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Back Plant of Melocactus intortus, growing on limestone in Puerto Rico. While it is young, it produces a green, photosynthetic body protected by large spines so sparse they do not shade the photosynthetic tissues. Once old enough to reproduce, it undergoes its phase change and continues to grow as an adult (the adult body is the cephalium) with narrower pith and cortex, no chlorophyll and covered with a dense layer of short, red spines and zillions of white trichomes. A single shoot apical meristem produces both bodies. A cephalium this long must be the product of many years of growth as an adult.
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