633 



CHAPTER IX 



FUXGI, as already mentioned, differ essentially from algae in the 

 absence of chlorophyll, and therefore in the absence of any power of 

 directly forming starch or other similar substance by the mutual 

 decomposition of carbonic acid and water, accompanied by evolution 

 of oxygen. They must therefore, in all cases, be either saprophytes 

 or parasites, deriving their nourishment from already organised food- 

 materials, either, in the former case, from decaying animal or vege- 

 table substances, or, in the latter case, from the living tissues of 

 other plants or of animals. Fungus-parasites are the cause of most 

 of the diseases to which plants, and of a large number of those to 

 which animals, are subject. 



The individual fungus always consists of one or more hyph(f, 

 slender filaments containing protoplasm and a nucleus (except possibly 

 in some of the most simple forms), but no chlorophyll and rarely any 

 pigment. The cell-wall is composed of a substance differing some- 

 what in its properties from ordinary cellulose, since it is not coloured 

 blue by iodine after treatment with sulphuric acid ; it is known as 

 fungus-cellulose. These hyphse may be quite distinct or very loosely 

 attached to one another ; those which penetrate the soil, or the 

 tissue of the ' host ' on which the fungus is parasitic, constitute the 

 mycele. In the larger fungi, such as the mushroom, the portion 

 above the soil is composed of a dense mass of these hyphae, lying 

 side by side, constituting a so-called pseudo-parenchyme, but never a 

 true tissue. In some families the hyphae have a tendency to become 

 agglomerated into balls of great hardness called sclerotes, which have 

 the power of maintaining their vitality for very long periods. The 

 modes of reproduction of fungi, both sexual and non-sexual, are 

 very various. Among the latter the most common are by non- 

 motile spores or gonids, and by zoospores. The former are very 

 minute bodies, each composed of a single cell, or less often of several 

 cells, which are either formed within a spore-case or sporange, or 

 are detached from the extremity of hyphae by a process of pinching 

 off or abstriction. From their extreme lightness they are wafted 

 through the air in enormous numbers, and thus bring about the 

 extraordinarily rapid spread of many fungi, such as moulds. The 

 zoospores are, like those of the lower algae, minute naked masses of 

 protoplasm provided with one or more vibratile cilia, by means of 

 which they move very rapidly through water, and finally force their 

 way into the tissue of the host, where the zoospore loses its cilia, 



