Palomar Alluaudia

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Alluaudia procera in Full Bloom at Palomar College Didiereaceae: An Unusual Family From South Africa & Madagascar

Alluaudia procera is a striking plant from Madagascar near the Performing Arts Complex on the main campus. It is drought deciduous like our local ocotillo (family Fouquieriaceae), shedding its leaves when water is scarce. Like ocotillo, the stems are covered with dense spines (technically thorns in Alluaudia); however, Alluaurdia belongs to a very different family, the Didiereaceae. Even more remarkable, the African succulent Portulacaria afra, often misidentified as "jade plant" also belongs to this family.

Alluaudia procera: A remarkable drought deciduous tree from Madagascar.

Alluaudia procera is a striking, spiny plant native to southern Madagascar. It is sometimes called Madagascar ocotillo because of its resemblance to our local ocotillo (Fouquiera splendens) native to the Anza Borrego desert region. Looks can be deceiving when it comes to botanical nomenclature. Alluaudia belongs to a completely different plant family, the Didiereaceae, while ocotillo belongs to the Fouquieriaceae, along with the Baja California landmark "Boojam Tree" (Fouquiera columnaris), formerly placed in the genus Idria.

Distantly related plants from similar climates on our planet, such as arid deserts, often resemble each other with similar adaptations. This is referred to as convergent evolution. The classic example in botany textbooks compares spiny, succulent euphorbias of Africa (Euphorbiaceae) with cactus of North America (Cactaceae). I prefer the term "homoplasy" because it includes convergent & parallel evolution, 2 terms that are often difficult to distinguish, especially in computer generated phylogenetic trees and cladograms.

Phylogenetic trees are based on shared, derived characters that are entered into a computer. In other words, the phylogenetic trees reflect shared traits derived from common ancestry. Homoplastic characters can introduce errors into phylogenetic studies because it is possible for such characters to provide false grouping information. Like bioluminescence in a variety of unrelated animals, spiny stems that store water also occur in unrelated plants. Spiny stems with drought deciduous leaves do not indicate a close kinship between Alluaudia and ocotillo. The close grouping between Alluaudia and "elephant plant" (Portulacaria afra) is based on flower structure and DNA sequencing. In fact, the DNA revolution has had a tremendous impact on plant systematics and plant taxonomists.

Modified Leaves (Spines) vs. Modified Stems (Thorns)

According to botanist Sean Lahmeyer at Huntington Botanical Gardens (HBG), most plants have spines to protect themselves from hungry animals. The development of true spines and spine-like structures, like thorns and prickles, is a recurring adaptation seen in plants from around the world. Spines are derived from leaf tissue and thorns from stem tissue. Prickles come from neither; they are simply corky projections from a plant’s skin, or dermal tissue. The spines of ocotillo are clearly modified leaves because they are derived from petioles of primary leaves on new growth. Drought deciduous leaves are produced in the axils of spines and are referred to as secondary leaves. They appear when adequate soil moisture is available. Depending on rainfall, there can be several crops of secondary leaves in one year.

Although the "spiny' stems of Alluaudia superficially resemble ocotillo, the anatomy of its "spines" is quite different. They are dervived from stem tissue and are botanically classified as thorns rather than spines. Unlike the woody thorns of many plants, Alluaudia thorns have a strong, corky shield made of dermal tissue that can be pried off to reveal a soft green thorn underneath (Sean Lahmeyer, HBG website). The following image shows each pair of Alluaudia leaves subtending a thorn.

Left: Stem thorns of Alluaudia procera. Right: Branch of elephant plant (Portulacaria afra). Both of these species are members of the family Didiereaceae. Their close kinship is based on flower structure and DNA base sequences.

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