Development in Agaricus
After the fruit body of Armillaria hinnulea expands, the partial veil remains as a ring on the stem.Species in the genus Agaricus have a partial veil that is made of two layers of tissue, although the two layers are not clearly distinct in all species. In the early 20th century, American mycologist George F. Atkinson investigated the development of the mushroom Agaricus arvensis by collecting young mushroom buttons (immature fruit bodies with the veil intact and the cap not yet expanded) and observing their growth in the laboratory. He determined that the partial veil originates from the tissue lying outside the annular cavity (the area containing the delicate developing hymenium and enclosed by the partial veil) and is not clearly separated from the universal veil. It is connected to both the margin of the cap and the surface of the stem. The partial veil increases in size as tension is applied to it from the expansion of the cap and stem. The lower portion of the partial veil (connected to the stem) has a looser texture, and is relatively porous to allow for air exchange. The upper portion of the partial veil (next to the gill cavity) is connected directly with the margin of the cap. It originates partly from fundamental tissue (actively dividing hyphae that comprise the bulk of the cap and stem tissue) and partly by growth from the margin of the cap. The looser portion of the veil is torn off from the surface of the stem as the mushroom expands and grows, and provides the looser lower portion of the duplex veil characteristic of this species and some of the other species of Agaricus, like A. augustus.
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