Answer: B (leaves)
cac·tus () Plural cacti () or cactuses. Any of various plants that have thick, leafless, often spiny stems and grow in hot, dry places, chiefly in North and South America. Photosynthesis takes place in the stems of cacti, as the leaves have evolved into narrow spines to prevent water loss. Some kinds of cacti have brightly colored flowers and edible fruit.
Did You Know?
The 2,000 species of cacti are known for living in extremely dry climates, such as the American Southwest. Cacti are excellent at conserving water. Their leaves are sharp spines, which have been known to cause great pain to animals interested in eating them. The spines also help the plant gather scarce water. Water vapor in the air condenses on the spines and then drips to the ground, where it is taken up by the roots. The roots are shallow and widely spread out to take advantage of this condensation and the rare desert rain showers. A cactus can be between 80 and 90 percent water, and its thick walls keep its water from evaporating. In fact, a cactus can be a thousand times better at conserving water than a different kind of plant of the same weight.
spine () 1. See vertebral column. 2. A sharp-pointed projection on a plant, especially a hard, narrow modified leaf, as on a cactus. See more at leaf. See Notes at cactus, thorn.
- Scientific name
- Common names
- Naturalised distribution (global)
- Introduced, naturalised or invasive in East Africa
- Reproduction and dispersal
- Similar species
- Economic and other uses
- Environmental and other impacts
- My Cactus Lost Its Spines: Do Cactus Spines Grow Back
- Do Cactus Spines Grow Back?
- What to Do if Your Cactus Lost its Spines
- Cactus Spines Can Affect You Deeply
- Why Do Cacti Have Spines?
- 1. Cactus Spines Trap Air, Regulating The Plant’s Temperature
- 2. Provide Shade
- 3. Spines Protect Cacti From Predators
- 4. Mobility And Reproduction
- 5. Cactus Spines Help Collect Water
- 6. Cactus Spines Are Modified Leaves
- What’s The Difference Between Thorns And Spines?
- Are Cactus Spines Poisonous?
- Ask a Scientist: Why does a cactus have needles?
Opuntia exaltata (Long-spine Cactus)
Opuntia exalta A. Berger
Austrocylindropuntia exaltata (a. Berg.) Backeb.; Opuntia subulata (Muehlenpf.) Engelm.
Long-spine cactus, cholla cactus
Naturalised distribution (global)
Locations within which Opuntia exaltata is naturalised include southern and eastern Africa.
Introduced, naturalised or invasive in East Africa
Opuntia exaltata is invasive in parts of Kenya (A.B.R. Witt pers. obs.) and Tanzania (Henderson 2002). (Global Invasive Species Database). It is likely to be found in as an ornamental in Uganda but it is not yet invasive there. Small, isolated infestations of O. exaltata are found throughout Kenya. Larger infestations are found in parts of Kenya including Naivasha, Nakuru and Nairobi.
Savanna, grassland, but usually cultivated as an ornamental or as live hedge. Currently, this species is spreading into surrounding wooded grasslands, along roads in disturbed soil.
Opuntia exaltata is a spiny, much branched succulent shrub 2-5 m tall. It is pale bluish-green or greyish colouration (glaucous). Its spines are strong, straight, up to 5 cm long, yellow-brown in colour and arise from the white woolly pits (areoles) in clusters of 1-2-3.
O. exaltata develops a trunk with age and its branches (cladodes) are curved, cylindrical, tuberculate (with small wart-like swellings or small rounded outgrowths) with knobbly projections.
Its leaves are elongated 30-60 or 120 mm long, curved, fleshy and persistent.
Flowers are terminal on cladodes, orange or greenish-yellow. The fruit is green, pear-shaped, about 90 mm long, but usually sterile.
Reproduction and dispersal
Opuntia exaltata reproduces vegetatively from the branches. It produces fruit but the seeds are usually sterile. It produces fertile seeds in native range.
There are no similar looking cacti in East Africa.
Economic and other uses
Opuntia exaltata is cultivated as an ornamental or as live hedge used to exclude livestock, large and small wild mammals. It can be used as a medicinal plant. However, these uses cannot compensate for this plant’s overall negative impacts.
Environmental and other impacts
Opuntia exaltata is a potential ecosystem transformer species. The spines and glochids can irritate the skin. The plant lowers the value of pastures since it cannot be browsed and it also curtails movement of grazing animals. In addition, the spines can injure people who stray in areas where they are growing and even cause tyre punctures to vehicles. Therefore, they can affect tourism in wildlife parks and ranches. The spines can also injure livestock and wild herbivores, especially when normal pasture is reduced by the invading cactus and these animals are forced to feed on O. exaltata. It displaces native species and prevents the free movement of wildlife.
O. exaltata has been listed as a noxious weed in South Africa (prohibited plants that must be controlled. They serve no economic purpose and possess characteristics that are harmful to humans, animals or the environment).
The precise management measures adopted for any plant invasion will depend upon factors such as the terrain, the cost and availability of labour, the severity of the infestation and the presence of other invasive species. Some components of an integrated management approach are introduced below.
The best form of invasive species management is prevention. If prevention is no longer possible, it is best to treat the weed infestations when they are small to prevent them from establishing (early detection and rapid response). Controlling the weed before it seeds will reduce future problems. Control is generally best applied to the least infested areas before dense infestations are tackled. Consistent follow-up work is required for sustainable management.
Manual control can be effective when numbers of plants are very low but must be done carefully otherwise plant fragments will resprout into new plants, thus exacerbating the infestation. Both the spines make this a difficult and uncomfortable process). Burning of uprooted plants will help minimise this risk if there is enough dry material to ensure that the material burns. Plants can be treated by herbicide stem injections. When using any herbicide always read the label first and follow all instructions and safety requirements. If in doubt consult an expert.
Controlled burns have been used to control Opuntia species. Such burns must be well-timed and coordinated to reduce the risk of creating a bushfire and there must be sufficient material to carry a hot fire. Fire could be used for small, isolate stands but it will not penetrate large stands.
The moth Cactoblastis cactorum feeds on this plant in its larval stages and can help control Opuntia exaltata as part of an integrated control programme. It was introduced to Tanzania but never established. It is likely that some cochineal species which feed on Opuntia species have been introduced to East Africa. This group offers some prospects for biological control of Opuntia species.
Not listed as a noxious weed by the state or governments in Kenya, Tanzania and Uganda.
In some parts of the world, the spineless cultivars of Opuntia are exempt from restrictions associated with the more invasive spiny species.
Henderson, L. (2001). Alien weeds and invasive plants. A complete guide to declared weeds and invaders in South Africa. Plant Protection Research Institute Handbook No. 12, 300pp. PPR, ARC South Africa.
Henderson, L. (2002). Problem plants in Ngorongoro Conservation Area. Final Report to the NCAA.
Agnes Lusweti, National Museums of Kenya; Emily Wabuyele, National Museums of Kenya, Paul Ssegawa, Makerere University; John Mauremootoo, BioNET-INTERNATIONAL Secretariat – UK.
This fact sheet is adapted from The Environmental Weeds of Australia by Sheldon Navie and Steve Adkins, Centre for Biological Information Technology, University of Queensland. We recognise the support from the National Museums of Kenya, Tropical Pesticides Research Institute (TPRI) – Tanzania and Makerere University, Uganda. This activity was undertaken as part of the BioNET-EAFRINET UVIMA Project (Taxonomy for Development in East Africa).
BioNET-EAFRINET Regional Coordinator: [email protected]
Etymology -From the Latin word mammilla, meaning “nipple” or “teat”. With nearly 200 recognized species, the genus Mammillaria is one of the largest of the cactus family. For the most part, these species are globose or ball-shaped plants which grow either solitary or in clumps. Some clumps may reach over 3 feet (1m) with many stems. Few species grow much over around 6-8 inches in height by 4-6 inches in diameter. All have nipple-like tubercles with dimorphic areoles on the ends. Spines may be stiff and stout, few or many, bristle-like, hair-like, pectinate (comb-like), and come in a wide range of colors. In the axils, that is between the tubercles, there may be wool or bristles or both or neither. Flowers come from second-year growth in these axils and often form a ring around the stem. In many species the flowers are small and pink at less than half-an-inch in diameter. Some are small and yellow or white, while a few species have showy flowers which stand on long floral tubes above the plant. In many cases, the petals will feature a darker midstripe. The fruits are typically red, tube-like structures resembling little candies and are edible. To describe so many different species in such general terms does not do justice to this genus.
In the wild, all but a few of the species are endemic to (only found in) Mexico. A handful of species reach up into the bordering United States and only a couple species find their way down through Central America and Northern South America as well as the islands of the Caribbean.
Mammillaria is arguably the most popular cactus genus in cultivation. The small-size and ease of growing and propagating for most species makes them perfect for mass distribution via large garden centers. The variation in spines are a novel selling feature and often, these plants fit the stereotypical expectations that most people have for a cactus plant. Although, some species are only going to be found among highly-dedicated cactus growers.
The popularity of this genus has led to a good deal of study and consequent literature on both growing and identifying Mammillaria species. Even so, disputes over nomenclature are still not eliminated and some species find their way back and forth between Mammillaria and other genera such as Cochemiea, Mammilloydia, Mammillopsis, and Solisia. Other similar genera such as Coryphantha and Escobaria which have both been included in Mammillaria in the past, seem to be more widely accepted as distinct by modern taxonomists.
My Cactus Lost Its Spines: Do Cactus Spines Grow Back
Cacti are popular plants both in the garden as well as indoors. Well-loved for their unusual forms and known for their spiny stems, gardeners can become unnerved when faced with broken cactus spines. Read on to learn what to do, if anything, for a cactus without spines and find out if these spines will regrow.
Do Cactus Spines Grow Back?
Spines on cactus plants are modified leaves. These develop from living spine primordia, then die back to form hard spines. Cacti also have areoles that sit on bases called tubercules. Areoles sometimes have long, nipple-shaped tubercles, on which spines grow.
Spines come in all kinds of shapes and sizes – some are thin and others thick. Some are ridged or flattened and some may be feathery or even twisted. Spines also appear in a range of colors, depending on the cactus variety. The most dreaded and dangerous spine is the glochid, a small, barbed spine commonly found on the prickly pear cactus.
A cactus without spines may have been damaged in the area of these areoles or spine cushions. In other cases, spines are removed from cactus plants on purpose. And, of course, accidents happen and the spines may be knocked off the plant. But will cactus spines regrow?
Don’t expect spines to regrow in the same spot, but the plants may grow new spines within the same areoles.
What to Do if Your Cactus Lost its Spines
As spines are an integral part of the cactus plant, it will make every effort to replace damaged stems. Sometimes things happen to the plant that causes broken cactus spines. If you find your cactus lost its spines, don’t look for them to regrow in the same spot. However, you may ask will cactus spines regrow in other spots? The answer is often yes. Spines may grow from other spots in the existing areoles.
As long as there is continued growth overall on a healthy cactus plant, new areoles develop and new spines will grow. Be patient. Some cacti are slow growers and it may take a while for this growth and the production of new areoles.
You may be able to speed growth somewhat by fertilization and locating the cactus in full morning sunlight. Feed with a cactus and succulent fertilizer on a monthly or even on a weekly schedule.
If your cactus is not located in full sun, adjust it gradually to more daily light. The right lighting encourages growth of the plant and may help the new spines to develop.
Cactus Spines Can Affect You Deeply
The cactus and foot in question | Photo: Chris Clarke
A few days ago I kicked a cactus with my bare right foot. Three dozen spines lodged deeply in my skin: in the ball of my foot, the ends of my toes, in the folds between my toes. I managed to remove about two thirds of them. The rest were driven into my flesh with no protruding ends to pull or scrape away. There they remain.
I didn’t mean to kick the cactus. I’m stupid, but not that stupid. We had a hard freeze a few days ago and a large cactus stem broke off from its mother plant. I let it stay where it had fallen, intending to let the break harden off for a few weeks so that I could plant it elsewhere in the yard without fungi invading the stem. It was well away from the sidewalk, I thought. It was too much trouble to put on shoes to take the used cat litter out to the trash, I thought. On my way back in I misjudged my step, and I literally saw stars for a moment.
The cactus in question — pictured above — was one of those cultivated Trichocereus hybrids you see in gardens all over California. Lucky for me. Its spines are only between a quarter- and a half-inch long, and relatively thin — but not too thin. Aside from having to hobble around for a week or so and making sure none of the points of entry get infected, I don’t need to worry much. If I’d kicked a cholla, I’d be in a lot worse shape.
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It’s a bad few months for my feet, desert wise. After more than two decades of wandering around in the desert, much of that time spent barefoot, and even more spent wearing hiking sandals, my feet had never found themselves the worse for a cactus encounter until the past six months. Last fall, heading out to check the mail, I trod unexpecting on a detached spine that had blown onto my driveway, tumbleweed style. That was more uneventful: the spine came out easily, and the pain subsided almost as quickly as the cursing.
This time did more damage. I’ve had to turn down two hikes so far, my right leg is developing a cramp from my late habit of walking on the inside of my foot, and I’m actually starting to consider that certain people around here might have a point about my putting on shoes before I go outside. Next thing you know I’ll be cutting my hair and getting a job in marketing.
They’re spines, by the way, and not “thorns,” or “prickles.” Thorns are modified branches, and prickles mere sharp hairs. Spines are highly modified leaves, growing from what were once leaf buds. In cacti, these modified leaves grow out of areoles, clusters of buds from which flowers and (in some cacti) leaves also grow.
Areoles on a Mojave Desert beavertail cactus | Photo: Chris Clarke
It’s the areoles that allow you to determine if a plant is a cactus. Lots of plants get called cacti that aren’t, sometimes because the plant is a succulent but more often because it hurts when you bump up against it. Agaves and yuccas aren’t cacti, nor are ocotillos, nor those picturesque and well-armed Euphorbias you see in office buildings.
Cactus spines serve an obvious defensive function, which becomes more important in the desert than in other parts of the world. Succulent plants are sources of water, which is in short supply in the desert pretty much by definition. A barrier of spines is often an effective deterrent to grazing animals that might otherwise rob you of your precious bodily fluids by chowing down on your leaves and stems.
It doesn’t always work. Black tailed jackrabbits and Audubon’s cottontails manage to eat prickly pear cacti despite the armaments, for instance. Many cacti have evolved subsidiary defense mechanisms, including bitter-tasting, emetic, and otherwise unpleasant chemicals in their flesh. Ironically, one species of mostly spineless cactus, Lophophora williamsi, has developed a chemical defense mechanism so effective that people have depleted it in quite a few parts of its range. It’s far from the only cactus species that uses hallucinogens as a defense mechanism. Some are even common landscape plants in California, a piece of information which I note for academic purposes only.
Defense against herbivores isn’t the only function spines perform. In many California desert species, spines also offer shade against the searing, implacable radiation coming from the desert sun. There are a few California desert cactus species, the above-pictured beavertail cactus among them, that rely on a thick waxy coating as protection rather than spines. But many desert species bear cloaks of spines so thick you can barely see the green skin beneath.
That’s not a bad hint as to the origin of a cactus species, incidentally. Cacti don’t just grow in deserts. The majority of cacti are native to habitats ranging from dryish, cold grasslands to tropical rainforests. Many of the most popular landscaping species originated either in the understory of montane conifer forests — the common nopal prickly pear being an example — or in deserts where coastal fogs provide some protection from the sun. The column cactus I kicked this week is probably an example there. In the Arizona Upland section of the Sonoran Desert, where rainy summer afternoons are far more common, many cacti show green through their spines, the saguaro being a fine example.
There’s one other important function spines perform in some cactus species, primarily those belonging to the genus Cylindropuntia — the chollas. If you’re a large hairy animal brushing up against a patch of cholla, with or without shoes, it’s very likely that some of that cholla will come along with you. Eventually you get it shaken loose and the section of cholla falls to the ground, where it can root out and start a new plant. In this way, the cholla’s fiendish spines serve as reproductive organs of a sort.
Chollas and their cousins the prickly pears, by the way, have a type of spine unique in all of cactusdom: little hairy devils referred to as glochids. Glochids look benign when you encounter them for the first time, and more than one young child has learned to his or her sorrow that that fuzzy prickly pear cactus is not to be petted despite its velveteen appearance. Often growing in the hundreds per areole, glochids detach with almost no effort and easily burrow into your skin, where their barbed shafts and sheer numbers make complete removal almost impossible. (See this piece in KCET’s SoCal Wanderer for tips on how to get most of them out.) As the pain glochids cause is slow to build, it’s hard to resist the conclusion that their main function is punitive. Vindictive, even.
Back in the days when I worked in plant nurseries for a living, I had enough glochids work their way into my skin permanently that I’m probably about 2 percent cactus at this point. Thankfully, these aren’t glochids in my feet. I work behind a computer now instead of moving large potted cacti back and forth, so I can manage to stay mostly off my feet for a day or five. Within a week the pain will ebb, and I’ll begin the process of slowly absorbing those dozen spines into the tissue of my foot. You can make that 2.5 percent cactus, I guess.
Chris Clarke is an environmental writer of two decades standing. He writes from Joshua Tree regularly at his acclaimed blog Coyote Crossing and comments on desert issues on KCET weekly. Read his recent posts here and follow him on Twitter.
I always assumed that cacti just had spines for protection, but it turns out they have many more functions and are really quite remarkable adaptations. They help cacti overcome the difficulties of living in desert conditions in a variety of interesting ways.
Why do cacti have spines? Spines help cacti to thrive in desert environments by providing multiple functions. They protect the plant from predators, provide shade, help regulate temperature, reduce water loss and even help the plant to spread and reproduce.
Read on and I’ll explain exactly how cacti use their spines to help them to survive and thrive in the inhospitable conditions in which they live.
Why Do Cacti Have Spines?
There are 6 main reasons why cacti have spines. I’m going to explain each reason in a little detail and I’m sure you’ll be fascinated by the interesting uses for these spiky adaptations.
1. Cactus Spines Trap Air, Regulating The Plant’s Temperature
Although cactus spines are small, they are often very numerous, covering the surface of the plant. In the same way as wearing a jumper traps an insulating layer of air next to your skin, the spines of a cactus trap a layer of air close to the surface of the plant, reducing ventilation.
This layer of air insulates the cactus and prevents large swings in temperature. This is useful both in the very hot conditions of the desert, but also protects them from large changes in temperature between day and night, and during the different seasons, where in some desert climates, it can get quite cold at night.
2. Provide Shade
Cactus spines are small and thin and not what you would normally expect from a sun shade. Hold a single cactus spine up and the shadow that it makes is tiny and seemingly insignificant.
However, because cacti spines are so numerous, the combined effect of all of them provides a surprising amount of shade. There aren’t too many places to find shade in a desert and the sun could easily damage a cactus without some ability to prevent some of this reaching the surface of the cactus.
In addition to reducing heat, the shade of cactus spines also reduces evaporation of water from the surface of a cactus, reducing water loss and helping a cactus survive more efficiently through a period of drought.
3. Spines Protect Cacti From Predators
Cactus spines are extremely sharp and can be very painful to touch unless you are careful. I’m constantly surprised by how sharp they are when I am repotting a cactus, and it reminds me why special care is needed around these plants.
Cacti are often one of the most numerous plants in a desert environment and, for the many animals that live here, a cactus would make a welcome meal, but the spines prevent the majority of animals from feasting on these plants.
Unfortunately for cacti, there are a number of animals that aren’t bothered in the slightest by the sharp spines of cacti, and go ahead and eat them anyway. Javelinas, camels and many rodents and insects will all eat cacti despite their spines.
4. Mobility And Reproduction
Some species of cactus have barbed spines which readily attach to any animals (or even humans) that get too close to them. This is a clever reproductive mechanism, most famously exhibited by Cholla (Cylindropuntia) cactuses.
When anything gets to close, the barbed spines will stick in the fur or skin of the animal. Part of the cactus will detach from the parent plant and be transported elsewhere.
When these parts of the cactus eventually detach from the animal, they will start growing a new plant where they fall.
This behavior isn’t unique to cacti, as many plants enlist the help of animals to help them reproduce and spread, but some cacti have assigned this role to their spines, with excellent results.
Interestingly, this means that when you see hundreds of Cholla cacti in the desert, they are likely to be clones of each other and genetically identical, due to the efficiency of this reproductive technique.
5. Cactus Spines Help Collect Water
The deserts that many cacti live in are notoriously arid and rainfall is unpredictable. It can be a long time before the next rain, so cacti and other desert plants must make use of adaptations to collect, store and conserve water.
Cactus spines have an important role in collecting water from the air and are able to do this very efficiently due to the large surface area in contact with the air.
Cacti don’t lose water from these spines as they are metabolically inactive, but they allow water vapor from the air to condense onto the spines and run down onto the surface of the plant and into the soil, to be absorbed by the roots.
Even when it has not rained for a long time, there can still be significant moisture carried in the air. Warm air holds more moisture, but as it cools, the air can’t carry all of the moisture, and some of this will condense onto surfaces. This is why dew forms in the evening, and cacti are ready and waiting to collect this source of water from the air.
6. Cactus Spines Are Modified Leaves
Cactus spines are actually heavily modified leaves, although they don’t have any of the same structures that are found in normal leaves and are completely inert. Cacti have adapted to the harsh conditions they live in by developing areoles instead of branches and spines instead of leaves.
The problem with leaves is that the large surface area causes a lot of water to be lost in a process called transpiration. Cacti have adapted by moving the functions of leaves into the body of the cactus. This means that photosynthesis is done in the trunk of the cactus, rather than where we normally think of it being done in other plants.
The adaptation and development of spines developed gradually, as the hot and inhospitable conditions of deserts led to survival advantages in having smaller leaves. Gradually, the functions of the leaves moved to the central stem, and the leaf remnants hardened and developed the other adaptations of spines that we see today.
What’s The Difference Between Thorns And Spines?
Plenty of plants have thorns, but only a few types of plants have spines, and yet a lot of the time they look quite similar. So what’s the difference? Spines are modified leaves, whereas thorns are modified branches.
All plants with thorns will also have leaves of some description. Plants with spines have no leaves, because the spines ARE the leaves.
Interestingly, many of the plants we characteristically associate with having thorns, actually don’t. The thorns of roses are actually prickles. Prickles are sharp extensions of the stem or leaf epidermis and are different from both spines and thorns.
Are Cactus Spines Poisonous?
Cactus spines are not poisonous. They can, however, cause significant damage to your skin, particularly those varieties with barbed spines. Barbed spines are designed to detach and lodge in the skin of the animal or human they come into contact with. These spines are difficult to remove and can cause localized dermatitis or a granulomatous reaction if they are left untreated.
Opuntia cacti have very short spines which will lodge in your skin and can be really difficult to get out. They will irritate the skin and be very uncomfortable. I’ve spent many frustrating hours trying to get prickly pear spines out of my skin with a pair of tweezers. Not a situation I would like to be in again.
New cactus spines developing
Cactus spines emerge from areoles as the plant grows. Mature spines are inert and will not grow back if they are damaged or removed. New spines will only grow from new areoles as the cactus grows.
If you are growing cacti as ornamental plants, handle them with care, as the cosmetic damage that is done from damaging or breaking spines is permanent.
I hope you’ve enjoyed learning why cacti have spines. This article is part of a series looking at some of the interesting characteristics of cacti and succulents. Check out some of the other articles in this series below.
- What Soil Do Cacti Need?
- How Fast Do Cactus Grow?
- How Long Do Cacti Live?
- What’s The Difference Between Succulents And Cacti?
Ask a Scientist: Why does a cactus have needles?
Mexico’s prickly pear cactus could turn out to be a great energy source for the future. Video provided by AFP
Meet the student asking the question:
Name: DonTino Gillespie-Pitts
School: Johnson City Intermediate School, Johnson City Central School District
Teacher: Mrs. Goroleski
Hobbies: Video games
Career interest: Scientist
Meet the scientist:
Answered by: Jessica Hua
Title: Assistant professor of ecotoxicology, Binghamton University
Research area: Ecology, evolution, ecotoxicology
Interests/hobbies: Basketball, hiking, reading
Question: Why does a cactus have needles?
Cacti are one of my favorite types of plants and their needles certainly make them stand out. The needles that cover cacti are critical to their survival in many ways. Many cacti are found in dry habitats, so in order to survive, these plants have to store a lot of water. In fact, scientists estimate that 90-94% of a cactus plant is made up of water. Because of the high water content of cacti, these plants are great snacks for thirsty organisms. In the wild, animals that eat cacti include: quail, kangaroo rats, sheep, desert tortoises and also many insects. So, one reason for why cacti have needles is to prevent thirsty or hungry animals from eating or damaging the plant.
You may also notice that cactus spines can differ in color and texture. Some spines are rigid while others are feathery. Spines can also range in color from white, to gray, to pink! These differences in spine color and texture allow cacti to better blend into their environment (i.e. camouflage). So, a second reason for why cacti have needles is to allow them to hide from organisms that might cause harm.
In desert environments where many cacti are found, heat is another major problem. In order to survive these hot conditions, needles on a cactus can provide shade from the sun. A single-needle may not seem like it can provide much shade, but in many species of cacti, the needles are in clusters that are really close together. These spine clusters almost act like mini-umbrellas that help to shade the plant. By shading the cactus, these spines also help prevent the cactus from losing water through evaporation.
So all in all, spines are adaptations that protect and help cacti hide from animals that may want to eat them. They also provide shade for the plant, which keeps them cooler and limits water loss.
Ask a Scientist runs on Sundays. Questions are answered by science experts at Binghamton University. Teachers in the Greater Binghamton area who wish to participate in the program are asked to write to Ask a Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, N.Y. 13902-6000, or e-mail [email protected] For more information, visit binghamton.edu/mpr/ask-a-scientist/.
The evolutionary conversion of leaves to spines in cacti
One defining feature of cacti is having clusters of spines. Numerous plants have spines of course, but in cacti, spines occur in clusters in the axil of leaves, even though the leaves are usually microscopic. Most cactus morphologists have concluded that cactus spines are either modified leaves or modified bud scales (the difference is inconsequential because bud scales themselves are modified leaves). The leaf-nature of spines is certainly understandable from the point of view of location: spine primordia look just like leaf primordia and are produced at a location where we would expect leaf primordia – at the base of the axillary bud’s shoot apical meristem.
Evolution appears to have been more complex than would be expected: mature cactus spines do not contain any of the cells or tissues characteristic of leaves, and conversely leaves lack all features characteristic of spines. The two organs have little in common other than developing from leaf primordia. Spines consist of just a core of fibers surrounded by sclereid-like epidermis cells. They have no stomata, no guard cells, no mesophyll parenchyma, no xylem, no phloem. When mature, all cells in a spine are dead, and even when the spine is still growing it has living cells only at its base. Cactus leaves on the other hand – even the microscopic leaves of Cactoideae – have parenchymatous epidermis cells, guard cells, spongy mesophyll, chlorenchyma, xylem and phloem. So the evolutionary conversion of cactus leaves into spines did not involve a mere reduction of the lamina and then further reduction of midrib and petiole, it instead involved the suppression of all leaf-cell type genes and activation of genes that control formation of fibers, the deposition and lignification of secondary walls, and then programmed cell death. These fiber morphogenesis genes are not activated in any cactus leaf (none at all has fibers), but they are activated of course in the development of wood. It would appear that after an axillary bud apical meristem initiates spine primordia, most leaf genes remain suppressed and instead wood fiber genes are activated. This does not involve all wood genes because vessels are never produced in the spines, just wood fibers. This would be a type of homeotic evolution.
| This hard woody trunk does not look like that of a cactus, but the spine clusters unmistakably indicate that this Pereskia pititache is a cactus.
Click on any photo for a larger image.
|The spines of this Ferocactus hamatacanthus provide very obvious protection.||All members of the subfamily Opuntioideae have glochids: numerous, very small spines that are deciduous. Opuntia polyacantha.|
|Plants of this Tephrocactus are always tiny, but they are fierce — their spines are 2 or 3 times longer than the rest of the body.||Not all cacti have spines. This peyote (Lophophora williamsii) has hundreds of trichomes in each axillary bud, but no spines. The same is true of Blossfeldia and a few other species.|
A puzzling aspect of spine development is the phyllotactic arrangement of spine primordia around the axillary bud apical meristem. To be truly leaf-like, spine primordia should occur at the points of intersection of two sets of Fibonacci spirals centered on the axillary bud apical meristem just as ordinary leaves occur at the intersection points of spirals centered on the shoot apical meristem. But many spines occur in two rows, or all spine primordia occur on one side of the apical meristem but not on the other two or three sides. This was studied by Norman Boke who concluded that spine primordia do indeed arise in normal phyllotaxy, it is just that parts of the axillary bud are so crowded that the spine primordia in those areas are suppressed. Perhaps so, but further study is warranted. Boke also noticed that very often spine primordia that were the last to be initiated were the first to develop: primordium initiation was centripetal, maturation was centrifugal. This does not happen with leaves: the first leaf primordia to be initiated are the first to develop.
| Notice that the spine clusters occur in two sets of intersecting spirals — phyllotactic spirals. Because spines develop from primordia produced by axillary bud apical meristems, we would expect that each spine in each cluster would also be located at intersections of phyllotactic spirals. Parodia aurespina.
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|The spines of this Ferocactus occur in two sizes and they do not appear to be located in phyllotactic spirals.|
Spines that act as extra-floral nectaries in cacti
Other aspects of spine biology are complex and in need of study. In several genera, some spines in each axillary bud develop as glands, known as extrafloral nectaries. They secrete a sugar solution that attract ants. These spines consist of loosely arranged parenchyma cells that secrete into intercellular spaces, and the accumulating nectar is then forced upward and out through small holes in the epidermis. Such spines are short and broad but still have a spine-like organization with the secretory cells looking like short, broad, thin-walled fibers.
| This is an extremely unusual plant of Ferocactus peninsulae, discovered by Dr. Jon Rebman. Typically, some spines in each areole develop as glands in this species, but this plant has an unusually large number.
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|The axillary buds of Coryphantha clavata are unusual — the bud’s apical meristem becomes extremely elongate, and the portion at the tip of the tubercle forms spines, the portion in the middle of the tubercle forms glands (modified spines), and the basal portion produces flowers (not shown here).|
Mauseth, J. D. 1982. Development and ultrastructure of extrafloral nectaries in Ancistrocactus scheeri (Cactaceae). Botanical Gazette 143: 273 – 277.
Spines that provide shade in addition to (or instead of) protection
Many cacti are protected from full sunlight by a dense covering of spines. Rather surprisingly, perhaps as many as half of all cactus species are adapted to dark rainforests (Epiphyllum, Rhipsalis) or semi-shady shrublands/grasslands (Echinopsis, Mammillaria, Notocactus) or cool, wet, cloudy highlands (Austrocylindropuntia, Oroya, Soehrensia) (Habitats are described in Cactus Odyssey). Such plants would be killed quickly by exposure to full sunlight in Phoenix, Arizona. Species in such cool or semi-shady habitats often have either just a few spines or only very short ones. However, cacti from intensely sunny, hot deserts often must have a complete covering of spines. In many cases, the spines are so strong and painful that they obviously offer both protection and shade, but in many species the spines are so soft that a hungry, thirsty animal would chew through them with no trouble. The benefit of such spines definitely appears to be blocking sunlight and thus preventing the plant from over heating, the chlorophyll from being bleached and the plant’s DNA from being damaged. For example, in Mammillaria plumosa, spine epidermis cells project outward as long trichomes, giving the spine a feathery appearance. In other species, the spines are flat, thin and papery, being too flexible to deter animals, but broad enough to shade the plant (as well as to camouflage the cacti among the grasses with which it grows).
| Austrocylindropuntia tephrocactoides grows at high altitudes in the Peruvian Andes. Intense sunlight and heat are rarely problems.
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|Matucana aurantiaca grows in such humid, foggy areas that keeping ahead of mosses is a challenge.||Oroya peruviana (yellow) growing with Austrocylindropuntia tephrocactoides at high, cool altitude.|
|The body of Epithelantha bokei is heavily shaded by its spines, but the spines are so soft they would not deter an animal from eating the cactus.||Epidermis cells on spines of Mammillaria plumosa grow out as trichomes, shading the plant.||Epidermis cells on spines of Mammillaria theresae also grow out as trichomes, seen here with backlighting.|
Mauseth, J. D. 1976. Cytokinin‑ and gibberellic acid‑induced effects on the structure and metabolism of shoot apical meristems in Opuntia polyacantha (Cactaceae). American Journal of Botany 63: 1295 – 1301.
Mauseth, J. D. 1977. Cytokinin‑ and gibberellic acid‑induced effects on the determination and morphogenesis of leaf primordia in Opuntia polyacantha (Cactaceae). American Journal of Botany 64: 337 – 346.
Part 2. The Cactus and Succulent Journal (USA) 74: 127 – 133.
Part 3. The Cactus and Succulent Journal (USA) 74: 212 – 215.