A cactus stem is not chemically uniform
San Pedro’s thick stem is built in layers. A tough outer covering protects it from damage and water loss; just beneath it lies green tissue that captures sunlight. Farther in, much of the stem is pale, water-storing pulp. Mescaline is unevenly distributed across these tissues, so a sample’s location can matter as much as the fact that it came from the same cactus.
That pattern makes the plant’s interior more than a single reservoir of water and fiber. Its living tissues also contain alkaloids—nitrogen-based compounds made by plants—that can affect animals and people. In San Pedro, the best-known of these is mescaline, a compound that changes perception partly by interacting with serotonin receptors in the brain, which help regulate mood and sensory experience.
A boundary between protection and chemistry
The concentration just beneath the peel puts the compound near the stem’s exposed living surface, rather than evenly through the soft core. Plant chemicals often serve ecological roles, including discouraging organisms that feed on them, though that broad pattern does not by itself establish why San Pedro places mescaline there. The cactus’s layered structure also means that statements about “the mescaline in a cactus” can conceal important differences between tissues.
Concentrations can vary among plants and growing conditions, so the skin-adjacent band is a useful description of where levels are highest—not a guarantee that every stem has the same chemical profile. The striking detail is anatomical: in this cactus, a potent compound is concentrated in a narrow living layer, only a small distance from the surface that protects the plant.
