CHAPTER IX 

 GALLS CAUSED BY FUNGI AND MYCETOZOA 



FUNGI are plants without a system of true tissues and 

 without chlorophyll ; the tissues are replaced by inter- 

 twining threads known as "hyphae," Oxygen is absorbed, 

 and carbon dioxide is given off. The order is a very large 

 one, comprising minute species known popularly under such 

 terms as rust, smut, mould, and mildew, also the larger 

 species, often spoken of as mushrooms and toadstools. The 

 majority may be arranged under two headings, saprophytes 

 and parasites. There are some intermediate forms. Certain 

 saprophytic fungi may become true parasites. Saprophytic 

 fungi flourish on dead organic matter, such as rotting wood, 

 leaves, etc. Parasitic fungi feed on living organic matter, 

 either plant or animal ; many that occur on plants give rise 

 to gall structures. 



A parasitic fungus lives at the expense of its host as a 

 rule, and confers no benefit in return, but many instances 

 may be adduced in which a kind of give-and-take arrange- 

 ment exists between host and parasite; such is known as 

 symbiosis or mutualism. It may be seen in all lichens, a 

 lichen being a composite plant, made up of a fungus and an 

 alga, the latter the host, the former its parasite. 



In some cases symbiosis favours vigorous growth and 

 the production of more seeds than in the case of plants not 

 affected by the parasitic fungus — e^., the hyphae which 

 occur in LoHum temulentum and other grasses of the same 

 genus. Spherical tumours occur on the root -fibres of 

 many leguminous plants — e.g., the Bird's-foot Trefoil and 



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