The filamentous hyphae of a fungus give it a unique relationship with its physical environment. The fungal mycelium has an enormous surface area-to-volume ratio compared with that of most large multicellular organisms. This large ratio allows the fungus to take in a great deal of water and mineral nutrients when soil moisture is high, which allows rapid growth in moist environments. The downside of the large surface area-to-volume ratio of the mycelium is its tendency to lose water rapidly in a dry environment. Thus fungi are most common in moist environments. You have probably observed the tendency of molds, toadstools, and other fungi to appear in damp places.

Another characteristic of some fungi is a tolerance for highly hypertonic environments (those with a solute concentration higher than their own; see Key Concept 6.3). Many fungi are more resilient than bacteria in hypertonic surroundings. Jelly, for example, does not support bacterial growth because its high sugar content makes it too hypertonic to those organisms, but it may eventually harbor mold colonies. Mold in the refrigerator illustrates yet another trait of many fungi: tolerance of temperature extremes. Many fungi grow in temperatures as low as –6°C, and some can tolerate temperatures higher than 50°C.