30 College of Forestry 



oiizyniatic digestion of the hemicellnlose reserve materials is 

 the explanation for the phenomenon of gnmmosis. 



As was stated earlier, the hemicellnlose, xylan, is difficult 

 of digestion and less active physio'logicallv than the trne 

 celhilose with which it is accompanied and combined. After 

 this hemicellulose is converted into sngar and giun by enzy- 

 matic action, it is not impossible that the gnm may be further 

 changed by an oxidative or other action into an insolnble by- 

 product while the sugar is utilized as food by the fungal 

 hyphae. 



EELATION OF PATHOLOGIC AND NOKMAL 

 HEAETWOOD. 



All who have concerned themselves with the question of 

 the relation of pathologic and normal heartwood have consid- 

 ered them from the anatomic and physiologic sitandpoints. 

 In normal heartwood formation it is to be suj)posed that the 

 inner, older parenchyma cells have died normally through 

 age, lack of function, deficient food supply, etc., and that 

 their contents, in part at least, seep through the cell walls 

 and even ooze out into the lumina of the neighboring cells. 

 The normal heartwood is also to be regarded as an initial 

 stage of decomposition when considered in this limited sense. 

 Where tyloses occur they may be considered to have arisen 

 from the parenchyma cells which remain alive longer than 

 the other elements. The older sapwood is always poorer in 

 water and richer in air than that last formed. It has been 

 found that heartwood, normal as well as pathologic, has 

 greater specific gravity and is richer in solid substances than 

 the sapwood. The transformation to heartwood therefore 

 must be regarded as an addition to the woody substance due 

 to the oxidation of the cell contents and the decomposition 

 of these residues within the woody elements. 



It must be remembered that, in the case of pathologic heart- 

 wood near wounds in living trees, an increase in weight has 



