Grow a Spine or Two - in Defense of Cacti

Cholla and saguaro cacti in the Superstition Mountains
Cholla and saguaro cacti dominate an Arizona landscape

The iconic saguaro cacti (Carnegiea gigantea)1 of the Sonoran desert2 symbolize the essence of “cactus” for many people. With their giant arms outstretched, saguaros have provided an unmistakable ambiance for many Western movies and TV shows.3 However, cacti are more than charismatic desert superstars and have fascinating biology. Some cacti even call rainforests their homes.

What's a cactus?

Cacti are flowering succulents4 that store water primarily in their stems. With a few “primitive” exceptions, cacti have no leaves. Let me rephrase that: cacti lack “true” leaves – their leaves having evolved into cactus spines. The spines can be variable in shape, size, and function: some are long and sharp while others are soft and feathery, like the outer spines of the feather cactus, Mammillaria plumosa,5 that shield against excess sunlight.

Cactus diversity ranges from the half-inch Blossfeldia liliputiana of the Argentine Andes mountains to the massive cardon (Pachycereus pringlei) of northwestern Mexico to the nearly spineless Christmas cactus (Schlumbergera) native to cool Brazilian rainforests. The majority of cacti are from the Americas.

Lacking leaves for photosynthesis, a cactus’ entire outer surface or skin has taken over this ability, giving cacti their green color.

Prickly pear cactus
A prickly pear cactus basks in the sun

Monumental cactus

Saguaro cacti dominate the landscape in the Superstition Mountains
Saguaros marching like a herd of triffids6

Until you see a saguaro cactus in person, it is difficult to appreciate its impressiveness. Standing up to a record 78 feet tall and too wide to wrap your arms around,7 it can hold over 5 tons of water and imbibes as much as 200 gallons of rainstorm water by expanding its water-filled tissue into its pleated sides. However, Hollywood cowboys notwithstanding, most cactus water can’t be consumed since it contains bitter chemicals that render it undrinkable, so bring water when you’re in the desert. The seasonal fruits of the saguaro, prickly pear, hedgehog, and barrel cacti, to name a few, are edible and have long been a staple in Mexican and Central American diets, but beware of the spines!

Typically, saguaros grow below 4,000 feet of elevation because of their low tolerance for freezing temperatures, although some may be found higher on south-facing slopes warmed by  sun exposure.

A giant, 78-foot-tall sack of water (a fully hydrated saguaro can be as much as 90% water, rivaling a watermelon) needs support to prevent it from collapsing under its own weight. The saguaro and many other cacti have woody structures that provide this reinforcement. Yet, unlike trees, cactus wood is arranged in ribs or cylinders embedded in the watery cactus flesh rather than in a single woody trunk.

Exposed ribs in a saguaro wound
Exposed ribs in a saguaro wound
Saguaro skeleton
Standing saguaro skeleton
Cylindrical cholla skeleton
Cylindrical cholla skeleton

You should never reveal your true age. - Eva Arnold

A saguaro can live on average for 150-175 years with some estimated to be over 200 years old. Depending upon the growing conditions branches may start appearing after 50-100 years. The animation to the left (you may need to press PLAY to start it) shows saguaro branch growth over 16 years from 2006 to 2022. Presumably, this cactus is around 70-120 years old now.

The photographs used to create this animation were generously provided by Doris Evans of the Tucson Audubon Society

A tree’s age can be determined by counting the annual growth rings in its trunk. Aging a cactus is more complicated because its ribs don’t have annual growth rings. The image below compares growth rings from a peach tree to a cross-section of a saguaro cactus rib. Age estimates can be made based on established growth-size relationships; however, there can be considerable variation depending on individual growing conditions.

Cross section of tree banch compared to saguaro rib
Cross section of a tree branch(top) and saguaro rib (bottom)

Hope is not lost for those who want to know the age of a cactus with more certainty. Spines are retained throughout the life of a saguaro8 with the oldest spines at the base and the youngest at the top of the cactus. The difference in carbon 14 (14C ) dating of the oldest and youngest spines reveals the age (unfortunately, 14C dating isn’t readily available to the average person – I did say this was more complicated!). 14C dating can only be performed on a living saguaro since the spines are shed in a jumble once the cactus dies and collapses.

A fascinating wrinkle in cactus aging is the observation of transverse bands across the spines of fishhook barrel cacti (and young saguaro cacti). Like our fingernails, cactus spines grow from their bases – most of the spine is non-living tissue. As the spine grows, it produces one band each day, so the series of a spine’s bands reflects daily growth during each spine’s growing season. This way, yearly growth measurements from the top to the bottom of the cactus, and daily measurements within each spine, are possible.

Fishhook barrel cactus showing diurnal growth bands
Fishook barrel cacti and a closeup of its spines' daily transverse bands.

Porcupine of the desert

While hiking in Arizona with my son Forest and his dog Ajax recently, we were all “jumped” by cholla cacti.9 Despite our curses, the cacti weren’t acting malevolently. The cholla’s loosely-attached, spine-covered stems hitch rides on any creature that casually brushes against them.
Cholla cactus stems and victims
Detached cholla cactus stems and the aftermath of cholla "attacks"

“Keep your friends and your enemies close” ~ cholla cactus motto

Most cacti’s spines are defensive and serve to protect against herbivores or other cactus-damaging animals. Conversely, chollas’ spines foster intimacy – the stems cling and are difficult to remove. In an intriguing example of convergent evolution, the spines share a striking similarity to the quills of the North American porcupine.10 In both cases, the backward-pointing scales make removing the spine/quill difficult. The cholla succeeds by fastening the broken-off stem to a convenient animal.
Microscopic comparison of cholla cactus spine and porcupine quill
Microscopic comparison of the scales of a cholla spine and a porcupine quill

Once attached to an animal chauffeur, the cholla stems can be spread widely. However, unlike seed-containing plant burrs that attach to fur or clothing, these hitchhiking cholla stems reproduce vegetatively, like propagating with a plant cutting. They are clones of the parent plant and don’t require pollinators or seeds.

In fact, although cholla species can have lovely flowers, many of their seeds are sterile. The detached stem provides spines to defend against roaming herbivores and an ample water supply for the new cactus.

The birds and the bees… and the bats and the moths

Many cacti reproduce by seeds rather than vegetatively like cholla. Viable seed production requires pollination, which in the case of cacti is usually performed by an animal. Hummingbirds are common pollinators during the day – the red, tubular flowers of Christmas cacti are favorites. Some cactus flowers bloom only at night and are frequented by moths and bats, with the latter accounting for the pollination of 25% of cacti. Finally, at least one cactus species, the Mexican organ pipe cactus (Marginatocereus marginatus), covers all its bases and uses nocturnal (bats) and daytime (hummingbirds) pollinators.

“I remember very clearly someone saying, 'Don't shake hands with the cactus,' and I thought, 'Well, why not? What could possibly go wrong?' Shaking hands is a friendly gesture.” ~ Benedict Cumberbatch

Cacti seem well suited for survival in the face of adversity – they are armed with spines, carry an ample water supply, and have a variety of reproduction strategies. Yet, many cacti, including the charismatic saguaro, face uncertain futures due to habitat loss accelerated by human encroachment, harvesting of shade-providing nurse trees, poaching of cacti, and drier, hotter local environments produced by climate change. So, if you have the opportunity, get out and shake hands – metaphorically – with a cactus.

NOTES

1  The plural of cactus can be cacti, cactusses or cactus. I hope that the purist who demands that cacti be used also uses musea to refer to the plural of museum. As in: “When we were in New York we visited many art musea.”

2 The Sonoran desert is located in southwestern Arizona and northwestern Mexico.

3 The ambiance is often misplaced, however, since the movie setting may be far from where saguaro cacti grow. See El Dorado movie mistakes

4 Not all succulents are cacti and many have specialized, thick leaves for the storage of water.

5 Don’t be fooled by the feather cactus’ soft spines – they hide sharp ones underneath.

6 Saguaros don’t really march and only superficially resemble the triffids of horror film fame: The Day of the Triffids

7 I advise against embracing even small spiny cacti.

8 This is true in many other cactus species as well.

9 Ajax is the the smallest of the three of us by far, however he responded the most bravely to the cactus assault.

10 Cholla cacti don’t actually jump or throw themselves. This is a false myth like that of quill-throwing porcupines. See my Nature Unveiled blog about porcupines: To Quill or not to Quill

References

Thanks

Special thanks to Zoe Weil’s editorial expertise

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Mary Harmon
Mary Harmon
1 year ago

Your beautiful photography and brief, humor infused, well researched commentary is a welcomed addition to my email in box. Thank you for your curiosity!

Zoe Weil
Zoe Weil
1 year ago

Once again, convergent evolution! With a plant and an animal no less. Nature is amazing. Thanks for another fascinating, illuminating post.

Barbara Acosta
Barbara Acosta
1 year ago

The notes are almost as good as the rest of the article!

Christine Rivard
Christine Rivard
1 year ago

Loved this article. Very informative. Thank you.

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