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Analogy: Desert-dwellers

Both of these two plants have thick, water-filled branches and sharp spines. You might guess that they are closely related — but they are not. In fact, one "cactus" is more closely related to a common weed, and the other is more closely related to a carnation. Their similarities are analogies — independently evolved adaptations that aid survival in the desert: succulent, waxy stems help store water, and spines provide shade and protect the plant from herbivores.

Hoodia succulent Cholla cactus

Hoodia and cholla with related plants
Hoodia/cholla phylogeny

Hoodia photo courtesy of Friederike Hübner & Ulrich Tränkle, Germany, www.asclepidarium.de; Cholla photo © Brother Alfred Brousseau, Saint Mary's College; Milkweed photo © 2003 Christopher L. Christie; Stephanotis photo courtesy of Ashland High School Horticulture Department, Ashland, Ohio; Carnation photo © Joseph Dougherty, Society for Environmental Education; Prickly Pear Cactus photo by Glenn and Martha Vargas © California Academy of Sciences.

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