''Agaricus arorae'' is a moderate-sized, forest-dwelling

mushroom A mushroom or toadstool is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground, on soil, or on its food source. ''Toadstool'' generally denotes one poisonous to humans. The standard for the name "mushroom" is t ...

that exhibits distinctive color changes. It fruits early in the mushroom season. Unusually within the genus '' Agaricus'', the mushroom's cap cuticle turns yellow when exposed to a base such as

potassium hydroxide Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exp ...

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

but stains red when cut. In the field, it has a brownish, fibrillose/squamulose cap, which turns red when bruised. ''Agaricus arorae'' was first described from Santa Cruz County but since has been found in San Mateo and Alameda counties.Kerrigan, Richard W. (1985) ''Agaricus arorae'' can be distinguished by its scales and a conspicuous stipe. ''Agaricus arorae'' was named after American mycologist and author David Arora.

Description

Pileus

The cap is 3–8 cm broad, and varies from hemispheric to convex. The margin starts curved, but then decurved. The cuticle slowly bruises a

red Red is the color at the long wavelength end of the visible spectrum of light, next to orange and opposite violet. It has a dominant wavelength of approximately 625–740 nanometres. It is a primary color in the RGB color model and a secondar ...

dish colour, and yellows with KOH. With a dry surface, the cap's disc can be glabrous or tomentose. Toward the margin, either minute

fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...

s or scales develop. The cap's fibrils are generally a brownish colour. The cap has an indistinct

odor An odor (American English) or odour (English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is caused by one or more volatilized chemical compounds ...

with a mild taste. The context can be as large as 5 mm thick, soft, and bruises irregularly a vinaceous colour where cut.

Lamellae

The gills are free, close, and broad. At first, they are buff-brown, but turn a darker brown in age.

Stipe

The stipe is 4–9 cm long and 0.5-2.5 cm thick. At the base, it is slightly enlarged and becomes stuffed at maturity. The white veil is rather membranous and yields a thin ring. When cut, the cortex discolours to pinkish-orange. The stipe base changes brown to rusty-brown from handling. The margin is sometimes light brown. The apex surface is white, and patchy fibrillose over a dull-buff ground color.

Spores

Spores are 4.5-5.0 x 3.0-3.5 µm, elliptical, and inequilateral in profile. In addition, they are moderately thick-walled, lacking a germ pore.Arora, p. 320

References

* * ;Specific {{Taxonbar, from=Q4691332

arorae Arorae (spelling variants: Arorai, Arurai; also known as Hope Island or Hurd Island“Captain Patterson, commanding the brig ''Elizabeth'', called it Hope Island: “Hope Island, in 2° 43′ S and 176° 56′ 25″ E, was the first discovery, t ...