For over a century biologists have commented on the heterophyllous nature of certain species in the Tillandsiodeae. Juvenile leaves of Tillandsia deppeana Steud. are similar morphologically to those of adult atmospheric tillandsioid epiphytes, while adult leaves resemble leaves of adult tank-forming tillandsiod epiphytes. Although both leaf types share a common photosynthetic pathway (C3), juvenile leaves maintain positive rates of net CO2 exchange during a period of desiccation long after CO2 exchange has ceased in adult leaves. Because the juveniles of T. deppeana are more drought-resistant, this life-form was probably a prerequisite to the movement, in an evolutionary sense, of the Tillandsiodeae into the epiphytic habitat. Conjecture regarding the derivation of modern tillandsioid epiphytes from xeromorphic or mesomorphic forms must await similar comparative investigations of juveniles and adults of other species in this group.
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