FUNGI, GENERAL CHARACTERS 247 



396. The vegetative or growing part of the fungi. In 



nearly all the fungi, the growing or vegetative part consists of 

 delicate filamentous or thread-like growths. These threads are 

 called mycelium, or a single thread of the mycelium is often called 

 a hypha, which may be simple or somewhat branched. A mat 

 or tangle of hyphae is spoken of as mycelium. In some fungi 

 the mycelium is scantily developed, while in others dense mats 

 or stout cords are formed. 



397. The fruiting stage or part. In most of the fungi special 

 hyphae, called sporophores (spore bearers), are developed from the 

 vegetative mycelium. These are simple or branched, single or in 

 fascicles, as in the molds and mildews; or united into large and 

 compact bodies, as in the mushrooms. Asexual reproduction takes 

 place, in most fungi, by the formation of asexual spores,* either 

 motile or non-motile. In many species there are several different 

 kinds of asexual spores on separate forms or parts of the fungus 

 (polymorphism); sometimes these different forms occur on dif- 

 ferent hostt plants (heteracism), as in the wheat rust. Sexual 

 reproduction is by conjugation in some of the lower forms, or by 

 fertilization of an egg by a sperm. The details must be sought in 

 a study of different types. 



398. The three classes of fungi. While little attention can be 

 given here to the classification of the fungi, it may be well to 

 outline briefly the three great classes. In the lower fungi the 

 mycelium is siphon-like, i.e., similar to the threads of the siphon 

 algae. The characteristic fruit structure is a spore case (sporan- 

 gium) with usually a variable and indefinite number of spores. 

 These fungi form what is called the sporangium series and make 

 up the Class Phycomycetes (alga-like fungi). Here belong the 

 molds, water molds, downy mildews, etc. 



399. In the higher fungi the mycelium is septate, i.e., there are nu- 

 merous cross walls in the mycelial threads. There are two classes. 



400. First. The characteristic fruit structure is a sac-like body 

 called an ascus, which contains usually a regular and definite 



* Conidia, or gonidia, or chlamydospores. 



t The host is the plant on which a fungus is parasitic. 



