INJURIES INDUCED BY PLANTS 



199 



due to my having access only to old dry sporophores, which 

 rendered the correct identification difficult. In the interval 

 Professor Magnus has correctly identified 

 the fungus as P. Schweinitzii. It appears 

 on the Scotch pine, the Weymouth pine, 

 and the larch. 



The decomposition which it produces 

 very closely resembles that due to the 

 preceding species, but in the present case 

 the white branching mycelial strands are 

 absent, the mycelium at most growing 

 out of the fissures as a fine chalky in- 

 crustation. The smell of the wood, which 

 is very characteristic and intense, reminds 

 one of the smell of turpentine, without 

 however being perfectly identical. 



The sporophores, which appear on the 

 dead wood or project from the bark-fissures 

 of living trees, take the form of reddish 



brown cushions, which afterwards assume 



FIG. 125. Tracheid of 



a somewhat bracket-like shape. The porous Pinus destroyed by P. 



layer, which is yellowish green when young, 

 assumes a deep red colour if ever so slightly 

 abraded. 



As decomposition advances the tracheids 

 exhibit spiral cracks and fissures (Fig. 125). 

 Apparently these cracks are due to the 

 shrinkage of the wall-substance,which always 

 remains fairly dry. It is owing to these 

 cracks that the wood is so easily pul- 

 verised. 



P. vaporarius also induces cracks and 

 fissures in the cell-walls, but, instead of 

 extending completely round the cell-lumen, 



these are small, and are arranged in large numbers in vertical 

 rows. 



extracted, the walls 

 consisting chiefly of 

 lignin. Cracks occur 

 in the secondary wall 

 when dry, while the 

 primary wall, a d, re- 

 mains intact. The 

 spiral structure of the 

 secondary wall is the 

 cause of the fissures 

 in the walls of adjoin- 

 ing cells crossing at the 

 bordered pits, c, and at 

 the punctures, d^ e. 

 Where pits and punc- 

 tures are absent the 

 fissm'es are simple, f. 



