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St. Louis, Missouri
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Each monthly issue of the Henry Shaw Cactus Digest includes club updates, columns and articles by members on their favorite aspects of cactus and succulent culture. Follow the link below this item to read select Digest articles -- or join HSCS to receive every article in the print version of the Digest.
By Joe Merkelbach
Everyone knows that cactus plants have sharp spines. But actually, not all species do, but spines are a true and defining scientific characteristic for the family. Less well recognized is why cacti have spines and how they benefit the plants.
To understand the why of spines, a good approach is to go back to the most primitive members of Cactacae and see what use they made of spines. Since soft, succulent plant tissue does not harden and fossilize, we do not have a fossil record of steps in the evolution of cacti.
The Pereskia genus, generally considered to be primitive due to the plants' retention of leaves and non-succulent stems, does have cactus-defining spines that arise from areoles. The spines were thought to protect the plants from browsing by animals, and this is how they were first beneficial and retained in the genetic line. More recent research has indicated that spines are modified forms of leaves, which also arise from areoles. Microscopic examination has shown venation in some spines.
As time has lengthened in the development of cacti, however, spines have become beneficial in other ways. In hot, dry environments, succulence is all about retaining moisture against the effects of the weather conditions. Coatings of spines provide a jacket against the drying effects of wind and help to reflect away the potential excessive heating effects of sunlight.
In extremely dry situations, such as the seaside deserts of Chile and Peru, available moisture is blown onshore as fog rather than rain. The pointed ends of spines serve as condensation points to pull water vapor from the air, and spined cacti are among the most successful plant forms in this area. The fuzzy, hairy forms of spination act as blankets, protecting stems from cold and frost exposure by retaining warmer microclimates next to the plant skin.
Spines are also mechanical aides to plants. In some clambering forms that sprawl and cling to other plants or rocks, they help the plants hold their places. The hooks on the ends of spines also help plants spread clones of themselves over a landscape. They attach to animals' coats and spread as they drop away and establish themselves in other appropriate habitat.
Spines, although they sometimes seem very formidable, are not really designed to inflict pain. Instead, the novel adaptations of spines in response to environmental challenges are worthy of a deeper sort of respect.
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