200 THE INDIVIDUAL ORGANISM 



food. They comprise a large subdivision of the thallophytes (the fungi) 

 and scattered examples among the seed plants. 



The fungi are defined as those thallophytes that lack chlorophyll. 

 They are of many sorts, the best known being the bacteria, molds, rusts, 

 yeasts, smuts, mildews, cup fungi, bracket fungi, coral fungi, mushrooms, 

 and puff balls. The various groups of fungi are not all closely related. They 

 probably originated at different times and from different kinds of algae, 

 so that they do not form a "natural" group in the sense of having had 

 a common ancestry. Functionally, however, they do form a natural 

 grouping sharply distinguished from other thallophytes by the absence of 

 chlorophyll and the modified nutritional methods that this lack en- 

 tails. Some fungi are saprophytes (Greek, sapros, "putrid," and phyton, 

 "plant"), which obtain their food from the dead bodies of plants or 

 animals, or from plant and animal products. Others are parasites, which 

 get food from the living bodies of plants or animals. Many bacteria and 

 some other fungi make little distinction between these two modes of life 

 and may exist saprophytically at one time, parasitically at another. Still 

 others, although these are relatively few, have developed symbiotic 

 (Greek, symbiosis, "a living together") relations with green plants, in 

 which both members of the association profit. Let us briefly examine a 

 few instances of these various modes of existence. 



Saprophytic bacteria and other fungi are largely responsible for the 

 destruction of the dead bodies of organisms and the liberation of their 

 compounds, which thus are made available for reutilization by the living. 

 Among the important saprophytic soil bacteria are the ammonifying, 

 the denitrifying, and the free-living kinds of nitrogen-fixing bacteria. The 

 ammonifying and denitrifying bacteria break down protein-containing 

 substances. Those of the first class liberate ammonia, which is oxidized 

 into nitrites and nitrates by the nitrifying bacteria already mentioned, 

 while the denitrifying bacteria liberate free nitrogen gas into the air. The 

 former increase soil fertility, the latter diminish it. The nitrogen-fixing 

 bacteria live upon organic materials but have the property of taking 

 nitrogen gas (which green plants cannot utilize) from the air and convert- 

 ing it into organic compounds, which may subsequently be broken down 

 and made into nitrates by other bacteria. Most of the common molds 

 (for example bread mold, Fig. A. 5), yeasts, mushrooms, puff balls (Fig. 

 A. 8), bracket fungi (Fig. A. 9) and coral fungi are saprophytes. 



There are thousands of parasitic species of fungi, and few of the higher 

 animals and plants do not at least occasionally serve as hosts 1 to some of 

 them. These fungus parasites include the bacteria (Fig. A. 4) that cause 

 disease in animals and plants, the fungi that kill fish, the mildly parasitic 

 mildews that grow upon leaves, the molds that cause ringworm and 



1 The organism that supplies sustenance to any parasite is termed its host. 



