Properties of a Cactus

Stems

• The succulent stems of cactus range in shape from round to cylindrical, and have many growth forms ranging from the branched or unbranched, tall, single columns of Arizona saguaro to the ground-hugging Mammillaria cushions of many stems sprouting from a common base. Cactus stems may also be flattened cladodes, such as the leaflike pads of Opuntia "prickly pear" that can generate roots or sprout new cladodes from their areoles. Many cactus stems are adapted for vegetative reproduction. Cladodes detached by wind or animals generate new plants, and when taken to the extreme in some Cholla species, the flowers produce no seed.

Spines

• Cactus spines are modified leaves rather than thorns, which are modified branches. Spines arise from cactus areoles and can be thin and hairy, flat and papery, straight, hooked, curved or bristly. Spines discourage animals from eating cactus, and they also shade the plant surface and baffle air movement to reduce evaporative water loss. In coastal deserts, spines harvest water from fog by condensation. Among vegetative reproducers, spines are adapted to catch a ride on the fur of passing animals. One areole may bear several types of spines, and among Opuntia, areoles bear tiny, deciduous glochids that easily embed themselves in skin but are not easily removed.

Wood

• In all woody plants, wood is formed by fibers in the xylem that, along with phloem, conduct fluid throughout the plant. In larger cacti supported by a skeletal framework, wood is formed by a secondary xylem along with living fibers that are wider and longer in the tall species and shorter and narrower in smaller species. In barrel and globular cacti, the xylem vessels are extremely short and narrow and lack fiber. The living fibers can be quite succulent, as in Opuntia, but in the closely related cylindropuntia Cholla, the wood is solid and resembles the wood of nonsucculent desert plants.

Water

• Cacti resist drought by storing water. In tropical rain forests, arboreal cacti catch water flowing on tree bark during the rainy season and conserve it to survive the dry season. Cacti have ribs or tubercles that ridge and wrinkle their surfaces during water stress. Cactus stems may be as much as 94 percent water, and when a cactus is fully hydrated, tubercles and the valleys between ridges swell until they're turgid and bow outward. Drought that reduces water content below 20 percent will cause cacti to die from the base upward, with upper shoots remaining alive for several years.