Even an inexperienced gardener can recognize succulents, with their fleshy bodies and easy-to-spot identifying features. This vast and diverse group contains approximately 60 plant families. The most extensive, and one of the most bizarre, is the cacti family. It includes nearly 2,000 species, almost all of which are native to both Americas. Cacti grow from the Arctic Circle to the mountains of Chile, but are most abundant in the southwestern United States and Mexico.

A relatively new addition to the plant kingdom, cacti are said to have emerged about 30 million to 35 million years ago. It wasn’t until about 20 million years ago that the cactus family diversified. Over the last approximately 10 million years, cacti spread across two American continents – an amazingly fast rate of expansion across such a vast geographic area. The evolution of cacti’s morphological, physiological and defensive traits all focused on one primary goal: water conservation.

Cacti vary in size and general appearance, from low button-like clumps to the upright columns of barrel cacti and the imposing saguaro. All have one common feature – a large proportion of internal tissue to external surface area to reduce moisture loss. Regardless of growth habit, cacti stems act as water reservoirs. The stem’s main attribute is its flexibility; it can expand and contract depending on the amount of water it holds. Many cactus stems are pleated like an accordion. This living water tank is covered by a waxy coating, further reducing water loss.

A glaucus barrel cactus with yellow spines and straw-like yellow blooms.

Cacti Characteristics

Superficially, cacti look like they lack branches and leaves. In fact, they have both but in highly modified forms. All cacti possess aureoles, areas that resemble tufts of white cotton from which spines emerge. In fact, aureoles are extremely reduced branches and the spines are modified leaves. These radical changes were necessary adaptations to particularly dry environments. Their transformed leaves range from microscopically small and pointy to large and barbed to silky and hair-like. They are composed of fibers made up of dead cells and contain little or no water. The primary function of the spines is protection from herbivores. They also shade the stem, keeping it cooler than the surrounding air. Furthermore, spines collect condensed water vapor during the night and, in some cacti, they are able to gather rainwater and funnel it to the roots.

Although cacti are associated with spines, a few species don’t have any. In some cases, they shed their spines when they reach adulthood. These spineless plants are not defenseless since they produce psychoactive substances that act as deterrents. An example is the infamous peyote (Lophophora williamsii), known for its powerful psychedelic effects.

Foliage modification comes at a high price, as cactus leaves can’t photosynthesize. Instead, the stems are responsible for photosynthesis. As a result, the photosynthetic area is limited; however, this helps minimize inevitable water loss.

A bed of potted white-spined golden barrel cacti.

Cacti adaptations don’t stop there. To further minimize photosynthesis-related water loss, cacti evolved a physiological mechanism for photosynthesis called CAM (crassulacean acid metabolism) photosynthesis. They leave their stomata (the little pores that allow gas exchange) closed during the heat of day and instead do their gas exchange at night. The evening’s lower temperature and higher humidity further help cacti retain water. In the dark, however, the plants can't use the sun's energy to make sugars out of carbon dioxide, so they store the carbon dioxide and wait for the sunrise.

Instead of developing deep root systems, cacti grow extensive shallow roots. These spread away from the plants and sit just under the ground’s surface to take advantage of the lightest rainfall. When it rains, cacti immediately shoot out more roots. During dry periods, roots shrivel up and break off.

The prickly pear cactus produces disproportionally large yellow blooms.

Blissful Blooms

Though they live in hostile environments, cacti are able to produce some of the most beautiful flowers in the plant kingdom. Like spines and stems, cacti flowers vary greatly but most blooms tend to be solitary, symmetrical and multi-petalled. Their colors range from white to every imaginable hue except true blue and black. Blooming, like all cactus processes, is aimed at minimizing water loss. Consequently, flowers are short-lived, and some cacti open their blooms at night. The flowers are oversized, brightly colored, strongly scented and nectar-rich. They are designed not simply to attract pollinators, but to do so quickly. Bee pollination is the most common; however, day-blooming cacti also attract butterflies and birds such as hummingbirds, while the white flowers of night-bloomers invite moths and bats. About a quarter of cacti are bat-pollinated, a record high proportion. These winged mammals specialize in the columnar growers such as the saguaro and organ pipe cactus.

Slow Growers

Cacti pay a high price for their water-saving adaptations – extremely slow growth. Some of the most sluggish growers may increase in size as little as 6 millimetres (1/4 inch) per year. Living rock (Ariocarpus fissuratus), an extremely slow-growing spineless cactus, can take up to 50 years to reach 12-1/2 centimetres (5 inches) in diameter.

A pincushion cactus with multi-colored blooms.

Care Requirements

Growing cacti is not difficult, as they are among the toughest of all houseplants. Most are well adapted to living indoors with low relative humidity. Because of their slow growth rate, they rarely need repotting. If one does need a new home, the pot should be filled with a fast-draining mix. A cactus’ water and fertilizer requirements are modest, particularly during the low-light winter months. Most cactus casualties are due to overwatering in winter, which can lead to fatal rot. As the amount of light increases in the spring, so does the plant's need for water and food. A cactus needs two applications of fertilizer when it is actively growing in the late spring and summer. An ingredient it needs in abundance all year round is light, so placement in a bright sunny window is a must.

With their peculiar anatomy, cacti provoke curiosity. They are admired for their stark beauty and sculptural quality. Various water-conservation traits reach a zenith in cacti. These evolutionary attributes and our interest in the unusual assure survival of these spiny, sun-loving succulents.

Text and photos by Gina Dobrodzicka

Gina Dobrodzicka is a freelance writer and trained horticulturalist who lives on Vancouver Island.

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