There is a myth that immediately after heavy rains in forests, mushrooms appear in large numbers and form a very large ring or circle, which may be several metres in diameter. These are called as "fairy rings". Can you explain this phenomenon of fairy rings in biological terms? Discuss the mycelial structure in $Agaricus$ and its soil-borne nature.

(N/A) $1$. Biological Explanation of Fairy Rings: The phenomenon of "fairy rings" is caused by the radial growth of the mycelium of certain fungi, such as $Agaricus$ $campestris$, in the soil. As the mycelium grows outward from a central point in search of nutrients, it depletes the soil of organic matter and nitrogen in the center. The fungus continues to grow at the periphery where nutrients are available, forming a circular pattern. When the environmental conditions (like heavy rain) are favorable, the fruiting bodies (mushrooms) emerge from the peripheral mycelium, creating a visible ring.
$2$. Mycelial Structure in $Agaricus$: The mycelium of $Agaricus$ is composed of branched, septate, and filamentous hyphae. It consists of two types: primary mycelium (monokaryotic) and secondary mycelium (dikaryotic). The secondary mycelium is the dominant, long-lived, and vegetative phase that forms the complex fruiting body (basidiocarp).
$3$. Soil-borne Nature: $Agaricus$ is a saprophytic fungus. It lives in the soil, feeding on decaying organic matter (humus). Its mycelium spreads extensively through the soil substrate, secreting extracellular enzymes to break down complex organic compounds into simpler absorbable nutrients.