32
THE ESSEX NATURALIST.
materials is easily the most resistant organic constituent ; in
woods it may be attacked fairly readily, if slowly, by certain
Basidiomycetes, but its removal is always accompanied by losses
of cellulose and hemicelluloses. Thus the simple explanation
of the difference between brown and white rots is no longer
entirely satisfactory, although it is, nevertheless, still convenient
to divide fungi attacking the wood substance into these two
categories.
The sap stains are caused by fungi which do not primarily
attack the wood substance, but which penetrate the wood and
obtain their nourishment from food materials, like starch, in
the lumina of the cells. They do not cause decay, although they
may bring about a slight decrease in certain strength properties
of the timber: their hyphae usually pass from cell to cell by way
of the pits, but they may penetrate the walls, and in the rays
the walls may be partly destroyed [20]. Despite their benign
habits they often prove undesirable additions to a piece of
timber.
The moulds, which were noted above as occurring on timber,
may for convenience be classed with sap stains, although, as their
hyphae are usually colourless, they do not cause serious stain.
They sometimes prove troublesome when, as may happen, they
occur in great masses during kiln seasoning and interfere with
the drying of the wood [22].
To the classification just outlined may be added that group
of fungi which attack the cambium. It may reasonably be
objected that their habitat brings them outside the scope of the
present paper, but since their activities may subsequently affect
the wood they may appropriately be considered.
Like other living organisms, wood-inhabiting fungi require
certain conditions before they can occupy a substratum. There
are four main requirements, the lack of any one of which will
make the substratum uninhabitable. These requirements are
a suitable supply of food, water, oxygen and a suitable range of
temperatures. To these may be added a means of penetrating
the substratum which they attack.
Being devoid of chlorophyll, fungi are dependent upon
complex organic materials for their nourishment. Some, the
parasites, obtain this from living organisms, others find their
food in dead matter—they are saprophytes. As far as fungi which