Stem

Lateral

When we picture a typical mushroom, the stem usually sits right in the center of the cap (like the handle of an umbrella). But for mushrooms growing on tree trunks or stumps, a stem in that position would be extremely inefficient. So evolution came up with the lateral stem (lateral stipe).

12 species in the database.

How does it work?

As David Arora explains, the cap's main job is to hold the gills or tubes (the hymenophore) strictly horizontal, openings facing down. Only in this position can the spores fall freely to the ground under gravity without getting stuck between the gills.

If a mushroom grows on a vertical tree trunk, its stem attaches not to the center but to the side of the cap (at its edge). This lets the cap stick out from the trunk like a little balcony or shelf while keeping a perfectly horizontal orientation.

The best-known mushrooms with a lateral stem

1. Oyster mushroom (Pleurotus ostreatus): The most popular and most delicious example. The oyster mushroom's stem is short, thick and attached to the edge of the cap (which is shaped like an ear or a shell). Oyster mushrooms grow in tiers, one above another, forming beautiful cascades on the trunks of broadleaf trees.

2. Bitter oysterling (Panellus stipticus): A small mushroom with a short lateral stem that grows in groups on wood. It's known for gills that can glow in the dark (bioluminescence). It's considered inedible because of its very bitter/astringent taste.

3. Dryad's saddle (Cerioporus squamosus): Also known as the "pheasant's back mushroom." Its large, yellow, scaly cap is attached to the tree by a short, thick lateral (or eccentric) stem, which often turns black at the base. Edible when very young.

Questions and answers

If an oyster mushroom grows on the ground instead of on a tree, where will its stem be?

Oyster mushrooms (Pleurotus) can sometimes grow from a piece of wood buried in the soil. In that case, if the wood lies horizontally, the stem will try to grow upward and may end up central or eccentric (off-center) rather than lateral. This shows that stem shape in such mushrooms is adaptive and depends on the direction of gravity and light during growth.

Are there poisonous mushrooms with a lateral stem?

Yes, there are toxic species. For example, angel's wings (Pleurocybella porrigens) — a pure white, oyster-like mushroom with a short lateral stem or none at all. It was long considered edible, but in Japan an outbreak of fatal poisonings by this mushroom was recorded (especially among people with kidney disease).

Sources

1.Зерова М.Я. та ін. Визначник грибів України. 1979
2.Roger Phillips. Mushrooms (Roger Phillips)
3.David Arora. Mushrooms Demystified
4.Index Fungorum