Randomly Selected Botanical Terms: Glochids

The spines of a cactus are an obvious threat. They are generally sharp, smooth, and stiff; as soon as you are stabbed by one, it is immediately clear that you’ve gotten too close. Sitting at the base of the spines – or in place of spines – on many species of cacti is a less obvious, but significantly more heinous threat. Unless you’re looking closely, this hazard is practically invisible, and the pain and irritation that can come as a result of close contact has the potential to last significantly longer than the sharp poke of a spine. This nefarious plant part is called a glochid, and if you’ve ever made contact with one (or more likely several dozen of them), it’s not something you will soon forget.

Opuntia polyacantha x utahensis

The spine of a cactus is actually a leaf. The area from which a spine emerges from the fleshy, photosynthetic stem of a cactus is called an areole, which is equivalent to a node or bud on a more typical stem or branch from which leaves emerge. In place of typical looking leaves, a cactus produces spines and glochids. Like spines, glochids are also modified leaves, although they appear more like soft, little tufts of hair. However, this unassuming little tuft is not to be trifled with.

Close inspection of a glochid (with the help of a microscope) reveals why you don’t want them anywhere near your skin. While the surface of a cactus spine is often smooth and free of barbs, glochids are covered in backwards-facing barbs. The miniscule size of glochids combined with their pliable nature and retrose barbs, make it easy for them to work their way into your skin and stay there. Unlike spines, glochids easily detach from a cactus stem. Barely brushing up against a glochid-bearing cactus can result in getting stuck with several of them.

Opuntia basilaris var. heilii

Because glochids can be so fine and difficult to see, you may not even be aware they are there. You probably won’t even feel them at first. Removing them is a challenge thanks to their barbs, and since you may not be able to remove them all, the glochids that remain in your skin can continue to cause irritation for days, weeks, or even months after contact. For this reason, cactuses are generally best seen and not touched, or at the very least, handled with extreme care.

Apart from being a good form of defense, the glochids of some cactus species can serve an additional function. Most cactus species occur in arid or semi-arid climates, where access to water can be quite limited. In order to increase their chances of getting the water they need, some desert plants are able to collect water from the air. A few species of cactus do this, and glochids are a critical component in making this happen.

Cylindropuntia whipplei

A study published in the Journal of King Saud University – Science (2020) examined the dew harvesting ability of Opuntia stricta, commonly known as erect prickly pear. As described above, the spines of O. stricta are smooth, while the glochids are covered in retrose barbs. Both structures are waterproof due to hardened cell walls and cuticles. However, due in part to the conical shapes of both the glochids and their barbs, water droplets from the air are able to collect on the tips of the glochids. From there, the researchers observed the droplets in their travel towards the base of the glochids. As they moved downward, small droplets combined to form larger droplets.

At the base of the glochids are a series of trichomes, which are small hair-like outgrowths of the epidermis. The trichomes do not repel water, but rather are able to absorb the droplets as they reach the base of the glochids. For a plant species that receives very little water from the soil, being able to harvest dew from the air is critical for its survival, and this is thanks in part to those otherwise obnoxious glochids.

See Also: Prickles

Drought Tolerant Plants: Prickly Pears

In the introduction to this series about drought tolerant plants, I defended water efficient gardens by claiming they don’t have to be the “cacti-centric” gardens that many visualize upon hearing terms like “xeriscape,” “water-wise,” and “drought tolerant.” And this is absolutely true. However, that won’t stop me from suggesting that such landscapes include a cactus or two. Despite their menacing and potentially dangerous spines, they are actually quite beautiful, and a cactus in bloom is really a sight to behold. Together with a variety of grasses, herbaceous flowering plants, and shrubs, cactus can add unique forms, textures, and focal points that will enhance the look and function of a water-wise garden. This is why I recommend considering cactus, particularly (as far as this post is concerned) one of the many varieties of prickly pears.

The cactus family (Cactaceae) has a native range that is limited to the Americas. Within that range it is expansive, and cactus species can be found in diverse regions from Canada down to Patagonia. The genus Opuntia (the prickly pears) is the most widespread of any genus in the cactus family consisting of at least 300 species found throughout the Americas. Even a brief investigation into Opuntia will reveal that there is considerable controversy as to how many species there actually are and what to call them. This is partly due to the large ranges that species in this genus can have and the diverse habitats they can be found in within those ranges, resulting in a single species having many forms, varieties, and/or subspecies. Hybridization is also common in this genus where ranges overlap, augmenting the challenge of identification.

Generally, prickly pears have flattened stems with spines and glochids emerging from small bumps called areoles. Their flowers are large, showy and a shade of either yellow, orange, or pink and sometimes white. They form fruits that are either fleshy and juicy with a red or purple hue or hard, dry and a shade of brown or tan. The flattened stems are called pads or cladodes and can be quite large in some species, while diminutive and sometimes rounded in others. Some species are without spines, but all have glochids – tiny, barbed, hair-like structures found in clusters on the stems and fruits. While the spines can be painful when they penetrate skin, the glochids are far more irritating as they easily detach themselves from the plant and work their way into the skin of their victims. The fleshy fruits, called tunas, can be eaten after first taking care to remove the glochid-infested outer layer. The young stems of many species can also be eaten – they are referred to as nopales and are common in Mexican cuisine.

Flowers of Opuntia sp. with bee inside flower on the left

Flowers of Opuntia sp. with bee inside flower on the left

Again speaking generally, prickly pears are very easy to propagate and cultivate. Their two main preferences are full sun and well-drained soil. If you are worried that the soil you are planting them in is going to stay too wet for too long, amend it with some gravel. This is especially important if you live in a climate that receives lots of precipitation or that has cold, wet winters. Once established, prickly pears will move around the garden. If that becomes a problem, expanding plants are easily pruned and traveling plants are easily removed.

I live in a climate that requires the selection of cold hardy prickly pears, so I am taking my specific recommendations from two books: Cacti and Succulents for Cold Climates by Leo J. Chance and Hardy Succulents by Gwen Moore Kelaidis. If you live in a warmer climate, your options will be greater. Still, the options for cold regions are pretty numerous, so for the sake of space I am narrowing my list down to a handful that stand out to me at this particular moment.

Three eastern United States species of prickly pears (O. compressa, O. macrorhiza, and O. humifusa) are, according to Chance, “more capable of dealing with wet and cold conditions than almost any other members of the cactus family.” They still require well-drained soil though. An appealing trait is their large, juicy, red fruits that can add garden interest in late summer and fall. Opuntia engelmannii is another species with the potential to tolerate cold, wet conditions. Its size is appealing to me, with pads that reach a foot wide and plants that grow several feet tall. Chance advises finding “a clone that is known to be cold tolerant” and making some space for it, “as it becomes huge in time.” The most cold tolerant prickly pear may be Opuntia fragilis. It is a diminutive plant with a large native range and a variety of forms, some with rounded pads “shaped like marbles.”

Fruits ("tunas") of Opuntia engelmannii - photo credit: www.eol.org

Fruits (“tunas”) of Opuntia engelmannii – photo credit: www.eol.org

Opuntia fragilis 'Frankfurt' - photo credit: wikimedia commons

Opuntia fragilis ‘Frankfurt’ – photo credit: wikimedia commons

Opuntia polyacantha is a prickly pear native to my home state, Idaho. It is found at high elevations throughout the Intermountain West and is also found on the Great Plains. It has many forms and varieties, and its flowers are various shades of pink or yellow. It is a fast growing species and spreads around easily. Other cold hardy species include Opuntia macrocentra (which has a very attractive yellow flower with a red-orange center), Opuntia erinacea (commonly known as hedgehog prickly pear for its abundant, long spines that can obscure the pads), and Opuntia microdisca (a tiny Argentinian prickly pear with pads that barely reach an inch across but, as Chance says, “works very well in a dry rock garden with other miniatures”).

Pads of Opuntia polyacantha

Pads and spines of Opuntia polyacantha

A post about Opuntia could go on indefinitely due to the sheer number of species and their diverse forms and attributes. This is meant merely to pique your interest. The flowers, if nothing else, should certainly interest you. In her book, Kelaidis calls them “improbably beautiful,” and goes on to say that they are “often papery, always glistening and showy.” Chance likens them to “any fancy rose” because they are “extraordinarily large, brightly colored, [and] eye catching.” Next week, as part of Awkward Botany’s Year of Pollination, I will present another reason to be fascinated with the flowers of Opuntia. For now, I will leave you to ponder this word, “thigmonasty.”

Want to learn more about prickly pears? Check out Opuntia Web.