others 
were so much affected that the leaves resembled 
tinder, and when the volumes were opened, fell 
out in dust or fragments.’ Many practical 
persons have written upon this disease; and 
