glochid [ GLOH-kid ] or glochidium [ gloh-KID-ee-uhm ] noun, plural glochids or glochidia: small barbed hair (modified spine) found only on members (like prickly-pears and chollas) of the cactus subfamily Opuntioideae
spine [ spahyn ] noun: a modified, sharp, pointed leaf
Glochids or glochidia, which grow in tufts, are prominently visible (and felt) by gardeners weeding or otherwise working around prickly-pear cactus, a plant native to large swaths of the United States, including our mid-Atlantic region. The fine hairs or sharp spines penetrate most gloves and many shoes, and stay there, or in the gardener’s skin, for an irritatingly long time, anchored by the fish-hook-like barbs at the ends. Glochidia imbedded in fingers or gloves can transfer upon contact, so do not touch eyes and face. Learn How to remove cactus spines (including ones stuck in your throat); yes, this can happen, if the edible cacti are not properly prepared. Internet cooking sites provide directions and recipes for using these nutritious plants as food.
The glochidia and spines grow from the plant’s flattened, water-storing stems, commonly known as pads (botanical term: cladodes), which are plump in ideal conditions, but shrink and wrinkle in drought or cold. The cladodes of some Opuntia, though somewhat shrunken, survive cold winters down to 20 below (Fahrenheit) or lower, thanks to their content of accumulated sugars and sugary alcohols that act as antifreeze.
Left to right: Opuntia gosseliniana, O. humifusa (flowering and fruiting), O. microdasys, O. polyacantha (en masse and single pad), O. santa-rita.
Although all cacti produce leaves, most are microscopic, so the enlarged stems serve as the green source of chloroplasts, allowing the stems to take on the task of photosynthesis, more commonly performed by leaves. In Opuntia, awl-shaped leaves are conspicuous on new pads (shoots), but they are promptly deciduous and drop off after several weeks.
“Most cactus morphologists have concluded that cactus spines [and glochidia] are either modified leaves or modified bud scales” (Mauseth, accessed 2022). Spines form in the areole (axillary bud) only on the side next to the leaf. Opuntia may or may not have spines–for example, some plants of O. humifusa (eastern prickly-pear) are spineless–but they always have high numbers of glochidia (short, thin, modified spines) that grow in the areole. A developing spine has three regions: at its base, actively dividing cells; an intermediate area where cells elongate, differentiate, and mature; and a terminal zone of dead, woody fibers (Mauseth, 2006). In a fully grown cactus spine, all the cells have turned into fibers and no living cells remain. After producing spines, areoles may produce shoots or flowers.
Besides protecting a cactus, spines may shade and insulate it as well as diffuse light and capture moisture. Spines also occur on the leaves and bracts of other succulent, herbaceous, and woody plants. In Opuntia, the glochidia serve as a dispersal mechanism. When glochidia catch on a passer-by, a new pad of the prickly pear cactus can break off. It hitch-hikes to a new place where it can set down roots and propagate.
Left to right: Spines on Cylindropuntia bigelovii, Echinocereus, Ferocactus cylindraceus tortulospinus, Agave parryi, Ilex opaca, Cirsium discolor.
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References
Baggaley K. 2022. How to remove cactus spines (including ones stuck in your throat). Popular Science.
Downie S. Integrative Biology 335: Systematics of Plants. Families of the Caryophyllales:
Cactaceae. University of Illinois.
Mauseth JD. Cactus Spines. Cactus adaptations to dry environments. The University of Texas. (accessed July 18, 2022)
Mauseth JD. 2006. Structure-function relationships in highly modified shoots of cactaceae. Ann Bot. 98(5): 901-26. doi: 10.1093/aob/mcl133. Epub 2006 Jul 4. PMID: 16820405; PMCID: PMC2803597.
Taylor D. Eastern Prickly Pear (Opuntia humifusa (Raf.) Raf.). Plant of the Week. U.S. Forest Service.
Vogelpohl S. 2018. Know Your Natives – Eastern Prickly Pear. Arkansas Native Plant Society.
Weakley AS, Ludwig JC, Townsend JF. 2012. Flora of Virginia. Botanical Research Institute of Texas. page 461.