76 



BOTANY. 



of the last, Collema (Fig. 44, C, D, E), a dark greenish, gelat- 

 inous form, growing on mossy tree trunks, and looking like a 

 colony of Nostoc, which indeed it is, but differing from an ordi- 

 nary colony in being penetrated everywhere by the filaments of 

 the fungus growing upon it. 



Not infrequently in this form, as well as in other lichens, special cavities, 

 known as sperm ogonia (Fig. 44, JD), are found, in which excessively small 

 spores are produced, which have been claimed to be male reproductive 

 cells, but the latest investigations do not support this theory. 



The last group of the Ascomy- 

 cetes are the "black fungi," Pyre- 

 nomycetes, represented by the black 

 knot of cherry and plum trees, 

 shown in Figure 46. They are 

 mainly distinguished from the cup 

 fungi by producing their spore sacs 

 in closed cavities. Some are para- 

 sites ; others live on dead wood, 

 leaves, etc., forming very hard 

 masses, generally black in color, 

 giving them their common name. 

 Owing to the hardness of the 

 masses, they are very difficult to 

 manipulate ; and, as the structure is not essentially different 

 from that of the Discomycetes, the details will not be entered 

 into here. 



Of the parasitic forms, one of the best known is the u ergot " 

 of rye, more or less used in medicine. Other forms are known 

 that attack insects, particularly caterpillars, which are killed 

 by their attacks. 



FIG. 46. Branch of a plum- 

 tree attacked by black knot. 

 Natural size. 



