OCCURRENCE OF BACTERIA 31 



organs, such as stems and leaves. Two groups stand out conspicu- 

 ouslyknown as algse and fungi, but there are other groups whose 

 relationship is not so clear. The main divisions of the Thallophytes 

 are (1) Myxomycetes, commonly known as slime molds, or slime 

 fungi, which combine characters of plants and animals; and (2) 

 Schizophytes or fission plants, characterized by cell divisions occur- 

 ring in rapid succession which is their only method of reproduction. 

 They consist of two groups: the Cyanophycese, or blue-green algae, 

 and the Schizomycetes, or bacteria. 

 The relationship is shown diagrammatically below: 



Thallophytes simple, 

 undifferentiated plants; 

 do not develop roots, 

 stems or leaves. 



_f Myxomycetes slime molds, or slime fungi. 



( Cyanophycese blue-green 

 Schizophytes fission plants \ algse. 



[ Schizomycetes bacteria. 

 Algae, including seaweeds, pond scums, water-silks, etc.; 



contain chlorophyll. 



f Yeasts. 



I Molds. 

 Fungi without chlorophyll, -j Mildews. 



Smuts. 

 [ Rusts, etc. 



Occurrence of Bacteria. Bacteria are ubiquitous, occurring as 

 they do nearly everywhere. They are found in soil to great depths, 

 their number decreasing with the depth and nature of the soil, being 

 more numerous in soil containing organic matter than in those 

 practically devoid of it. Although they occur in the atmosphere, 

 it is not their normal habitat, for growth and multiplication cannot 

 take place in it under ordinary conditions. The number and kind 

 found in air vary with a number of factors, chief among which is 

 locality. The air of some high mountains is practically devoid of 

 bacteria; city and country air also differ from each other in the 

 number and kind of bacteria they contain. Other controlling 

 factors are moisture, presence or absence of injurious substance, and 

 minute particles in the atmosphere. 



Most natural waters contain great numbers of bacteria. In 

 sewage and polluted water they are especially numerous, but occur 

 only in small numbers or not at all in deep wells and springs. The 

 kind of organism varies with the composition of the water and with 

 the original contamination. Milk as secreted by the milk glands 

 of cows is practically free from bacteria, but the vessels in which it 

 is handled so contaminate it that it rapidly gains in bacteria. Often 

 by the time it reaches the consumer it contains millions in every 

 cubic centimeter. In short, all food except that recently cooked 

 contains bacteria, the number and kind of which vary with the 

 nature and age of the food. 



Living as we do in a world which is teeming with bacteria, we can 

 expect to find them on the surfaces of the skin and mucous mem- 



