38 



REACTION OF HOST TO PARASITIC ATTACK. 



men other substances make their appearance in the lignified 

 walls, chiefly tinctorial phlobaphenes. 



The walls of the wood-elements are, however, not lignified 

 to the same extent. The primary layer of the wall is, as a 

 rule, lignified most and contains but little cellulose. In con- 

 sequence, on treatment with lignin-solvents, it becomes first 

 dissolved while the secondary and tertiary membranes, although 

 their lignin is also partially dissolved out, remain behind as a 

 distinct framework of cellulose. With longer treatment destruc- 

 tion of the tissue proceeds till only the pure cellulose membranes 

 of the isolated cells remain. The ferments of many fungi act 

 in this way ; for example Trametes pini, as shown in Fig. 1 2 ; 

 at a the wall is in its normal condition, showing a primary 



Fig. 12.— Section of tracbeides of pine-wood in process of dissolution by the 

 ferment of Trameta pini. ij^. (After R. Hartig.) 



wall and two striated secondary membranes ; at b the fungus- 

 ferment has caused a splitting of the primary wall, which 

 formerly appeared as a single layer, and the elements are 

 separating from each other ; the " filling-material " of the inter- 

 cellular spaces (under c), and the ring of lime surrounding the 

 cavity of the pit d, remain for a longer time ; the right wall 

 of the cell b consists only of cellulose, (as indicated by the 

 striation being no longer shown, although still present) ; in 

 the cell e the primary wall has disappeared, and the secondary 

 and tertiary membranes thin off towards / in which only 

 the ash constituents remain as fine granules, better seen in 

 Fig. 13. 



In contrast to the ligniu-dissolving fungi, there are those 

 which dissolve cellulose. When wood is treated with sulphuric 



