METABOLISM OF ORGANISMS 43 



3. The Hay Infusion Microcosm 



The importance of the complex nutritional interdependence of 

 organisms in general as well as the cosmical function of green 

 plants — the link they supply in the circulation of the elements 

 in nature — may be emphasized and summarized by a brief de- 

 scription of a 'hay infusion.' 



Probably nowhere is the 'web of life' more conveniently or 

 convincingly exhibited than in the kaleidoscopic sequence of events 

 — physical, chemical, and biological — which are initiated when 

 a few wisps of hay are added to a beaker of water. Apparently 

 the chief components of a hay infusion are hay and water, but 

 these merely supply the matter and energy for the interplay of 

 various forms of life. Most of these are beyond the scope of un- 

 aided vision though chiefly responsible for the obvious changes 

 which occur from day to day in their environment. 



Ordinary tapwater, for instance, contains free oxygen and vari- 

 ous inorganic salts in solution, and not infrequently different 

 species of Bacteria, unicellular green plants, and Protozoa. The 

 hay soaking in the water contributes soluble salts, carbohydrates, 

 proteins, etc. It also supplies many microscopic animals and plants 

 which have adhered to it in dormant form and are only awaiting 

 suitable surroundings to assume active life again. (Fig. 18.) 



A microscopical examination of an infusion when it is first made 

 shows very few active organisms, but within a day or so, depend- 

 ing largely on the temperature, it reveals countless numbers of 

 Bacteria which have arisen by division from the relatively small 

 number of dormant and active specimens originally present. At 

 first the Bacteria are fairly evenly distributed in the infusion, but 

 as conditions change, largely through the chemical and physical 

 transformations which they themselves bring about, those species 

 which can employ oxygen in combined form (that is, in chemical 

 compounds) find existence possible and competition less keen at 

 the bottom of the beaker, while those types of Bacteria which 

 are dependent upon free oxygen gather nearer the surface 

 where the supply is being replenished constantly from the 

 atmosphere. 



Up to this point the life of our microcosm is largely bacterial — 

 unicellular saprophytic plants which employ as food the complex 

 decomposition products of the proteins, etc., of the hay. The proc- 



