DISEASES, INSECTS, AND ANIMALS INJURIOUS TO FOREST TREES. 279 



fungus, ^Ecidium elatinum. It is noticeable at different heights 

 up the stem ; the bark gradually dies off froro excrescences formed, 

 often of very considerable size, and the wood is laid bare and 

 begins to rot. Sometimes two or three cankerous spots may be 

 seen on one tree, rendering the bole more or less unsuited for 

 technical purposes, and, under any circumstance, there is consider- 

 able loss in value. 



Nf'ctrias. 



Spruces, and to a less extent silver fir and pines, are often 

 affected with a disease caused by the ''^ Nectria cucurhitula,^' the 

 hypha? of which invade the small sieve-tubes of the bast. As long 

 as the fir is doing well the parasite is confined to the resting parts 

 of the bark, and cannot make its way into the active cambial 

 region, the living cells of which go on dividing and growing quite 

 normally. Thei'e is considerable reason • for believing that it 

 makes all the difference to the fungus what kind of start it gets. 

 If the mycelium is young and feeble the active tissues of the 

 cortex may cut it out very soon, and the ordinary observer can 

 find no trace of the invading fungus or of disease ; but if it starts 

 in a bed of dying cells capable of yielding it sufficient food 

 materials (the hyphie can grow in a cavity flooded with turpen- 

 tine), its rate of spread depends entirely on what resistance is 

 offered by the vegetable activity of the cells around. This raises 

 the interesting question of the mutual actions and reactions 

 between the hyphtie and the living cells of the tree. All I can 

 say here is that the hyphaj may excrete some poison-like sub- 

 stance whose action the living cells of the cortex and cambium 

 may resist so long as they are strong, well fed, and vigorous, and 

 may even break up and destroy. Once let such cells, however, fall 

 below a certain standard of health and activity, and the hyphaj 

 make their way in and carry all before them. Factors in the 

 environment (soil, temperature, light, etc.) determine the result. 



HysteriMin pinast7'i. 



This is a disease which is very often noticeable on young pines 

 when in the seed-bed. In autumn the leaves first show a slightly 

 mottled appearance, occasioned by the mycelium of the fungus 

 developing in the interior of the foliage. In the following spring 

 the needles rapidly assume an entirely brown colour and die ofi', 

 and the spermogonia of the fungus show as black pimples. Pines 

 so affected are of very little or no use for shifting, as they are so 



