Cacti ARE Succulents

 
Succulents are plants that have organs such as leaves, stems or roots that are capable of storing water during the rainy or wet season in order to survive extended periods of drought. All the plants in the cactus family (Opuntiacea = Cactacea) are considered stem succulents. During periods of moisture, the stem swells and then during droughts slowly contracts. Cactus that have ribs are particularly well adapted to this as the ribs fill in and contract like an accordion.

One of the first things that one notices about a cactus is that it does not have any leaves but rather is covered with lots of sharp spines. The spines are what family is named after as the name cactus comes from the Greek word kaktos meaning thistle. These spines are highly modified leaf or leaf parts. Leaves are normally very poor at conserving a plant's water supply and are generally intolerant of high temperatures two characteristics which would make survival in very hot and dry areas impossible. What leaves are good at is photosynthesizing and cooling the plant off. To replace the leaf's involvement in photosynthesis, the epidermis of the cactus remains green and acts as the photosynthetic organ. By being tolerant to high temperatures, cacti do not depend on a cooling system.

Although other succulents have spines, it is the grouping of spines on areoles that distinguishes cacti from other cactus-like plants. An areole is the radial arrangement of spines on padlike buds where shoots and flowers may arise. The areoles themselves are arranged in a regular pattern either along the ribs of columnar or barrel cacti or at equally spaced intervals over the face of pad-like cacti. In some cactus species, such as the christmas cactus (Schlumbergera sp.), the areoles are difficult to see as they are grouped at the tips and along the sides of the stem segments and are very small. One plant family that is often confused for the cactus family is the euphorbia family (Euphorbiacea) which contains such plants as Cow's Horn Cactus (Euphorbia grandicornis), African Milk Bush (E. trigona) and Crown of Thorns (E. milii). While all these plants have spines, few to no leaves and have green stems, they do not have areoles. If still in doubt whether you have a euphorbia or a cactus, make a small cut in an inconspicuous place: if the sap is milky white and sticky, then you have a euphorbia as cactus sap is clear and watery.

Although spines do not perform any of jobs that leaves do, they do have three very important functions. The first one is to protect the plant against herbivory. Cacti grow very slowly in usually harsh environments and cannot afford to lose any of their plant mass to the occasional browser. There are some rodents and birds however that rely on cacti as an important source of food and water or as a home and have figured out how to get around the prickly problem of spines. Another function that spines perform is to slow down blowing winds thus reducing evaporative water losses.

Old Man Cactus (Oreocereus sp.) with its long hair-like spines does this very well as air is trapped in its spines and cushions the effects of the wind. Thirdly, spines allow any moisture in the cool night air to condense on them (much like dew on grass) and then that water drips to the ground where the roots can absorb it. Although the typical desert is very hot and dry during the day, during the night, the temperature drops and the relative humidity rises dramatically. The moisture content of the air does not increase at night, but as the air cools, its ability to hold water decreases. (Relative humidity is the measure of the air's moisture content of the air over its ability to hold water at a given temperature.)

One common misconception about cacti is that they are only found in true deserts. While a few species can survive in the pure sand of deserts like those found in Arizona where rainfall is sporadic, cacti are found in habitats where moisture is not quite as limiting and the soil is, although nutrient poor, not pure sand. Habitats where cacti can be found range from semi desserts to dry grasslands where rainfall is periodic and soils are generally poor and from sub-alpine mountainous regions to tropical humid jungles where moisture is either from snow melt in the first case or from high humidity in the second case and the soil is poor to non-existent.

The key to understanding where cacti survive is noting that they are adapted to regions where the environment is limiting and that there is a lack of competition from other vegetation in terms of light, moisture and nutrients. Depending on their habitat, cacti have a variety of survival mechanisms to overcome their harsh environment. In the drier regions, cacti go dormant during the hot dry season and only grow and flower when there is moisture. Sometimes, the desert cacti flower, set seed and then go dormant again over the space of only a few days. In their mountainous and northern range, cacti go dormant during the cold season surviving temperatures of -30 C and below and only grow during the warm summer months. In humid jungles, cacti are epiphytic and grow in trees and have adapted to growing in shaded, nutrient poor environments by having flat stem segments to capture the filtered light efficiently and by having roots that absorb moisture and nutrients from water dripping off the trees that they're living on. In the home, the key to growing cacti successfully is to mimic some of these environments. For example, when your cactus is going dormant usually as response to daylength, restrict watering to only once per month and only increase to weekly to biweekly watering when it is showing signs of growth

Cacti are native only to the Americas and Columbus is purported to be the first European to discover this spiny leafless plant. American natives of course knew all about cacti and it was incorporated into many cultures. Cactus sap has been used medicinally; some cactus sap has narcotic effects and has been used in religious ceremonies; the sap of Stenocereus gummosus is toxic and is thrown into streams where it stuns fish which are then easily fished out by hand; the stems of some cacti themselves can be used as a source of food either baked or raw; cactus fruit of some species can be eaten raw, jammed or candied; the long soft spines of Oerocereus celsianus are used as pillow and bed stuffing; spines of other cacti are used as toothpicks, combs, sewing needles and fishhooks; yet other cacti are used as building materials and as living fences or hedges.

One the earliest recorded purposeful cultivation of a specific species of cactus is by the Aztecs.They grew Opuntia coccenillifera which acted as a host to the cochineal scale insect which was harvested and crushed to produce a either a rich purple (from the female scales) or brilliant scarlet (from the male scales) dye used in fabrics and cosmetics.
In conclusion then, two things to remember about cacti are that: 1) although it looks like a cactus and feels like a cactus (OUCH) it ain't necessarily a cactus and 2) all cacti are succulents but not all succulents are cacti.

© 1995 Erl Svendsen
(Permission requested for reprint)

Cacti are Succulents
Growing a Healthy Cactus
Propagating Your Cactus
Cacti and Succulents
Growing Cacti