About Opuntia engelmannii Salm-Dyck ex Engelm.
Opuntia engelmannii Salm-Dyck ex Engelm. is generally a shrubby cactus that forms dense clumps up to 3.5 metres (11 ft) high, and usually has no visible trunk. Its flat stem segments, called pads or cladodes, are green (rarely blue-green), shaped obovate to round, and measure about 15–30 cm long and 12–20 cm wide. In the Sonoran Desert, the terminal pads of this cactus are predominantly oriented east-west, an adaptation that maximizes absorption of solar radiation during the summer rainy season. As the main stem structure, the cladodes also act as the cactus's primary photosynthetic organ. O. engelmannii typically grows in high-altitude deserts, where lower atmospheric pressure increases water stress and can make gas exchange difficult. Because of this, the species relies on CAM photosynthesis, which enables more efficient gas exchange and reduces water loss. The species's small, barbed glochids are yellow when new, and turn brown as they age. The larger cactus spines are non-vascularized modified leaves that grow in clumps from the cactus's areoles. Spines are highly variable: there are between 1 and 8 spines per areole, and spines are often absent from lower areoles. They are yellow to white, slightly flattened, and 1–6 cm long. Spines harden from the tip downward and develop a waxy cuticle; they do not perform photosynthesis. Their primary function is defense, but spines also facilitate the cactus's asexual reproduction by catching on the fur of passing animals. As the animal attempts to remove the clinging spine, it can carry away an entire cladode, which may later dislodge in a new location and grow into a new individual plant. Like other species in the Opuntia genus, O. engelmannii has both taproots and adventitious roots; some taproots function as water storage organs. Deeper roots anchor the plant and absorb water from deep underground reserves, while shallow roots spread over a wide area to absorb rainfall quickly. Opuntia species are notable for being able to grow both root types directly from areoles on detached cladodes, an adaptation that makes propagation very easy: separated cladodes can develop new roots and shoots from their areoles to form a whole new independent plant. Most Opuntia species, including O. engelmannii, form associations with arbuscular mycorrhizal fungi. These fungi colonize Opuntia plants more heavily in summer than in winter, and more heavily when water is abundant. The mycorrhizae help the cactus cope with drought stress. The flowers of O. engelmannii are yellow, occasionally reddish, and measure 6 to 9 cm in both diameter and length. The flowers are hermaphroditic and have inferior ovaries, and their stamens are thigmotropically sensitive. Flowering occurs in April and May, and individual blooms last between 1 and 2 days. Single-day blooms open around 11 AM and remain open for 6 hours. Two-day blooms open around 10 AM and stay open until 4:30 PM on the first day, then open again from 8:30 AM to 2:00 PM on the second day, giving an average total open time of 12 hours per bloom. Flowers produce nectar to attract pollinators, and O. engelmannii produces more nectar than most other Opuntia species. Like other Opuntia, O. engelmannii undergoes cross-pollination but is also self-compatible. Cross-breeding between different Opuntia species has also been observed. Known pollinators include solitary bees and sap beetles. Most pollinators of O. engelmannii are polylectic, meaning they pollinate many unrelated plant species, but several pollinator species are specialized to Opuntia. These specialists are mostly solitary bees from the families Anthophorini (formerly classified as Anthophoridae), Andrenidae, and Megachilidae. Large and medium-sized bees are considered the most effective pollinators for transferring pollen between different O. engelmannii plants. Beetles and smaller bees are usually classed as "pollen thieves", as they often collect pollen without successfully pollinating the flower. The fleshy fruits of O. engelmannii are purple, 3.5 to 9 cm long, and 2 to 4 cm wide. They are spineless and glabrous, most often shaped ovate-elongate or barrel-shaped. The fruits are sweet and eaten by many animals, including humans. The seeds are ovoid or lens-shaped, averaging 0.45 cm long, 0.35 cm wide, and 0.16 cm thick. The seed funiculus is thick and white, and the embryos are typically curved. Fruits of O. engelmannii usually contain an average of 144 to 172 seeds. Seeds can remain viable in soil for at least 19 months before germinating. Within the species's native North American range, seeds are most often dispersed by coyotes. O. engelmannii is native to subtropical regions of North America. It prefers hot, dry conditions, but is more tolerant of frost than other Opuntia species. Its native range extends from California to Louisiana in the United States, and from the Mexican states of Sonora and Chihuahua east to the Tamaulipan matorral in northern and central Tamaulipas. While it is occasionally found in the Mojave Desert, it is largely replaced there by Opuntia basilaris, which does not require summer rain. It has become naturalized and invasive in southern and eastern Africa, including Loisaba in Kenya, and was reported in Spain in the early 2000s where it also remains invasive. The fruits of O. engelmannii were a dependable summer food source for Native American tribes. The Tohono O'odham people of the Sonoran Desert specifically classified the fruits by their color, ripening time, and storage life. Today, O. engelmannii is cultivated as an ornamental plant for drought-tolerant gardens, container plantings, and natural landscaping projects. It has also been used as a living hedge and as cattle fodder. These uses, combined with its ability to survive drought, led to it being imported to many locations across Africa.
