Question: While Opuntia is from the genus of the cactus family, what genus is the Houttuynia?
Context: Opuntia is a genus in the cactus family, Cactaceae. Cylindropuntia whipplei (formerly known as "Opuntia whipplei", common name whipple cholla) is a member of the cactus family, Cactaceae. Opuntia cacanapa is a cactus in the "Opuntia" genus.  In the United States it is found on the South Texas Plains in Brewster County and adjacent counties to the southeast.  It also occurs in nearby Mexico. Opuntia anahuacensis is a cactus species in the "Opuntia" genus and a member of the Opunitioideae.  It grows along the Gulf Coast of Texas, and perhaps northern Mexico.  The plants are short, perhaps 1- or 2-ft tall, but they are wide.  Some thickets may be 20- to 40-ft across and composed of multiple plants.  The fruit is purplish.  The cladodes are uniquely shaped, obovate with a neck.  The original description claimed the plants were yellowish green, but they may be green or rarely blue-green. Opuntia alta is a cactus species in the "Opuntia" genus.  It is a large plant, with some older specimens forming trees to 15 ft tall.  It resembles "Opuntia lindheimeri", and immature specimens can be mistaken for that taxon.  Mature O. alta plants form discrete trunks and have small fruits. Unlike O. lindheimeri, the flowers have pale stigmas. Opuntia macrocentra, the long-spined purplish prickly pear or purple pricklypear, is a cactus found in the lower Southwestern United States and Northwestern Mexico.  A member of the prickly pear genus, this species of "Opuntia" is most notable as one of a few cacti that produce a purple pigmentation in the stem.  Other common names for this plant include black-spined pricklypear, long-spine prickly pear, purple pricklypear, and redeye prickly pear. "Opuntia trichophora" (Engelm.  & J.M. Bigelow) Britton & Rose, is a species of cactus in genus Opuntia, more commonly known as prickly pears or nopal.  "O. trichophora" is distributed throughout parts of New Mexico, Colorado, and Utah, and may have disjoint populations in Wyoming, southern Montana, and southern Idaho.  "Opuntia trichophora" is a diploid (2n=22) but has sometimes been treated as a variety of "Opuntia polyacantha" a tetraploid (2n=44).  "O. trichophora" tends to have longer spines than "O. polycantha" or "O. macrorhiza". Opuntia atrispina is a cactus species in the "Opuntia" genus.  It has limited distribution in the United States.  In Texas it can be found from near Uvalde to Del Rio/Langtry--a small strip of area just 50 miles long. Houttuynia is a genus of two species in the Saururaceae native to Southeast Asia.  One species, "H. cordata", is widely cultivated as a culinary herb.  The genus was originally described in 1783 by Carl Peter Thunberg when he formally described "H. cordata" as the only species.  It remained a monotypic genus until 2001 when Zheng Yin Zhu and Shi Liang Zhang discovered and described a second species native to China, "H. emeiensis". Opuntia aciculata, also called Chenille pricklypear, old man's whiskers, and cowboy’s red whiskers, is a perennial dicot and an attractive ornamental cactus native to Texas.  It belongs to the genus "Opuntia" prickly pear cacti.  It is also widespread in Nuevo Leon, Tamaulipas (northern Mexico).
Answer: two species in the Saururaceae