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. Tin's criticism is valid up to a point. The screen 

 wliich the root makes is of two kinds : firstly, a layer of 

 cork across the phloem and cortex, i. e. 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 cOnsecpience 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 tlie 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, whicli, 

 being washed down by rain, will reach the lower strata, 

 where, as shown in the next section, dead roots most cora- 

 moidy occur. 



I 2 



