Mushrooms are the reproductive structure, or fruiting body, of a larger organism existing mostly out of sight as a network of filaments called mycelium. This temporary structure emerges to produce and disperse the next generation of spores. The most recognizable form features a cap supported by a stem, and beneath the cap lies the gills. These thin, blade-like sheets are engineered to multiply the surface area available for spore production. Understanding these structures, technically known as lamellae, is fundamental to appreciating the biology and diversity of gilled fungi.
Defining the Gill Structure
The gill is a delicate, blade-like structure radiating from the stem, or stipe, to the cap’s edge, or pileus. This arrangement is a biological strategy to vastly increase the fertile surface area within the confined space beneath the cap. Compared to a flat surface, the formation of gills can increase the spore-producing area by as much as 20 times.
Each gill possesses a complex microscopic anatomy composed of three main layers. The central core, known as the trama, provides structural support to the entire blade. Surrounding the trama is the subhymenium, which is a layer of cells that gives rise to the outermost, most significant layer, the hymenium.
The hymenium is the fertile layer that covers both faces of the gill. This layer is densely packed with two primary cell types: basidia and cystidia. Basidia are club-shaped cells responsible for producing and releasing spores. Cystidia are larger, non-spore-producing cells that may serve protective or regulatory functions, and their distinct shapes are often used as identifying characteristics by mycologists.
The Reproductive Function of Gills
The biological purpose of the gill structure is the creation and dispersal of microscopic reproductive units called basidiospores. These spores are created through sexual reproduction within the basidia cells lining the hymenium. As the spores mature, they sit atop tiny prongs called sterigmata protruding from the basidium.
The mushroom must employ a mechanism to launch the spores clear of the gill surface and into the air currents below. This is achieved through a phenomenon known as ballistosporic discharge, or the spore-shoot mechanism. A small droplet of fluid forms at the base of the spore, and when this fluid fuses with a film of water on the spore’s surface, the sudden shift in the center of gravity causes the spore to be forcefully ejected.
Once launched, the spores fall into the dead air space between the vertically oriented gills, a clear path for them to drop. The mushroom’s stem lifts the cap and gills high enough off the ground to allow wind currents to catch the falling spores and carry them away. This efficient vertical orientation and spore-launching system ensures that trillions of spores can be dispersed widely to establish new fungal colonies.
Variations in Gill Attachment and Appearance
The way a mushroom’s gills connect to the stem is a highly reliable feature used for species identification. Gills are classified into several distinct attachment types based on their connection point:
- Free gills do not attach to the stem at all, leaving a noticeable gap between the gill end and the stipe.
- Adnate gills attach broadly and squarely to the stem, meeting it at a right angle.
- Adnexed gills are narrowly attached, often curving slightly upward before meeting the stem.
- Sinuate or emarginate gills are notched or scooped out near the stem.
- Decurrent gills run down the length of the stem below the point where the cap attaches.
Beyond attachment, the spacing of the gills (crowded or distant) and their color are also important identification features. Gill color often relates to the color of the spore print, which is the mass of spores left when the cap is placed on a surface.