40 



TECHNICAL PROPERTIES OF WOOD. 



qther words, the wood becomes "rotten." The rotten elements 

 may fall away in dust and produce a hollow. For our purpose we 

 may consider separately concealed rot and external rot. 



The internal rot caused by parasitic fungi, popularly termed 

 ''wet rot," "red rot," "white rot," &c., may find entrance into 

 the tree either by the roots or through a dead branch, or through a 

 wound in the stem. Rot that enters through a dead or broken 

 branch of some size {see Fig. 8) is the most fatal of all to the 

 value of the tree, as it always extends down the entire stem. 

 Both parasitic and saprophytic fungi attack the broken jagged 

 end, which moreover absorbs large quantities of atmospheric mois- 

 ture. The fermenting action of the fungi converts the wood into 

 a mixture of acid substances, which are carried down into the 

 portions below by the rain soaking into the branch, and which 

 are poisonous to the living parts of the tree. Thus the rot spreads 

 rapidly downwards to the base of the tree. Often a callus grows 

 over the edge of the broken branch, and forms a constantly 

 deepening cup to catch and retain the rain water {see Fig. 

 9). Rot that enters by way of the roots is the most dangerous 



Fig. 9. 



Hollow formed by callus-formation over edge of a dead 

 branch, and progressive rot. (After Boppe). 



