CHAPTER XVIII 

 PHYSIOLOGY OF FUNGI, LICHENS, BACTERIA 



MANY of the Fungi play a very important role in the economy 

 of nature. The saprophytes, in association with Bacteria, are 

 largely responsible for the decomposition of vegetable remains, 

 and without them the whole surface of the earth would become 

 buried under the bodies of plants and animals. Through their 

 agency the material locked up in the raw humus of the soil is 

 transformed into simpler chemical compounds, and rendered 

 available for the use of higher plants. A striking instance of 

 this function is afforded by those cases in which the Fungus, 

 responsible for the decomposition of humus, is intimately asso- 

 ciated as a mycorrhiza with the underground organs of Flowering 

 Plants (cf. p. 72 and Fig. 31). The processes of decay initiated 

 by saprophytes are, however, often detrimental to the interests 

 of man, as in the case of the Basidiomycete (Merulius lacrymans) 

 responsible for so-called " dry rot " of timber, and the diverse 

 and almost ubiquitous Moulds which all too readily develop upon 

 articles of food. 



The parasitic species frequently do great damage to cultivated 

 plants and to animals. In some cases the parasite sooner or 

 later brings about the death of its host (e.g. Silver-leaf Disease 

 of Plum), but more commonly (e.g. Mildew, Rust) the diseased 

 plant, though injured, continues to live as an unhealthy indi- 

 vidual and to maintain the parasite which grows at its expense. 

 A considerable number of parasites can also exist as saprophytes, 

 so that they remain alive after the host has died (e.g. Pythium 

 and many Smuts). Conversely, certain saprophytes (e.g. Mucor) 

 can occasionally act as parasites, generally after access has been 

 obtained at a point of injury. The so-called wound-parasites 



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