Within the genus Agaricus, the Section Xathodermatei contains a number of species that are commonly known as yellow stainers and they are known to contain phenol which causes quite nasty gastric upsets if consumed. I have been meaning to put together a post about these but it was only this morning that I found a substantial patch of them on a street verge to do some images and experiments. A few members of the patch are shown in the picture below.
Yellow stainers on street verge
The yellow staining reaction is seen both on the cap and on the stem of the mushrooms and manifests itself as a bright chrome yellow stain that quickly fades. Once picked, the yellow stain on the cap may not continue to show itself. The picture below shows the sort of stain that occurs when you first pick one of these mushrooms. This was completely gone within 2 minutes.
Yellow stain on edge of cap when first picked
The partial veil on these mushrooms has a fluffy appearance that I believe can be called flocculose. The appearance of the partial veil at various stages is shown in the following set of images.
Partial veil at various stages of growth
The yellow stain on the cut stem can also been seen in these images as can the white core in the centre of the stem, a feature that is also seen in supermarket mushrooms.
Another feature that tends to be a characteristic of mushrooms in this Section is the ‘boxy’ cap shape. That shape can be seen in the first image above. This is where the analysis gets interesting. As I have mentioned elsewhere, the yellow stain can be made permanent on these mushrooms by applying an alkali. The yellow colour is due to 4,4-dihydroxyazobenzene. While looking closely at these mushrooms I noticed that they have a very distinct internal structure featuring a very dense section in the cap above the stem. This is revealed in a sectioned piece developed with Napisan solution which provided the necessary alkalinity and perhaps some oxidizing power that might have had an effect. An example of a sectioned mushroom developed in this way is shown in the pictures below, compared with a supermarket version.
Yellow stainer (top) compared with normal supermarket mushroom (bottom), both developed with Napisan solution
In this view it is immediately apparent that there is a significant difference in the internal structure of the two mushrooms. The hard core in the centre of the cap of the yellow stainer shows up clearly. I strongly suspect that this structure is responsible for the boxy shape of these mushrooms. Beyond that however, the flesh of the cap of the yellow stainer remains refractory to the effects of the Napisan while the flesh of the supermarket mushroom saturates and slightly darkens. The hard core also influences the way the cap separates from the stem. In the case of the supermarket mushroom the separation is very clean but in the case of the yellow stainer the stem breaks away with a rather ragged edge, as shown in the picture below with two yellow stainers on the right and two supermarket varieties on the left.
Cap separation on supermarket mushrooms (left) versus yellow stainers (right).
I have in mind a few more experiments on these interesting mushrooms but I will finish this post off for now.
A quick postscript. After about an hour the difference between the two mushrooms became even more stark.
