About Opuntia engelmannii var. dulcis (Engelm.) J.M.Coult. ex K.Schum.
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 pads, also called cladodes, are green (rarely blue-green), obovate to round, and measure around 15–30 cm long and 12–20 cm wide. In the Sonoran Desert, its terminal pads face predominantly east-west, which lets them maximize solar radiation absorption during summer rains. The cladodes form the plant’s stem and act as its primary photosynthetic structure. This species typically grows in high-altitude deserts, where lower atmospheric pressure increases water stress and can make gas exchange difficult. This leads Opuntia engelmannii to rely on CAM photosynthesis, which supports more efficient gas exchange and reduces water loss. Its glochids are initially yellow and turn brown as they age. The cactus spines are nonvascularized modified leaves that grow from areoles, usually in clumps. Spines are extremely variable: there are 1 to 8 spines per areole, and spines are often absent from lower areoles. They range from yellow to white, are slightly flattened, and measure 1–6 cm long. Spines harden from the top down and develop a waxy cuticle, and do not carry out photosynthesis. Their primary function is defense, but they also facilitate asexual reproduction: spines latch onto animal fur, and as the animal disentangles itself, it may carry away a cladode that can later dislodge and grow into a new individual plant. Members of 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 underground stores, while shallow roots spread over a large area to absorb rainfall quickly. Opuntia species are notable for developing both root types from the areoles of their cladodes, which is especially useful for propagation: separated cladodes are able to grow new roots and shoots from their areoles to produce a new independent functional plant. Opuntia species are commonly colonized by arbuscular mycorrhizal fungi. Mycorrhizae colonize Opuntia plants more in summer than in winter, and more when water is abundant. They help reduce drought stress for the cactus plants. The flowers of Opuntia engelmannii are yellow, occasionally reddish, and measure 6–9 cm in both diameter and length. The flowers are hermaphroditic with inferior ovaries, and their stamens are thigmotropically sensitive. Flowering occurs in April and May, and individual blooms last 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; on the second day they open from 8:30AM to 2:00PM. This gives an average total bloom time of 12 hours per flower. Flowers produce nectar to attract pollinators, and Opuntia engelmannii produces more nectar than other members of the Opuntia genus. Opuntia species undergo cross pollination, and 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 species, but several known pollinators are specialized to Opuntia. These specialists include members of the families Anthophorini (previously classified as the family Anthophoridae), Andrenidae, and Megachilidae, and are primarily solitary bees. Large and medium bees are considered the most effective for inter-plant pollination. Beetles and smaller bees are typically classified as "pollen thieves," because they often collect pollen without effectively pollinating the plant. The fleshy purple fruits are 3.5 to 9 cm long and 2–4 cm wide. They are spineless and glabrous, and often shaped as ovate-elongate or barrel-shaped. The fruits are sweet and eaten by many animals, including humans. Opuntia seeds are ovoid or lens-shaped, with average dimensions of 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. Opuntia engelmannii fruits usually contain an average of 144 to 172 seeds. The seeds can stay viable in soil for at least 19 months before germination. Within its North American range, the seeds are often spread by coyotes. Opuntia engelmannii is native to subtropical regions of North America. It prefers hot, dry conditions, but is more frost-tolerant 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 to the Tamaulipan matorral in northern and central Tamaulipas. While it is occasionally found in the Mojave Desert, it is mostly replaced there by Opuntia basilaris, which 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 and remains invasive there. The fruits of this species were a reliable summer food source for Native American tribes. The Tohono O'odham people of the Sonoran Desert specifically classified these fruits by color, time of ripening, and storage ability. Opuntia 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 its import to various locations across Africa.
