THE FUNGI WHICH CAUSE PLANT DISEASE 



401 



cheid of pine 

 decomposed by 

 Trametes pini. 

 The primary 

 wall dissolved 

 as far as aa; 

 in the lower 

 part the sec- 

 ondary and ter- 

 tiary layers are 

 only of cellu- 

 lose ; c, myce- 

 lium making 

 holes at d and 

 e. After Har- 

 tig. 



but must make entrance through some wound, as those due to 

 hail, wind, snow, insects, men and other animals, 

 etc., which exposes the inner bark, cambium, sap 

 wood or heart wood without its natural outer 

 protecting tissues. 



Within the tissues the mycelium may cause the 

 disappearance of substances,^^' e. g., Fomes ig- 

 niarius consumes the tannin, or the mycelium 

 may secrete enzymes which penetrate the host to fio. 288.— Tra- 

 long distances. These may dissolve first one com- 

 ponent of the cell, e. g., the lignin, next the 

 most lignified residue, the middle lamella, re- 

 sulting in dissolution of the tissue. In other cases 

 the parts of the cell walls other than the middle 

 lamella are first affected and soon shrink resulting 

 in cracks. Fig. 289. Some fungi cause character- 

 istic color changes particularly in those cell walls 

 which are rich in carbon. Parasitism in this 

 group is old since good examples of agarics 

 growing on wood are found as early as in the Tertiary 

 period.*' 

 These fungi spread to new hosts by spores borne in various 

 ways; by insects (Trametes radiciperda) animals, 

 wind (Polyporus pinicola) etc., or in a purely 

 vegetative manner by the mycelium which in the 

 form of rhizomorphs (Armillaria mellea) travels 

 through the ground to considerable distances.^*" 

 An excellent summary of the early history of our 

 knowledge of wood destroying fungi is given by 



Fig. 289.— Pine i,®338 



tracheid acted JiUlier. 



TOTus ''schwa- T^^ number of species of Agaricales which af- 

 nitzii. The fg^t jiyg plants in the ways mentioned above is 



cellulose has 7 , , . • . i • .1 • 



been extracted very great but m many mstances research m this 

 w^Hs "chiefly field has not yet revealed the true relation existing 

 lignin. Drying between the fungi and the woody plants upon 



has caused , , . V . 1 



cracks. After which they are found growmg; whether they 



^^'^' occur as parasites or as saprophytes; whether 



actually injurious or not. The species given below are mainly 



