INJURIES INDUCED BY PLANTS 217 



is usually only noticed in a building when it, and consequently 

 the woodwork also, has become practically dry. 



Frequently, however, dry-rot appears in new buildings to such 

 an extent that not only the joists but also the boards of the 

 false and true floors decay. When this is the case the cause 

 is usually to be found in gross negligence on the part of the 

 contractor. Most frequently the mistake is committed of placing 

 wet deadening material ("pugging") on the false floor and 

 covering it over too soon, either with the boards of the sub-floor 

 or of the true floor. I have thoroughly discussed the subject of 

 deadening material in my work on M. lacrymans. It must be 

 as dry as possible, and free from humus or anything that will 

 condense moisture. Clean gravel or coarse dry sand suits best. 

 Anything of the nature of coal-dust should on no account be 

 used. 



It is a great mistake to cover the floor too soon with oil 

 paint or with parquet, because this prevents the evaporation of 

 any moisture that may have been originally present in the 

 boards, or that may have been imparted to them by the packing 

 material. The water that is contained in the packing material 

 and in the woodwork cannot afterwards escape upwards. All 

 that is possible is an extremely slow evaporation downwards 

 that is to say, through the ceiling of the room beneath. Between 

 the false ceiling and the matchboard ceiling the air becomes 

 saturated with moisture, and this space offers conditions which 

 are extremely favourable for the growth of fungi. The flooring 

 boards, being saturated with moisture derived from the packing 

 material, decompose under the action of the spores which are 

 brought from the forest in the cracks of the wood. In two years' 

 time, when the building has become perfectly dry, the moisture 

 n the boards also disappears. The withdrawal of water induces 

 very great shrinkage in the already decomposed wood of the 

 lower side of the boards, while the upper side, being exposed to 

 the air or protected by paint or varnish, is not similarly affected. 

 The result is that the upper side of each board becomes convex 

 in the middle, and the nails are easily wrenched out of the 

 partially rotten joists. Open joints are thus formed, which may 

 be large enough to admit of the entrance of one's finger. 



The repairs thus rendered necessary are very expensive, and 



