Anatomical Adaptations to Xeric Conditions in Maihuenia (Cactaceae), a Relictual, Leaf-Bearing Cactus
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and Pereskia, constitute Pereskioideae, the subfamily of Cactaceae with the greatest number of relictual features, but the two genera differ strongly in habit and ecological adaptations. Plants of Maihuenia occur in extremely xeric regions of Patagonia and are small cushion plants with reduced, terete leaves and soft, slightly succulent trunks. Plants of Pereskia occur only in mesic or slightly arid regions and are leafy trees with hard, woody trunks and thin, broad leaves. Maihuenias have many more anatomical adaptations to arid conditions than do pereskias: maihuenias lack sclerenchyma in their phloem and cortex (M. poeppigii also lacks xylem sclerenchyma and can contract during drought); their wood consists of vessels, axial parenchyma, and wide-band tracheids and can store water as well as minimize embolism damage; one species channelizes water flow by producing intraxylary bark; and at least some stem-based photosynthesis occurs because maihuenias have small patches of persistent stem epidermis that bears stomata and overlies a small amount of aerenchymatous chlorenchyma. Pereskias lack all these features. Although closely related, maihuenias have fewer relictual features than do pereskias, and plants of Pereskia probably are more similar to the ancestral cacti.
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