THE CONTENTS OF THE SOIL 35 



Exp. Sta. The terms germ, microbe, bacterium (plural bacteria), 

 are popularly used in the same sense as micro-organism. These 

 beings are usually unicellular (each one consisting of only a 

 single cell). They are generally classified with plants. The role 

 of micro-organisms in rendering soil elements available to plants 

 is very complex and not yet well understood. A general dis- 

 cussion of these organisms will be found in Lipman's " Bacteria 

 in Relation to Country Life." The relation to germs in nitrification 

 is briefly discussed in King's "Soil," pp. 125-134, and Roberts' 

 "Fertility," 244-248. Fig. 6 illustrates one of the 

 common bacteria, very much magnified. This ^ 

 species (Bacillus ubiquitus) is abundant in water, ** 

 air, and decaying substances. < 



38a. Observe the deposits of sand in the quiet 

 side (usually the concave side) of streams, and Fig. 6. Micro-or- 

 also the delta where a rapid rill flows into a slow Kanisms, greatly 

 one. When the rill flows into a rapid stream, magni 

 the larger current carries away the deposit so that it may not be 

 seen. Recall how sand-bare form again and again in lakes, and 

 how streams must be frequently dredged to keep the channel 

 open. The slower the stream the more quickly does it drop its 

 sediment ; and the more winding, also, is its course, lying in the 

 bed of its own deposits. (See Fig. 4.) 



386. Dip a glass of water from a roily stream, and observe 

 the earth which settles to the bottom. 



39a. Glaciers are still abundant in alpine and arctic regions. 

 It was from the study of glaciers in the Alps that Agassiz con- 

 ceived the hypothesis that large parts of the earth had once 

 been subjected to glacial action. A good popular discussion of 

 glaciers and their action may be found in Chap. xvii. of Tarr's 

 "Elementary Physical Geography." Delightful readings may also 

 be made from Agassiz's "Geological Sketches." 



40a. Let the pupil catch a few rain drops on a perfectly clean 

 and clear pane of glass, and observe if any sediment is left when 

 the drops have evaporated. Is there any difference in the amount 

 of dust brought down after a " dry spell " and after a period of 

 rainy weather, or at the beginning and end of a shower ? The 



