DURABILITY OF WOODS 163 



cellulose contains approximately 44 per cent of carbon, the lig- 

 nified tissues contain upwards of 68 per cent of this element. 

 From this we are led to conclude that tissues yield to or resist 

 decay just in proportion to the extent of such modifications, 

 which are to be regarded as of a protective character. If this 

 principle be extended to lignified tissues in general, we must then 

 admit that since the extent and quality of the lignification do 

 not develop equally in all species, these latter must exhibit corre- 

 sponding differences with respect to their ability to resist the 

 disintegration attending what is commonly called decay, whether 

 such decay arises primarily as a process of slow oxidation, or 

 whether it is initiated through the operation of active enzymes. 

 The general law thus stated has an illustration in a very striking 

 instance of the relation which the special character of the cell 

 wall bears to agents promotive of decay, as recorded by von 

 Schrenk (68, 49), who shows that while Polyporus versicolor 

 readily attacks the living catalpa tree and produces widespread 

 decay, there is no fungus which will attack the timber when once 

 it has been cut and seasoned, a fact which serves to explain the 

 astonishing durability of this wood in spite of its great porosity. 

 Another factor of great importance is to be found in the con- 

 ditions which immediately surround a given timber, since it is a 

 well-known fact that the same species of wood does not exhibit 

 the same degree of durability under all conditions. Thus wood 

 in a well-drained and well-aerated soil will have a much longer 

 term of life than it would in a wet and badly aerated soil. Or, 

 again, the same difference would hold true as between a com- 

 paratively sterile soil and one which is rich in organic compounds. 

 The life of a timber in salt water is far greater than in fresh 

 water, or even than in well-drained soil, owing to the specially 

 preservative action of the salt ; while the durability may be 

 indefinitely prolonged if the wood be hermetically sealed in an 

 impervious matrix such as clay. These differences are readily 

 susceptible of an explanation by reference to the relation which 

 the various media bear to the growth of fungi and bacteria, since 

 we recognize in these two groups of plants the active agents 



