About Opuntia engelmannii var. cacanapa (Griffiths & Hare) D.Weniger
Opuntia engelmannii is generally a shrubby cactus that forms dense clumps up to 3.5 metres (11 ft) high, and usually has no visible trunk. Its stem segments, called pads or cladodes, are green (rarely blue-green), obovate to round in shape, and measure around 15–30 cm long by 12–20 cm wide. In the Sonoran Desert, terminal pads of this cactus are predominantly oriented east-west, an orientation that maximizes solar radiation absorption during summer rains. Cladodes make up the cactus's stem and act as its primary photosynthetic structure. Opuntia engelmannii typically grows in high altitude deserts, where lower atmospheric pressure increases water stress and can make gas exchange difficult. This environment leads the species to rely on CAM photosynthesis, which supports more efficient gas exchange and reduces water loss during the process. The cactus's glochids start yellow and turn brown as they age. Cactus spines are nonvascularized modified leaves that grow from areoles, and usually occur in clumps. Spines of Opuntia engelmannii are extremely variable: they range from 1 to 8 per areole, and are often missing from lower areoles. They are yellow to white, slightly flattened, and 1–6 cm long. Spines harden from top to bottom and develop a waxy cuticle; they do not photosynthesize, and their primary function is defense. However, spines also help the cactus reproduce asexually: they can catch on animal fur, and as the animal struggles to untangle itself, it may carry away a cladode that later dislodges and grows into a new plant. Species in the Opuntia genus grow both tap roots and adventitious roots, and some taproots function as water storage. Deeper roots anchor the plant and absorb water from deeper groundwater stores, while shallow roots spread over a large area to absorb rainfall quickly. Opuntia species are notable for being able to develop both root types from cladode areoles, which is especially useful for propagation: separated cladodes can grow new roots and shoots from their areoles to form a new independent plant. Opuntia species are commonly colonized by arbuscular mycorrhizal fungi. These mycorrhizae colonize Opuntia plants more heavily in summer than in winter, and more when water is abundant. The fungi help reduce drought stress in the cactus plants. The flowers of Opuntia engelmannii are yellow, occasionally reddish, and measure 6–9 cm in both diameter and length. They are hermaphroditic with inferior ovaries, and their stamens are thigmotropically sensitive. Flowering occurs in April and May, with individual blooms lasting only 1 to 2 days. Single-day blooms open around 11AM and stay open for 6 hours. Two-day blooms open around 10AM and remain open until 4:30PM on the first day, then open from 8:30AM to 2:00PM on the second day, giving an average total bloom time of 12 hours. Flowers produce nectar to attract pollinators, and Opuntia engelmannii produces more nectar than other Opuntia species. Species in the Opuntia genus undergo cross-pollination, but are also generally self-compatible. Cross-breeding between different Opuntia species has also been observed. Known pollinators include solitary bees and sap beetles. Most pollinators of Opuntia engelmannii are polylectic, meaning they pollinate unrelated plant species, but several known pollinators are specialized to Opuntia. These specialists include members of the tribes and families Anthophorini (previously classified as the family Anthophoridae), Andrenidae, and Megachilidae, which are mostly solitary bees. Large and medium bees are considered the most effective for inter-plant pollination. Beetles and smaller bees are typically categorized as "pollen thieves", since they often collect pollen without successfully pollinating the plant. The fleshy fruits of Opuntia engelmannii are purple, 3.5 to 9 cm long, and 2 to 4 cm wide. They are spineless and glabrous, most often shaped like an elongated oval or a barrel. The fruits are sweet and are eaten by many animals, including humans. Opuntia 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 Opuntia engelmannii usually contain an average of 144 to 172 seeds. The seeds can stay viable in soil for at least 19 months before germinating. Within its native North American range, Opuntia engelmannii seeds are often dispersed by coyotes. Opuntia 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 typically replaced there by Opuntia basilaris, a species that does not require summer rain. It has become naturalized in southern and eastern Africa, including Loisaba in Kenya, where it is considered invasive. It was also recorded in Spain in the early 2000s, where it remains invasive today. The fruits of Opuntia engelmannii were a dependable summer food source for Native American tribes. The Tohono O'odham people of the Sonoran Desert, in particular, classified the fruits by color, ripening time, and storage quality. Opuntia engelmannii is cultivated as an ornamental plant, used in drought-tolerant gardens, container plantings, and natural landscaping projects. It has also been used as a living hedge and as cattle fodder. These useful traits, combined with its ability to survive drought, led to it being imported to multiple locations across Africa.
