MICRO-ORGANISMS 9 



FUNGI 



Among the largest of the categories of micro-organisms are the 

 fungi, which perhaps contain more complex morphological forms and 



more species than any of the other groups except possibly the algae. 

 According to A insworth (1 ) , there are more than 4,000 genera of fungi 

 and probably 100,000 species. Fungi are in air, soil, water, pla 

 and animals, and decaying organic matter. Mostly they are aerobic 

 Yeasts, molds, and mushrooms are common names used for some 

 types of fungi. Yeast usually refers to fungi that reproduce asexual ly 

 by budding, but may include forms from any of the four major class 

 of fungi. Mold implies any fungus with a mycelium. Mushroom 

 refers to fungi that produce large macroscopic fruiting bodies. In 

 culture, mushrooms produce mycelia and can be called molds. In 

 scientific writing it is better not to use these three terms. Two gen- 

 eral modern textbooks on mycology are Bessey {3) and Alexopoulos 



Fungi are usually divided into the following classes : Phycomycetes, 

 Ascomycetes, Basidiomycetes, Fungi Imperfecta and Myxomycei 



The system most generall}- followed is that of Martin {22). 



Phycomycetes 



The class Phycomycetes contains the most primitive fungi and rep- 

 resents about 245 genera and more than 1,300 species. The mycelium 

 is coenocytic: i.e., there are many nuclei within the cell walls, and septa 

 in the mycelium are relatively few or absent. The lower forms of 

 Phycomycetes are chiefly aquatic and usually require special tech- 

 niques for isolation involving baits. Reproduction is by means of 

 motile sporangiospores, or zoospores, except in the four orders where 

 nonmotile conidia or sporangiospores are formed. Sexual reproduc- 

 tion is unknown in some genera, but when present it is represented by 

 oospores or zygospores. In any studies with Phycomycetes it must be 

 remembered that there are few septa, and consequently care must be 

 taken not to break the mycelium into too small fragments or much of 

 it will be killed. 



Xot typically encountered in water are three of the higher orders — 

 Peronosporales, which are mostly obligate parasites on higher plants; 

 Mucorales, which are mostly saprophytic forms, although a few are 

 parasitic on other fungi ; and Entomophthorales, which are often para- 

 sitic on insects, although they can be grown in pure culture. Unlike 

 other classes of fungi Phycomycetes are named without regard to 

 whether the sexual stage is known or unknown. 



Much work on morphogenesis and physiology has been done on mem- 

 bers of the order Blast ocladiales, especially the genera AUomya s and 

 BJastodadiella. Among many of the genera producing motile cells, 

 much work remains to be done on the conditions for growth in pure 

 culture and on the problems of producing sufficient material for physi- 

 ological and biochemical studies. 



The groups most widely studied in pure cult tire for their products 

 and physiology are found in the order Mucorales, Genera such as 

 Rhizopus produce large amounts of fumaric and lactic acids. Mu- 



