MICRO-ORGANISMS 13 



asci, as in the family Gymnoascaceae, through such complicated si ruc- 

 tures as those in the morels, in which there is differentiation into a 

 stipe and cap, much as in mushrooms. The four main types of 

 ascocarps are as follows: The cleistothecia are sterile hypnae or 



pseudoparenchymatous cells enveloping the asci completely and 

 leaving no opening for escape or discharge of the ascospon The 

 perithecia are similar to the cleistothecia, but they have pore- or -lit.-, 

 for spore discharge. The locules superficially resemble the perithecia 

 immersed in a rather extensive sterile hyphal mass, or stroma, but 

 they are really just cavities hollowed out of the stroma. The apothecia 

 are disk- or cup-shaped structures largely exposing the asci upon 

 maturity. 



The Euascomycetes contains 14 orders, 60 families (##), and many 

 species that have never been studied from an industrial standpoint 

 or even grown in culture. However, many important plant pathogei I s, 

 such as the powdery mildew, apple scab, Dutch elm disease, and ergot 

 organisms, belong here, as do the highly edible truffles and morels. 

 Like the downy mildews of the Phycomycetes, the powdery mildews 

 are obligate parasites and have never been successfully cultured. The 

 morels and truffles have a mycorrhizal relationship with higher plants, 

 but the morels, and possibly the truffles, can be grown in culture in a 

 mycelial state. 



Several members of this subclass are of industrial interest, but they 

 are mostly airborne species or ones readily isolated from soil, and 

 they often have prominent conidial states. The genus Chaetomium 

 has long been important as an agent of deterioration. Neurospora is 

 extremely useful for genetic studies. Emericellopsis produces an anti- 

 biotic of the penicillin type useful against typhoid fever. Gibherella 

 produces a plant-growth regulator. Several organisms belonging to 

 the genera Ophiobolus, Sporormia, Enrotiuni, Peziza, Calonectria, and 

 Gibherella are utilized in the microbiological conversion of steroids. 

 As fermentation research expands, the number and variety of As- 

 comycetes useful to industry undoubtedly will increase. 



Basidiomycetes 



The class Basidiomycetes is estimated to encompass about 550 genera 

 and 15,000 species. All Basidiomycetes have in common a basidium, 

 which typically produces four basidiospores. In the basidium nuclear 

 fusion is followed by reduction division, so that the basidiospores are 

 haploid. Hence, the basidium represents the perfect or sexual state 

 of all fungi in this class. 



When basidiospores germinate, the mycelium is formed, which is 

 known as the primary or haploid mycelium. Later, diploidization oc- 

 curs and the mycelium becomes dikaryotic; i.e., nuclei of opposite 

 mating reaction are associated in the same cell but continue to divide 

 independently and multiply without nuclear fusion. This dikaryotic 

 mycelium is often characterized by clamp connections or swollen re- 

 gions at the septa, in which the earlier cell formed is swollen and ap- 

 pears to bend or clamp over the next cell beyond it. The mycelium 

 in nature may become very extensive and may live for years. During 

 periods of proper temperature and moisture, it produces fruiting 



