Left: Hoodia curroii only blooms after a good rain. The blossom’s scent is like that of rotting meat and feces—just the thing to attract the flies and other insects that pollinate these plants. Right: The pleats on this cardón Pachycerus pringlei reduce exposed surface area, which helps the plant lose less moisture during dry spells. When it rains, however, the pleats allow the plant to expand to store as much water as possible.
Form and Function
To the Point
A plant is considered a succulent if it has water-storing tissue in its leaves, stem(s), or roots and is able to remain independent of soil moisture for an extended period of time. A cactus stores water in its stem, which happens to be the largest part of the plant. Many succulents cache moisture in their thick leaves, which may take a familiar leaf shape, appear as a rosette of petals, or form a cluster of fingerlike projections. This range of visual variety is one of the things that make succulents popular with people. That, and their perceived low maintenance—no need to ask a friend to water your plants if you take a vacation!
When it comes to the difference between cacti and fat-leafed succulents, the sticking point is those sharp spines. The threatening thorns of a cactus are believed to be modified leaves. Some cacti, like the prickly pear, also have defensive weapons called glochids. These specialized spines usually bear a barbed tip, which lodges into the skin of any animal (or human) that brushes against it. Glochids detach from the plant, while primary thorns do not. By the time the pain registers with the unfortunate victim, it’s already too late— they’re hooked. The glochid got you! Since most people envision cacti as a threatening form of flora in the desert, they are surprised to learn that one member of the family, the epiphyllum, has a tropical address. Also known as orchid cacti because of their large, jewel-toned blossoms, epiphyllums (or epis, as they are called by aficionados) live a much different life than other cacti. Epis make a living in the forks of trees and in rock crevices in the tropical jungles of Mexico and Central and South America. Their unassuming, leaf-like stems don’t grow as thick with moisture as those of a desert cactus, but they do store their share of water. And while at first glance epis appear to be thornless, they do have quite short and nearly painless bristles along the edges of the stem. Epiphyllums are an example of a plant living where one least expects it. The succulent known as donkey’s tail, a popular hanging houseplant, adapted to thrive on steep cliff faces. Wherever there is flat space, the advantage of a trailing habit like that of donkey’s tail becomes an important factor in survival. The huge diversity of forms found among succulent plants is one of the reasons we find them so fascinating. n
Hold It! For cacti and succulents, once water is stored, the goal becomes keeping it. Green plants lose moisture through transpiration, and leaves, stems, and even flowers bear microscopic openings, called stomata, through which the plant takes in carbon dioxide and releases oxygen and water vapor. The leaf of a non-succulent plant can have about 100 to 860 stomata per square millimeter, while a succulent has dramatically fewer: 18 to 33 stomata per square millimeter. Fewer openings means less moisture lost. Succulent forms of flora have another trick up their stalk: they keep their stomata closed during the driest part of the day. At night, when temperatures drop and humidity rises, these plants open their stomata and load up on CO2, storing it for the next day’s production of energy. In most other plants, that process takes place during the day. Succulents also have a thick, hardened epidermis overlaid with a waxy coating. This double-layer of protection gives them their shiny look, and in a desert habitat, it can help the plant stay cool by reflecting the sun’s rays.
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