254 THE FUNGI 



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most destructive rots are sac fungi, though frequently caused 

 by some kind of conidial fructification rather than by the sac 

 fruit. Among them are the bitter rot of apples (Glomerella), 

 brown rot of peaches and plums (Sclerotinia), and plum pockets 

 (Exoascus). 



270. The imperfect fungi. Some other spot diseases and rots 

 are caused by fungi which are known only through conidial 

 or other types of asexual fructification. More species of these 

 forms have been described than of all the sac fungi together, 

 and they are assembled in a group called the Fungi imperfecti. 

 Some of them are very important economically, causing such 

 diseases as the potato scab (Oospora), tar spots (anthracnose) on 

 beans (Colletotrichum), black rot of tomato (Macrosporiiim), and 

 black rot of apples (Splicer opsis). Most of the imperfect fungi, 

 however, are saprophytes, and play an important part with 

 other saprophytic fungi in bringing about the decay of vege- 

 table debris. 



271. The lichens. The lichens deserve special consideration 

 as a very remarkable group. They are not single plants, but 

 composite organisms made up of algae which are contained in 

 an enveloping mesh of fungal filaments. The algal cells show- 

 ing through the fungal layers frequently give the lichen a 

 greenish color, but other pigments may be present, and some 

 lichens have brilliant yellow, orange, brownish, and reddish 

 tints. Lichens have a great variety of forms. Some grow closely 

 pressed against rocks and tree trunks (crustaceous, Plate V, J), 

 some are leaf-like ( foliose, Fig. 226), and some are much branched 

 (fruticose, Fig. 227). 



Tlie fructification of a lichen is most commonly a saucer- 

 or cup-shaped structure. The inner surface is a fruiting layer 

 (Plate V, 7>), and contains numerous eight-spored sacs, or asci 

 (Plate V, D), showing clearly that the fungi concerned in the 

 lichen are sac fungi, or Ascomycetcs. The fructifications are 

 therefore sac fruits, or ascocarps, and these are known in some 

 forms to develop as the result of a sexual process. Most of the 



