FUNGI. 233 



groups : the Mucors, which are very common moulds 

 (see Chap. X.) and the Oomycetes, which includes a very 

 great number of destructive parasites. Amongst the 

 most interesting are the Salmon-disease Fungus, the Vine 

 mildew which gives a " dead-leaf" taste to claret and has 

 been exceedingly destructive in the Medoc country, the 

 Damping-off Disease of Seedlings {Pythmm de Baryanuvi), 

 Mildews of many cultivated plants, of the common Nettle 

 and of Brooklime ( Veronica Beccabu7igd). 



The Potato -disease i^PhytophtJiora infestans) is perhaps 

 the most important of all, and its history is fairly well 

 understood. Like most of the mildews, the fungus 

 produces whitish spots on the under side of the leaves. 

 On examining these with a strong lens a whitish bloom 

 or fluff may be noticed, which is the spore-bearing part 

 of the fungus. This consists of little branching fungus 

 threads upon which the oval spores are seated (see 

 p. 211). These threads emerge from a stoma of 

 the leaf and run back in every direction through 

 the tissues of the leaves and stalks, absorbing food 

 material from every cell that they touch. The tissue 

 of the leaf becomes broken up and disorganised 

 and the leaf begins to rot and decay. The fungus 

 is spread from leaf to leaf by the spores which are 

 blown by the wind, or carried by animals, from one 

 leaf to another. They generally fall upon the upper 

 surface of a leaf, and if the weather is wet so that the 

 atmosphere is moist, the spores germinate. Each 

 generally breaks up into seven or more small rounded 

 bodies {zoo-spores) each with two thin, hair-like tails. 

 They are able to swim through the water on the 

 leaf, as the tails contract or lash about, so that the 

 zoo-spores can travel over the surface. After a time 

 they come to rest and the tails are lost : the zoo-spore 



