454 ANNUAL REPORT SMITHSONIAN INSTITUTION, 196 2 



is not always indiscriminate. B. N. Singh at Rothamsted has shown 

 that the soil protozoa are very selective in the bacteria they feed upon, 

 taking some species readily and refusing other. Fungi also comes into 

 the struggle : some are taken by predators, some are themselves preda- 

 cious, capturing nematodes either by hyphal nooses or sticky networks, 

 or producing sticky spores which are picked up by the nematodes. The 

 victim's body is then penetrated by an outgrowth from the fungus, and 

 its proliferations absorb the body contents (see pi. 5). C. L. Dudd- 

 ington has made interesting studies of these groups. There is also 

 vigorous chemical warfare: some of the fungi eliminate competitors 

 by discharging poisonous antibiotics. These have been studied by 

 S. D. Garrett and F. W. Brian ; it is not known how they affect the 

 soil fauna. Some of these enchytraeids, according to Jegen, poison 

 the nematodes on which they feed and then consume the decomposing 

 mass. Some plants excrete from their roots poisons to kill parasitic 

 fungi, others, curiously enough, excrete stimulants that arouse destruc- 

 tive pests from dormancy; potatoes thus become infested with wart 

 disease and parasitic nematodes. 



There are numerous cases of external parasitism and of phoresi 

 (transport of smaller animals by hosts), and internal parasites occur 

 in the larger animals, such as earthworms, but there is no record of 

 their occurrence in the small animals, so Dr. Macfadyen tells me. 

 This seems extraordinary : is it possible that they are there, but too 

 small to be seen with an optical microscope? Soils contain such enti- 

 ties, the phages. They attack and dissolve bacteria and actinomy- 

 cetes, but very selectively, each group attacking only certain strains of 

 one group of their victims. They are far smaller than bacteria, about 

 0.01 to 0.1 microns in diameter, and can be seen only by an electron 

 microscope. They are a class of virus, and are able to reproduce 

 themselves although they consist only of DNA enclosed in a protein 

 coat. They are studied by J. Kleczkowska at Rothamsted Experi- 

 mental Station. 



Bacteria appear to have the most enemies but are saved from ex- 

 termination by their colossal power of reproduction and their small 

 size, which presumably enables them to find refuge in the minute re- 

 cesses of the soil. This is fortunate because they are very efficient 

 scavengers and can produce and secrete enzymes which will decompose 

 many harmful organic substances; they also oxidize the poisonous 

 ammonia contained in the secretions of predators that feed on orga- 

 nisms rich in nitrogen, such as bacteria, which may contain 3 to 15 

 percent. This scavenging action of bacteria is assuming increasing 

 importance now that farmers are using increasing quantities of or- 

 ganic poisons to control weeds, pests, and plant and animal diseases; 

 these poisons get into the soil and if they accmnulated might do seri- 



