THE FUNGI 253 
on account of their economic importance. Forms differ- 
ing enormously in general appearance, size, and life habits 
are included in this class. : 
Here belong two genera of large, edible fungi, the morel 
(Figs. 176, 177) and the truffle. Many injurious genera 
are found, some of them saprophytes - ¢ 
and others parasites. Best known ; 
among the former are the green and 
the yellow molds, more properly 
called mildews (Fig. 175). A few ex- 
amples of the latter are the grape- 
vine mildew, the black-knot fungus of 
“plum and cherry trees, the peach-curl 
fungus, the bitter rot of apples, the 
brown rot of peaches, and the plum 
pocket fungus. 
LICHENS, SAC FUNGI AS MESS- 
MATES WITH ALG 
327. Definition, Occurrence. 
— Lichens (Figs. 178, 179) were ; 
formerly supposed to be a dis. 7, 117 A Yeu! Sno of 
tinct class of plants, and itisless pig 176. (x 240.) 
than forty years since their real 4 asci, or spore-sacs; m, myce- 
nature began to be understood. lium; p, paraphyses, or sterile 
A lichen is a combination of two oa 
plants. The green cells, called the gonidia (Fig. 180, g), be- 
long to some species of alga, and the remainder, the larger 
portion of the growth, is a fungus (usually a sac fungus) 
parasitic upon that alga. The groups of lichens correspond 
in structure to certain groups of fungi, but the genera are 
