i8 THE BIOLOGY OF FLOWERING PLANTS 



both in species and individuals, and possess an importance 

 the magnitude of which is being more and more realised. 

 The earthworm has already been mentioned as one of the 

 chief agents in soil formation. Numerous other animals 

 occur. Buckle (1923) has found representatives of forty-nine 

 genera of insects or insect larva? on or in the soil of agri- 

 cultural land. These include carnivorous, phytophagous, 

 and scavenging species. Some are pests ; others play a 

 part in breaking down organic matter and incorporating it 

 with the soil. There is an abundance of other arthropods, 

 and there are many worms (in the wide sense) and Protozoa. 

 Some are agricultural pests ; probably all are active in 

 breaking down organic matter into -particles and simpler 

 compounds. 



The study of soil Protozoa is in its earlier stages and 

 offers many difficulties. According to Kopeloff and 

 Coleman (1917), about twenty-seven species, flagellates, 

 ciliates, and rhizopods, have been identified. Some of these 

 may exist only encysted, while others are certainly active. 

 A gramme of soil may include anything from 10 to 100,000 

 individuals. Interest centres in the relation to the bacterial 

 flora which forms the chief food of many species. Too 

 vigorous a protozoan fauna may seriously reduce the 

 numbers of the bacteria which deal with organic matter, 

 especially nitrogenous compounds, and a decrease of fertility 

 may result. Excessive numbers of Protozoa are also 

 associated with the " sickness " to which the rich soil of 

 hot-houses is subject. This condition can be controlled 

 by partial sterilisation of the soil by heat or volatile disin- 

 fectants. Cutler, Crump, and Sandon (1922) found about 

 thirty species of Protozoa in an English field soil ; of these, 

 six (two amoebse and four flagellates) were constantly present 

 in considerable numbers. There were great fluctuations 

 in the numbers from day to day and a distinct seasonal 

 change, the highest number being recorded in November 

 and the lowest in February. On the whole bacteria were 

 least numerous when amoebae were most abundant. Russell's 

 book and the papers quoted in it should be consulted. 



