CactusMuseum.com - Cactus survival characteristics

Cactus Survival

One only needs to sit back and admire God's creation - the cactus. Most cacti were designed for life in the hot and dry deserts. Listed below are some of the characteristics of these plants that help them survive in this harsh climate.

Leaves: Cacti do indeed have leaves. You would probably mistake the Pereskia as a leafy shrub unless the spines underneath the leaves got to you first. The Opuntia have short-lived, cylindrical leaves that dry up and fall off as the pad or joint matures. While these two genera have leaves that actually help with photosynthesis, leaves on most cacti are absent or extremely tiny.

How does this help the plant?

In normal plants, small pores in leaves - the stomata - take in carbon dioxide for the production of sugars via photosynthesis. These stomata also provide an exit route for the water vapor inside the leaf. A plant with many leaves presents a large surface area capable of losing water vapor. In the desert, a leafy plant would quickly exhaust its water reserves. The vast majority of cacti carry on photosynthesis inside the stem instead of inside leaves. This characteristic preserves their water stores, but limits photosynthesis efficiency and results in slow growth. Cacti cry out patience!

Stem: The cactus stem serves as the plant's main photosynthetic organ and is used for water storage. A freshly hydrated plant stem is almost 100 percent water. Two cactus structures, the ribs and tubercles, help the cactus stem expand and contract as water availability changes. The ribs also help to channel water from rainfall to the roots and help to shade portions of the stem throughout the day. Many cacti stems have a globular shape, which for water storage, is the optimal shape. A globe maximizes volume while minimizing surface area. Minimizing surface area keeps water losses through the stomata to a minimum.

In some cacti, the water that is taken up into the stem is converted into a mucilaginous substance that does not evaporate as readily as water. This helps the cactus survive the cold of winter. In nature, as winter approaches, water to the plant is reduced. This forces the water-mucilage solution inside the stem to become more concentrated. The more concentrated solution lowers the freezing point of the stem. You could say that these cacti have their own anti-freeze solution.

Skin: The cactus skin is translucent and acts as the first line of defense against fungi, bacteria, and foraging animals. The skin has two parts: the epidermis and the hypodermis. A waxy layer of cells known as the cuticle covers the skin’s epidermis. The wax in the cuticle helps the stem to hold in its water vapor reducing water loss. On some cacti, the waxy cuticle is also lightly colored and reflects some of the incident light. This theoretically reduces the temperature of the stem, however, internal temperatures as high as 70C can be endured without fatal consequences. The cactus skin contains numerous stomata; the number of stomata per square area, however, is less than the number for normal plants - another water-saving characteristic. The skin's hypodermis layer provides mechanical support for the plant. Crystals in the hypodermis of some plants deter small animals from foraging on the plant.

Roots: Cactus roots help to gather and preserve water in several ways. In some cacti, shallow, extensive root systems spread laterally away from the plant (e.g. some prickly pear roots spread 10 to 15 feet away). In brief showers which only wet a few inches of soil, the shallow roots help the plant maximize water intake from a large area. Cactus roots also change characteristics as the water supply fluctuates. After a rainfall, existing dehydrated roots become more water conductive and new rain roots are formed to help soak up water. In times of drought, the rain roots shrivel and fall off and the existing roots dehydrate. The shrinkage of the existing roots creates an air gap that helps to prevent water in the roots from escaping back to the soil. A corky layer on the roots also helps to prevent water loss.

Areoles: New growth, spines, flowers, and glochids originate from pad-like structures on the cactus stem named areoles. Unlike most other spiny succulents, these areoles are independent structures not integrated into the stem. The benefit to the plant: If an areole is detached, the stem is not damaged and water inside the stem is preserved.

Spines: Spines help the cactus in several ways. The most obvious use of the spines is for protection against foragers. The cactus basically states "How much pain are you willing to indure to get a drink in this desert?"; Surprisingly, some animals do partake. In some cactus varieties the spines participate in water collection. Water from dew condenses on spines and, in some cactus species, downward-pointing spines help to direct rainwater to the roots of the plant. Spines help to reflect light away from the cactus stem theoretically lowering the stem temperature. Some spines also trap in a layer of air next to the cactus stem preventing loss of water via evaporative cooling.

CAM Photosynthesis: Cacti use a special form of photosynthesis called Crassulacean Acid Metabolism (CAM). CAM is named for the crassula family (Crassulaceae), a group of plants exhibiting the process. In plants, the pores, or stomata, of plants allow for gas and water vapor exchange with the environment. Arid plants need to conserve water vapor during the day, so these stomata remain closed until nightfall. The lower temperatures, lack of sunlight, and lower wind speed at night provide optimal conditions for CAM plants to open their stomata without risk of excessive water loss. At night, the opened stomata take in carbon dioxide (CO2) for use by the plant. Without the sun, the energy source for photosynthesis, CAM plants can't make sugars at night, so, the CO2 is processed into malic acid for storage in the cell vacuoles. During the sunlight hours, the plants breaks down this acid and the resulting CO2 is used to produce carbohydrates (sugars) for the plant via a process known as the Calvin cycle. The Calvin cycle produces sugar for the plant according to the photosynthesis equation: 6CO2 + 12H2O + Sunlight --> C6H12O6 + 6O2 + 6H2O, or carbon dioxide + water + light energy yields sugar (Glucose) + oxygen + water.

Sources:
Cacti of the Southwest: W. Hubert Earle, 1980.
Cacti and Succulents: Publications International, Ltd., 1976.
Familiar Cacti: North America: Kenneth D. Heil, 1993.
The Desert World: David F. Costello, 1972.
The Cactus Primer: Arthur C. Gibson, Park S. Nobel, 1990.

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