Reading 2
       Compared to the atmosphere, soil is a place where temperature fluctuations are small and slow.Consequently, soil animals are generally intolerant to sudden temperature changes and may not functionwell over a very wide range. That’s why leaving bare earth exposed to the hot summer sun often slows plantgrowth and why many thoughtful composters either put down a thin mulch in summer or try to rapidlyestablish a cooling leaf canopy to shade raised beds. Except for a few microorganisms, soil animals breatheoxygen just like other living things and so are dependent on an adequate air supply. Where soil is airlessdue to compaction, poor drainage, or large proportions of very fine clay, soil animals are few in number.
         The soil environment is generally quite moist, and even when the soil seems dry, the relative humidityof soil air usually approaches 100 percent. Soil animals consequently have not developed the ability toconserve their body moisture and are speedily killed by dry conditions. When faced with desiccation, theyretreat deeper into the soil if there is oxygen and pore spaces large enough to move about. So we seeanother reason why a thin mulch that preserves surface moisture can greatly increase the beneficialpopulation of soil animals. Some single-cell animals and roundworms are capable of surviving stress byencysting themselves, forming a little “seed” that preserves their genetic material and enough food toreactivate it, coming back to life when conditions improve. These cysts may endure long periods of severefreezing and sometimes temperatures of over 150 degrees Fahrenheit.       Inhabitants of leaf litter reside close to the surface and so must be able to experience exposure to dryerair and light for short times without damage. These are called primary decomposers. They spend most oftheir time chewing on the thick reserve of moist leaves contacting the forest floor. Primary decomposers areunable to digest the entire leaf. They extract only the easily-assimilated substances from their food:proteins, sugars and other simple carbohydrates and fats. Cellulose and lignin are the two substances thatmake up the hard, permanent, and woody parts of plants； these materials cannot be digested by most soilanimals. Interestingly, there are a few larvae whose digestive tract contains cellulose-decomposing bacteriabut these larvae have little overall effect.
        By the time the primary decomposers are finished, the leaves have been mechanically disintegratedand thoroughly moistened, worked over, chewed to tiny pieces and converted into minuscule bits of moistexcrement still containing active digestive enzymes. Many of the bacteria and fungi that were present on theleaf surfaces have passed through this initial digestion process alive or as spores waiting and ready toactivate. Digestive wastes of primary decomposers are thoroughly inoculated with microorganisms that canconsume cellulose and lignin. Even though it looks broken down, it has not yet fully decomposed. It doeshave a water-retentive, granular structure that facilitates the presence of air and moisture throughout themass creating perfect conditions for microbial digestion to proceed. Both secondary and primarydecomposers are necessary to complete the composting process.
【題組】36. According to the reading, which of the following is true about primary decomposers?
(A) Most of them are unable to digest cellulose and lignin.
(B) Most have a digestive tract that contains cellulose-decomposing bacteria.
(C) They can encyst themselves to camouflage.
(D) They desiccate leaves, creating mulch.