544 TEXTBOOK OF BOTANY 



lighted and well-aerated water where there is the largest number of host 

 plants and animals. Any aquatic organism may be invaded, injured, or 

 destroyed by parasitic fungi at some stage of its life cycle. The growth 

 of aquatic fungi may be checked by epidemics of bacteria. 



Terrestrial fungi occur everywhere on, or in, most kinds of plants and 

 many animals, as well as on their dead bodies. It would be difficult to 

 find a twig Iving on the ground in a forest that is not being invaded 

 and disintegrated by some fungus, and one may easily discover the cob- 

 webby fungous hyphae among the fallen leaves on the forest floor. Soil 

 fungi are most abundant in the upper foot of soil. Fungi digest the woody 

 tissue bv enzyme action. They injure the living hosts largelv through the 

 destruction of tissues, through the production of toxic substances, through 

 the consumption of food, and through interference with phvsiological 

 processes. 



There are about 75,000 species of fungi and thousands of chemically 

 different substrates; hence some fungi grow where others cannot. Fungi 

 are most numerous and grow best in moist warm situations, but they 

 mav be found also on the desert, in refrigerators, and on the arctic 

 tundra. 



Millions of bushels of fruits and vegetables in storage are destroyed 

 each year by fungi. Most of the apples, for example, that spoil in storage 

 are destroved by a single fungus, a blue mold which causes a soft rot. 

 In northern states where snow covers the ground throughout the winter, 

 much of the grass on golf greens is killed at the soil surface by another 

 fungus, the so-called "snow mold." The spores and other domiant struc- 

 tures of fungi often survive the natural extremes of temperature and 

 drought for long periods. 



We shall now consider in more detail a few of the common species 

 of fungi, in order to secure a better understanding of their structures, 

 growth, and reproduction, and also of their biological significance. 



Bread mold. If a moist piece of bread is placed under a bell jar for a 

 few davs, a tangle of colorless hyphae may cover the bread and even fill 

 the jar. This is the mycelium of the bread mold, Rhizoptis nigricans. 

 Its hyphae are much branched and have no cross walls; some of them 

 (rhizoids — root-like hyphae) penetrate the bread for short distances, 

 while others form the visible web. Bread mold, of course, grows on many 

 other substrates. The fungus secures food and water from the bread, and 

 as a result of enzyme action the bread is digested. Decay is nothing 



