442 



PROTECTION AGAINST FUNGI. 



The empty sporocarps then appear white. As a rule only 

 bark infected in the previous year produces aecidia. The 

 colourless, septate hyphae of this fungus grow perennially in 

 the intercellular spaces of the bark, bast and medullary 

 rays of its host, sending short haustoria through the cell-walls 

 to the cell-cavities. The mycelium is developed chiefly in the 



bark and bast, but penetrates into 

 the wood to a depth of a hand. 



The mycelium converts the starch 

 in the wood-cells into turpentine, 

 which becomes infused by drops in 

 the tissues, and cuts off the supply 

 of sap. Every year it spreads chiefly 

 longitudinally from the diseased to 

 the sound wood, so that the canker, 

 which is covered with resin, con- 

 stantly increases in size. The sap 

 being confined to the sound portions 

 of the wood, produces abnormally 

 large annual zones on the side of the 

 tree away from the canker (Fig. 212), 

 and when the infection has gone 

 nearly round the tree, its crown dies 

 above the point of attack, sometimes 

 within a year, but in other cases a 

 long period up to sixty or seventy 

 years may elapse before the crown 

 is killed. 



Hot dry summers accelerate the 

 death of the crowns of infected trees, 

 as the wood surcharged with resin 



cannot pass on enough water to supply the loss by transpira- 

 tion. Although the summit of the tree is dead, the lower 

 part of it may continue to live, provided there are enough 

 living branches below the canker to nourish the tree. 



Whether or not infection must always proceed from a wound 

 in the cortex of the tree is as yet undetermined. Parts of 

 the stem older than twenty to twenty-five years appear 

 incapable of being infected. 



Fig. 213. Peridermium pini, 

 Wallr. (corticola), on a 5-year- 

 old shoot of a mountain pine. 

 The sporocarps are closed 

 (#), or have already burst (b). 

 (Natural size.) 



