DEVELOPMENT OF PLANTS 205 



contents of the two tips fuse, forming a thick-walled gametospore 

 (Fig. 136, B, C). After a resting period, this gametospore 

 grows directly into a new mycelium. In some of the black 

 moulds reproduction is only effected by the union of hyphae from 

 different plants which must differ therefore in their nature, just 

 as you found to be the case in some species of Spirogyra. 



81. A Fly Fungus. — An interesting form allied to the mucors 

 is seen in the parasitic fungus Empusa, that produces an epi- 

 demic among flies at certain seasons when they may be seen dead 

 and clinging to the woodwork and window panes surrounded by 

 a white halo (Fig. 137, A). This appearance is caused by the 

 discharge of numerous white sporangia that are found at the 

 ends of the hyphae that project from all sides of the fly's body. 

 (Fig. 137. B). If, by chance, a fly should come within range 

 of these discharging sporangia, it would become infested and 

 later, branches of the hyphae would project from its body and 

 repeat the process of sporangia formation. 



Class B. Ascomycetes or Sac Fungi 



82. General Characters. — The Ascomycetes are the largest and 

 most variable group of the fungi. They are illustrated by the 

 powdery mildews which affect the leaves of a variety of plants ; 

 the brown and blue moulds that occur on preserves, decaying 

 fruit, old leather, etc. (Fig. 138) ; the cup fungi and morels 



(Figs. 156, 158) ; the black knot of the cherry and plum trees 



(Fig. 150), and the ergot of rye, etc. 



The mycelium resembles in structure that of preceding orders, 

 but its hyphae are composed of numerous cells instead of being 

 tubular as in the Phy corny cetes (Fig. 138). The sporangia (Fig. 

 142) are reduced in size and contain but a single spore each, 

 which is never discharged from the sporangium. This body will 

 be referred to in the future as a spore or conidium (plu. conidia). 



The gametangia are often suggestive of those of the downy 

 mildews (Figs. 147, A ; 157), though in many forms they appear 

 to be lacking. The fusion of the gametes results in the forma- 

 tion of a gametospore which germinates at once, forming in the 



