g6 Types of Aquatic Environment 



Muck is formed by the more com.plete decay of such 

 water plants as compose peat. The process of decay is 

 furthered either by occasional exposure of the beds to 

 the air in spells of drought, or by the presence of lime 

 in the surrounding soil, correcting the acidity of the 

 water and lessening its efficiency as a preservative. 

 Muck is soft and oozy, paste-like in texture and black 

 in color. In the openings of marshes, like that shown 

 on page 89 are beds of muck so soft that he who ven- 

 tures to step on it may sink in it up to his neck. In 

 such a bed the slow decomposition that goes on in hot 

 weather in absence of oxygen produces gases that 

 gather in bubbles increasing in size until they are able 

 to rise and disrupt the surface.* So are formed marsh 

 gas (methane) which occasionally ignites spontaneously, 

 in mysterious flashes over the water — the well known 

 "Jack-o-lantem" or "Will-o-the-wisp" or "Ignis 

 fatiius'' — and hydrogen sulphide which befouls the sur- 

 rounding atmosphere. 



The presence in marsh pools of these noxious gases, 

 of humous acids, and of bitter salts, and ^ the absence 

 of oxygen — except at the surface, limits their animal 

 population in the main to such creatures as breathe air 

 at the surface or have specialized means of meeting 

 these untoward conditions. 



m 



High and Low Water — Swamps being the shoalest of 

 waters are subject to the most extreme fluctuations. 

 That they retain through most dry seasons enougli 

 water for a permanent aquatic environment is largely 

 due to the water-retaining power of aquatic plants. 

 Notable among these is sphagnum, which holds en- 

 meshed in its leaves considerable quantities of water, 

 lifted above the surrounding water level. Aquatic seed 



*See Penhallow, "A blazing beach" in Science, 22:794-6, 1905. 



