Lophophora williamsii, the peyote cactus.

the heffter review of psychedelic research vol.2, 2001
a couple of interesting articles, especially "Visions of the Night: Western Medicine Meets Peyote".
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* "The Heffter Research Institute promotes research of the highest scientific quality with psychedelic substances in order to contribute to a greater understanding of the mind, leading to the improvement of the human condition, and the alleviation of suffering."
visionary cactus guide
a cacti growing blog
botany of peyote
"Peyote is one of the slowest growing plants in existence, taking 13 years or more
to mature. Older cacti are generally much higher in alkaloids than young plants.
The very oldest plants are worshiped by the Indians as "Father or Grandfather
Peyote". They are often kept as amulets or placed on a crescent alter to be
revered as a sacred object. A baseball sized Cactus can be well over 30 years
old.
...
There have been over sixty alkaloids discovered in this plant which has been
described as a " little green chemical factory ". This Cactus is known to contain
56 nitrogen containing compounds derived from a tyrosine base, as well as 20
tyramine-like alkaloids. Mescaline content usually ranges from about 3 - 6%, by
dry weight, averaging around 4%, but is highly variable. Most of the alkaloids
present can be classified as B-phenethylamines like mescaline or
tetrahydroisoquinolines like hordenine. These chemicals differ in structure from
LSD in that they don't have a complete indole ring. Mescaline content of fresh,
undried cactus is reported at 0.4 %.
...
Furthermore, our more “enlightened” culture
naturally assumes that the effects of peyote on a Red
Indian will be more or less the same as on a White
Suburbanite, because our minds run with Pavlovian
predictability down the well-worn rut of
pharmacological reductionism.
...
There are three Ameuropeans, two Americans and
a Norwegian, who each independently discovered
peyote and became interested enough in the plant to
sample it themselves. Chronologically, the first was a
Texas physician, John Raleigh Briggs (1851-1907),
who experimented with peyote in 1886. The second
was Carl Lumholtz (1851-1922), a Norwegian with a
passion for exploration who was the first white man to
live alone with the Australian aborigines, as well as
the Tarahumara and Huichol of northwest Mexico,
where in 1892 he first became aquainted with and
sampled híkuli, as these natives call peyote. The third
was James Mooney, an Irish-American with a lifelong
passionate interest in and sympathy for Indian history
and culture, whose field trips as an ethnologist working
for the Smithsonian allowed him in 1891 to be the first
non-Indian to attend a peyote religious ceremony and
report on it."