DEVELOPMENT OF PLANTS 



215 



88. Order e. Sphaeriales or Black Fungi. — This is the largest 

 group of the ascomycetes, over 2,000 species being known in the 

 United States alone. Scarcely a fallen twig or bit of old wood 



Fig. 150. 



Fig. 149. A common black fungus, Hypoxylon: A, habit of the fungus 

 as it appears on dead branches and logs. The round black bodies are an 

 association of the mycelium, stroma, and numerous ascocarps. B, a single 

 ascus enlarged, showing character of the ascospores. 



Fig. 150. The black knot, Ploivrightia, infecting a branch of cherry. At 

 the bottom of the branch is shown the early summer or spore-bearing 

 stage, c, and above a black warty mass of ascocarps, as, produced the 

 previous season. 



Fig. 151. A, several ascocarps enlarged, taken from region c in Fig. 

 150. B, diagram of an ascocarp as seen in section, showing the asci and 

 the opening for the escape of the ascospores. 



can be examined without revealing the minute hard black asco- 

 carps (Figs. 149; 152, A). Many of these fungi are quite con- 

 spicuous since the ascocarps are found in large compact masses 

 and also because they are often associated with a more or less 

 conspicuous outgrowth from the mycelium called the stroma 

 (Figs. 150; 152, D). The majority of the genera are saprophytic 

 upon dead and decaying vegetation, though some of them are 

 destructive parasites. The black knot, Plowrightia, the cause of 

 a serious disease to plum and cherry trees, illustrates very well 

 the characteristics of this order. The mycelium grows in the 



