HEART-ROT 115 
conditions were not accurately reproduced, that by steam- 
ing a portion of a root and allowing it to become infected 
immediately time was not allowed for the plant to develop 
its normal power of resistance against fungal attack. For 
when a portion of a root dies, the plant to which it belongs 
inserts a screen between the living and the dead parts 
which is made as far as possible impermeable to fungal 
hyphae. This criticism is valid up to a point. The screen 
which the root makes is of two kinds: firstly, a layer of 
cork across the phloem and cortex, i.c. all the tissues 
outside the cambium ; and secondly, various devices in the 
wood, which, having non-living as well as living elements, 
is less able to make a protective layer. In conifers the pro- 
tection of this region is chiefly, if not entirely, limited to 
plugging the tracheides with resin, and even this amount 
of protection is confined to the sap-wood, which contains 
living cells. The heart-wood can adopt no such measure, 
and in consequence the heart-wood remains as a free channel 
for the growth of any fungus which can live in it. It thus 
appears that dead roots, if they are small and contain no. 
heart-wood, may be safely delimited from the living portions 
of the tree, whereas large roots, when dead, present an 
open path by which any of the heart-rotting fungi can gain 
admission to the trunk. 
Dead roots may become infected in a variety of ways 
by spores or conidia, by contact with diseased roots, 
and, what is probably more important than either of these 
methods, by mycelium growing in the soil. Pure cultures 
of the fungus only proved successful on soils when the 
latter had been sterilized, and it was shown that other 
organisms, which were present in unsterilized soils, were 
sufficient to prevent the growth of Fomes annosus. But in 
spite of being unable to produce a rigid demonstration, 
I think it is probable that under suitable conditions the 
fungus may grow in forest soil and bear conidia, which, 
being washed down by rain, will reach the lower strata, 
where, as shown in the next section, dead roots most com- 
monly occur. 
12 
