Partial veils

The critical mission of a mushroom is to disperse its spores. So it is not surprising that many features of the mushroom exist to support this mission. One such feature is the partial veil. The partial veil is a membrane that covers the spore-bearing surface of the mushroom while it is developing. In effect, this encloses the young spore-bearing surface in a small chamber where it’s easier for the mushroom to maintain the right humidity and temperature for the developing spore-bearing cells.

While this is a simple function, partial veils come in many different forms, and recognizing these forms can be a big help in identifying the mushroom. The partial veil in the mushroom shown on the right, for instance, has two layers. The lower, smoother layer has peeled away except the area right around the stem — and a few smooth scraps, for instance at about five o’clock and eleven o’clock. The rest of the partial veil is showing the more pebbly surface of the upper layer. You can see the same process at work in the left-hand mushroom. This two-layered partial veil is characteristic of the section Arvenses of the genus Agaricus, and suggests that this mushroom belongs to that group.
Other mushrooms’ partial veils can have very different forms; it all depends on how the hyphae are woven together. The Agaricus partial veil above is also what we call membranous – it really looks like a skin, or membrane. But of course the hyphae can also be more loosely woven, giving us cottony partial veils – a veil that is clearly made from individual fibers, but is still quite stiff on its own.
and filamentous partial veils, which are thinner:
An extremely fragile, cobwebby partial veil is known as a cortina. There is a whole genus that is named after this feature, called Cortinarius, but it is present in other groups as well. The distinction between a fibrillose veil and a cortina is sometimes a matter of disagreement.
There are also mushrooms that have slimy veils. In these, the cells of the partial veil dissolve themselves, forming a gooey layer that protects the young gills much in the same way that a regular partial veil does.


 
The more sturdy, membranous veils, of course, leave a more solid ring. The most common of these is called a skirt-like ring, or pendant ring (that is, it hangs down), or annulus.


Another type of ring is even more sturdy: it sticks out flat horizontally, stiffly. This sort of ring is typical of the large parasols. Different people have made all sorts of analogies and names for it. It has been compared to a napkin ring and a wedding band.
Some authors make much of the fact that under ideal conditions it can be carefully worked free from the stem and slid up and down. This is fun to try, and helps you remember how this sort of ring looks; but if the mushroom is at all dried up it’s going to tear instead. So it’s not really a reliable identification feature. But it does show how distinct a ring this is from the pendent ring, which is completely interwoven with the stem, and can’t be separated from it like this under any circumstances. Technically, this sort of ring is also called an annulus, so this is one case where everyday language allows more precision than the technical vocabulary.
The more filamentous veils, like cortinas, may disappear almost entirely, and be visible (if at all) only because they have been colored by falling spores before collapsing. We saw this a bit above with the Lacrymaria cortina, and will see it again below. Finally, some very shredable partial veils may end up in tiny pieces strewn all along the stem. These pieces are often sticky and end up stuck all over your fingers when you handle the stem of the mushroom.

Annulus versus armilla

Another interesting kind of ring is a sheathing ring, which used to be called an armilla. The idea is that this partial veil is the extension of a layer that covers the whole stem and simply extends up and connects to the edge of the cap. So an armilla doesn’t hang down from the stem like a pendant ring, nor does it stand out stiffly from it like the parasol-style ring; instead, it has the form of a floppy upswept vase that the stem emerges from.
[At one point, mycologists thought that the distinction between an annulus and an armilla was so important that they named a whole genus after it: Armillaria. Nowadays this is a little misleading, as all the armillate mushrooms that originally made up that genus have been transferred out of it. However, if you know that the former Armillarias have pretty much all been transferred to Tricholoma, Cystoderma and Echinoderma, it gives you a head start on identifying them.
This shows another way that the armilla is different from the rings shown above: it is often pigmented like the surface of the stem, and indeed like the surface of the cap. In other words, the layer that gets torn open and folded out to form the armilla is actually the skin for the whole mushroom. In this respect, it is more like a universal veil. However, because it leaves a sort of ring high up on the stem, and because tearing it only exposes the gills, it has traditionally be classified as a partial veil; this is how you’ll see it in most reference sources. But inasmuch as it is simply a stretched portion of the regular skin of the rest of the mushroom, it is a developmentally unique sort of veil and ring.
Because the surface layer that forms the armilla carries all the mushroom’s pigmentation and texture, the portion of the stem above the armilla is often smooth and white, in clear contrast to the stem below it. If you see this, it’s another indication that you have a mushroom with an armilla. It’s not a term that is used a great deal in the mycological world right now. But it is part of a very distinct kind of mushroom development, so I present it here so you’ll at least have the concept available.

Partial veils: nature’s spore printers

Partial veils can bear other important implications for mushroom identification: they also catch falling spores. This is important in that spore color is often important for identifying your mushroom. Instead of having to take the mushroom home and do a spore print, one can learn the spore color of the mushroom by looking at the deposit on its partial veil – in this case, reddish brown:

This example also brings up another issue, which is the fate of the partial veil once the cap has expanded. Sometimes the veil may be left as a ring on the stem; with veils that break up more, shreds may be left hanging on the edge of the cap; but if it is a more wispy veil, the only sign that it ever existed may be this zone on the stem where some hyphae hang, invisible in their own right, but colored by spores. This is called an annular zone.

Partial veil fragments on the cap

Finally, the stem is not the only place where you may find evidence that your mushroom once had a partial veil. Often the edge of the cap is hung with clumps of hyphae that once were part of the partial veil.