The name of the species was chosen in honor of the American Maurice Machris. P. machrisii is a rather rare cactus, usually formed by several columnar stems reaching remarkable heights in nature. It is crossed by deep longitudinal ribs that are symmetrically spaced, along which small dark areoles emerge, rich of thick, short and very thin thorns of a nice delicate yellow color. The contrast between the glaucous green of the body and the bright colour of the thorns is remarkable, and gets even more accentuated when, at the sides of the stems, big white flowers with red tips bloom, very showy and extremely decorative!