20 



XI. EUMYCETES. 



Till I' l NCI. 



The Fungi tall naturally into four classes : 



I. Phycomycetes {Algal Fungi)) which exhibit reasonably clear 



affinities with the Algae. An example of this class is the 

 common white mould. 



II. Ascomycetes, including such forms as the edible morels and 



truffles. 



III. Basidiomycetes, to which the edible mushrooms, the poisonous 



Amanitas, and all the "toadstools" and "puff-balls"' belong. 



IV. Uredineae {Rust Fuugi), which include many of the worst 



disease-causing fungi, such as the rust fungi of cereals, the 

 coffee disease, etc. 



These classes differ widely from one another in many respects, but all 

 agree in the fact that the vegetative part of the plant consists of a number 

 of slender, branched filaments, called individually the hyphae and collectively 

 the mycelium of the fungus. All fungi agree also in the entire absence 

 of chlorophyll, and hence are obliged to obtain their food from organic 

 materials. If they obtain this food from other living organisms they are 

 known as parasites {e.g. rust of wheat) ; if from non-living organic 

 substances they are called saprophytes {e.g. mushrooms). 



Class I. Phycomycetes. 



There are two well marked families of this class, and both are of some 

 importance. The first is the Oomycetes, including a number of parasites, 

 most of which destroy seedlings. The disease caused by them is known 

 as the "damping off" of seedlings. Perhaps the best known member of 

 the family, however, is that which causes the potato disease, Phytophthora 

 infestans. The spores of this fungus germinate in wet weather on the 

 leaves of the potato plant, and produce a mycelium which grows first in 

 the tissues of the leaf, and from there spreads to the stem and finally to 

 the tubers. After growing in the leaf for a few days, some of the hyphae 

 begin to grow out of the stomata. These are stouter than the ordinary 

 vegetative hyphae and branch a few times after leaving the stoma. At the end 

 of every branch a single pear-shaped sporangium (Fig. i 6 H) is formed, and 

 then immediately below it a branch is given off, which soon grows in such 

 a way as to appear merely a continuation of the original branch, ami at its 

 end a second sporangium is formed, after which the process is repeated. This 

 successive formation of lateral branches, which become straightened, gives a 

 very characteristic bead)' appearance to the Sporavigiophore, sometimes 



{continual i,)i p. 24) 



