Colorado Cacti
By Jennifer Ackerfield
Well, this past summer wasn't
exactly the best for wildflower viewing!
The hot, dry weather may have
desiccated some wildflowers, but not
the cactus! Cacti are specially adapted
to survive these dry conditions, and are
widespread in Colorado. This article
examines the morphology, habitat and
distribution, species found in Colorado,
and survival strategies of cacti.
Morphology
There are several features which are found on all members of the family Cactaceae. All cacti have a succulent stem, inferior ovary, and unique structures called areoles. Areoles are a unique feature found in a wide number of positions on the cacti and are composed of two perpendicular buds. From the upper bud come the flowers and fruit or new branches, and from the lower bud come the spines. For the genus Opuntia, the areoles have groups of minute barbed bristles or glochids that can be very painful indeed - they become detached at the slightest tough to penetrate the skin - ouch!
> The flowers of cacti are generally solitary. The flowers are quite beautiful, appearing in shades of red, yellow, orange, and pink. There is no clear distinction between the sepals and petals in the perianth, but a gradual transition from sepals to petals. In addition, the ovary can bear areoles, scales, spines, or hairs for protection of the flowers and fruit.
> Cacti have numerous stamens
with long filaments. Interestingly, some
genera such as Opuntia have sensitive
stamens: when touched by an insect or
even a finger, they close over the top
of the pistil and straighten again a
moment later. This makes for an
effective and efficient pollination
strategy!
Habitat and Distribution
Except for the genus Rhipsalis which is indigenous to tropical American and Africa, the cactus family is exclusively New World in origin, with native species growing all the way from Canada to Chile! Most researchers concur that the origin of the family was somewhere near northern South America. Now, cacti can be found in other parts of the world as well, and are especially common in the Mediterranean, South Africa, and in Australia. Cacti in Colorado generally grow in exposed, dry areas such as pinyon-juniper communities, sagebrush communities, rocky hills, mesa slopes, and desert scrub communities.
> The occurrence of cacti in the
wild is gradually disappearing because
of the rise of collecting these plants for
use in the home garden. This should
not be practiced, and cacti should only
be bought from nurseries and garden
supply stores.
Species
The name "cactus" originally belonged to a prickly plant found in Italy called Spanish artichoke or Cynara cardunculus. Now, of course, we know that just because a plant is prickly doesn't necessarily mean it is a cactus!
Dr. George Engelmann was the American authority on cacti in the middle 19th century, and named most of the cacti in Colorado. Echinocereus engelmannii was named for Engelmann by Charles Parry, and Opuntia engelmannii was also named in honor of Engelmann's work with cacti.
> There are approximately 25 taxa of cacti in Colorado. Two of these species are rare plants. Sclerocactus mesa-verdae is only known to occur in the vicinity of Mesa Verde National Park in the southwest corner of the state. Sclorocactus glaucus is endemic to the western slope of Colorado. Both are classified as threatened by the U. S. Fish and Wildlife Service. Pediocactus knowltonii is rare in New Mexico and occurs up to 50 yards of the border of Colorado, but has not been collected in Colorado yet (collections of this species from Colorado are actually in New Mexico).
Opuntia is the largest and most widely distributed cacti genus in Colorado. Ten taxa of Opuntia can be found in Colorado, with Opuntia polyacantha being the most common. Opuntia's can be difficult to separate into species. The color of the flowers, size of the joints, and number and size of the spines show considerable variability and often depend on the amount of rainfall received during the growing season.
> One of the most ornamental
Opuntias in Colorado is the tree
cactus, Opuntia imbricata. It grows in
a shrub-like form up to 6 feet in height,
has trunks with a woody skeleton, and
has beautiful rose-pink flowers. You
can find the tree cactus in the
southeastern part of the state by places
such as Pueblo and Trinidad.
Survival Strategies
Cacti have many unique adaptations to their environment that enable them to survive in harsh conditions. These plants live in dry areas, so keeping enough water on hand and protecting this supply of water are big priorities! Plants lose water primarily through stomates on the leaf and stem surfaces. Cacti cannot afford to lose water during the hot daytime temperatures, so their stomates open at night! This allows the plants to take up CO2 at night when it is cool. At sunrise, as the temperature rises, the stomates close to prevent water loss and the stored CO2 is used for photosynthesis. This process is called Crassulacean acid metabolism, or CAM for short. Their stomates are also normally sunken such that they are not flush with the stem. This allows the stomates to remain protected from the heat as well.
As previously mentioned, cacti have unique features called areoles that produce spines and glochids. These sharp structures help protect the plant from animals that would surely like to eat the cactus for its abundant water content! In some cases, these spines are light-colored and reflect as much as three-fourths of the sunlight striking them. This helps keep the plant cool in the hot desert sun. The spines can also act as insulation on cold winter nights, raising the tissue temperatures above those that can damage cells.
> Many species of cacti have wide-spreading, shallow root systems. These roots are ready to absorb the first rain to penetrate the ground! This moisture is then stored in the soft stem tissues (parenchyma) for use when no water is available. This helps the plants to survive long periods of drought - some columnar and barrel cacti contain such an enormous volume of stored water in their stems that they can survive a drought ranging from just a few months to a few years!
Several species of cacti such as barrel or columnar cacti have stems with parallel ridges on their surface. These ridges serve many purposes - first, they partly shade the stem, helping to keep it cool. These ridges also allow the stems to swell with the uptake of water after a rainfall, much like the pleats of an accordion. The ridges expand quickly when water is stored and contract slowly as the moisture is used. These ridges also increase the stem's surface area to promote maximum photosynthesis.
> To prevent the cacti from
drying out, a thick waxy coat or cuticle
on its above-ground parts helps keep
evaporation of moisture to a minimum.
Conclusion
Cacti are indeed fascinating
plants with many survival strategies that
allow them to grow in harsh
environments. Try locating a few cacti
next summer!
References
- Boissevain, C. H. & C. Davidson. (1940). Colorado Cacti. CA: Abbey Garden Press.
- Nobel, Park. (1988). Environmental Biology of Agaves and Cacti. Cambridge University Press.
- Nobel, Park. (1994). Remarkable Agaves and Cacti. Oxford University Press.
- Schuler, Stanley (ed.). (1985). Simon & Schuster's Guide to Cacti and Succulents. Simon and Schuster.
- Spackman, S. et al. (1997). Colorado Rare Plant Field Guide. Prepared for the Bureau of Land Management, the U. S. Forest Service and the U.S. Fish and Wildlife Service by the Colorado Natural Heritage Program.
Cactus Fun Facts !
Cacti are such interesting plants - here are a few neat facts about them:
- Cacti occur natively in almost every state, and in more than half the states the only genus represented is Opuntia.
- Bees pollinate 80 to 90 percent of agaves and cacti.
- The stems of certain barrel-shaped cacti in the southwest tend to point southward toward the equator, while barrel cacti in the Southern Hemisphere tilt northward toward the equator!
- It takes approximately 75 to 100 years for a saguaro cactus to produce an arm.
- Specimens of saguaro cactus 30 feet tall can
contain hundreds of gallons of liquid!