INJURIES INDUCED BY PLANTS 



197 



cell-wall begins at the lumen and proceeds outwards, being pre- 

 ceded by the conversion of certain layers of lignin into cellulose. 



FIG. 124. Decomposition of spruce timber by P. borealis. #, a vigorous mycelium 

 in a tracheid containing a brownish yellow solution which has originated in the 

 medullary rays. In b and c the mycelia are still brown in colour and very 

 vigorous. At d and e the walls are much attenuated and perforated, and the 

 mycelia, not being so well nourished, are very delicate. At / the pits are almost 

 entirely destroyed. At g and h only traces of the walls remain. The destruction 

 of the bordered pits is to be followed from i to r. At i the bordered pit is still 

 intact, at k one wall of the lenticular chamber has been largely destroyed, its 

 inner boundary being marked by a circle. At / one side of the bordered pit 

 has entirely disappeared. A series of pits is shown from /;/ to ;/, in each of 

 which only a single delicate wall has been preserved namely, that which is pro- 

 vided with the closing membrane. In preparing the section a crack has formed 

 in this wall. From o to r pits are shown where both of the walls have been 

 partially or completely dissolved, and only at p and q does one perceive the 

 thickened portion of the closing membrane. At s one can plainly see the spiral 

 structure of both cell-walls, which when united form the common wall between 

 two tracheids. At / mycelia are to be seen traversing the tracheids horizon- 

 tally. 



The thin middle lamella persists longest, being converted into 

 cellulose and dissolved only after the internal portion of the wall 

 has entirely disappeared. 



