The way the gills attach to the top of the stalk is an important feature of mushroom morphology. Mushrooms in the genera "Agaricus", "Amanita", "Lepiota" and "Pluteus", among others, have free gills that do not extend to the top of the stalk. Others have decurrent gills that extend down the stalk, as in the genera "Omphalotus" and "Pleurotus". There are a great number of variations between the extremes of free and decurrent, collectively called attached gills. Finer distinctions are often made to distinguish the types of attached gills: adnate gills, which adjoin squarely to the stalk; notched gills, which are notched where they join the top of the stalk; adnexed gills, which curve upward to meet the stalk, and so on. These distinctions between attached gills are sometimes difficult to interpret, since gill attachment may change as the mushroom matures, or with different environmental conditions.