FUNGI. 85 



appear as parasites, " obligate parasites " (Mildew, Brand- and Rust- 

 Fungi, Cordyceps). 



The parasites which live on the surface of the host-plant are 

 termed epiphytic (Mildew, Fusicladium) ; and those living in its 

 tissues are termed endophytic (Ustilago, Peronospora}. Epizoic 

 (Oidium tonsurans, Laboulbenia) and endozoic Fungi (Cordyceps, 

 Entomophthora}, are distinguished, in the same manner, as those 

 which live on. the surface or in the interior of animals. The Fungi 

 designated pathogenic are especially those which produce disease 

 in human beings and in animals. 



Most of the diseases of plants are attributed to the parasitic 

 Fungi. These force their way into the host-plant by piercing the 

 outer wall of the epidermis, as in the Potato-disease ; or by grow- 

 ing in through the stomata, e.g. the summer generations of the 

 Bust of Wheat ; or they can only penetrate through a wound, 

 e.g. Nectria. Some effect an entrance into the host-plant by the 

 secretion of a poison (toxin) and a ferment, which softens and 

 destroys the cell-walls (Sclerotinia) . Some Yeast and Mould 

 Fungi secrete ferments (enzymes), which, for example, convert 

 cane-sugar into a sugar capable of fermentation. 



The relation of the parasitic Fungus to the host-plant is mainly 

 of two kinds. In the one case, the cell-contents are destroyed, 

 the protoplasm is killed, and the cellular tissue becomes discoloured 

 and dies (Peronospora, Armillaria mellea, Polyporus) ; in the 

 other case, the parasite has an irritating effect on the cellular 

 tissue, whereby the affected organ grows more rapidly and be- 

 comes larger than normal, producing hypertrophy. Such malfor- 

 mations are termed Fungi-galls (Mycocecidia) ; in this manner 

 " witches' brooms " are produced by ^Eridiuitlj " pocket-plum " by 

 Taphrina, and other deformities by Exobasidium and Cystopus 

 candidus. This hypertrophy may either be produced by a vigorous 

 cell-multiplication, which is most frequently the case, or by the 

 enlargement of the individual cells (Synchytriwm, Calyptospora). 

 The relation between host and Fungus among the Lichens is of 

 a very peculiar nature, termed " symbiosis." 



Vegetative Organs. The vegetative parts of a Fungus are 

 termed its mycelium. 1 This is formed of a mass of long, cylindrical, 

 branched cells resembling threads (and hence termed hyphai), 

 which have a continued apical growth. The mycelium, in its 

 early development, shows a well-marked difference between the 

 1 From the Greek ^175 = Fungus, hence " mycology." 



