426 PROTECTION AGAINST PLANTS. 



The walls of the hyphae are at first always soft and colour- 

 less, but when older they may be considerably thickened and 

 coloured brown or greenish-blue, as in rotten spruce- or beech- 

 wood. Sometimes the hyphae unite into compact bundles with 

 hard walls, termed rhizomorphs, which resemble roots, and 

 serve to carry the mycelia through unnutritious or dry media. 

 These are very conspicuous in Armillarea mellea, Vahl. Hyphae 

 also sometimes unite into small tuber-like bodies termed 

 sclerotia, which have thick cell-walls, and are richly supplied 

 with protoplasm and oil, and, as in Rosellinia quercimi, 

 R. Hrtg., may remain for some time dormant and resist desic- 

 cation, but under favourable conditions develop new mycelia 

 or sporocarps. 



In this way the mycelia of parasitic fungi live on the tissues 

 and nutritive material of their host, and interfere with its 

 transpiration and assimilation; they also dissolve the cell- 

 walls and their contents, often causing hypertrophy or excessive 

 formation of cells, and chemical change in the cell-wall. In 

 the latter case, they cause the death of the host. Insects 

 frequently attack trees which have become weakened by fungi. 

 Eventually the fructifying organs, which are characteristic for 

 each species of fungus, break out on leaves, twigs, bark or at 

 the scars of dead branches, sometimes through perforations 

 made by bark-beetles, sometimes on the roots of the host, or 

 on rhizomorphs, as in Armillarea mellea, Vahl. Innumerable 

 spores issue from the sporocarps, some of which find suit- 

 able resting-places, and the fungus-life recommenc.es in fresh 

 hosts. 



Most fungi are very transitory, and their life occupies only 

 a few months or weeks. In the case of others, the resting 

 spores hibernate, and the mycelia of some fungi may live for 

 two, three, or many years. Most of the destructive forest 

 fungi have the latter character. The polymorphy which exists 

 in the case of certain fungi requires an explanation here. 

 From the spores of certain fungi the same form does not 

 always appear, but sometimes one perfectly distinct, unlike 

 the parent fungus, and living on a different host ; its spores 

 may even produce a third form, though eventually the original 

 fungus is reproduced. Thus, many fungi, formerly considered 



