For a small subset of epiphytes in deep evergreen forests, an optical phenomenon purportedly promotes shade tolerance. Some aroids and gesner-iads, as well as the bromeliads Aechmea fulgens, Nidularium bruchellii (Fig. 4.2 5G), and several species of Vriesea and Tillandsia, possess abaxial mirrors which are thought to reflect unabsorbed photons back into overlying chlorenchyma (Lee, Lowry, and Stone 1979). Similar cyanic epidermal layers are fairly common among low-growing herbs in the same forests. Maroon to red backing is most appropriate, given the long-wavelength enrichment as well as overall depletion of light passing through dense overhead foliage. Not surprisingly, the bicolored leaves of such plants usually grow horizontally with minimal overlap (Fig. 2.17)- more about this later. Essentially monolayered Nidularium bruchellii exhibits a faint bluish-green irridescence reminiscent of the thin-film optical interference phenomenon reputedly responsible for greater relative absorption of red light in deep-shade-adapted Selaginella willdenovii (Lee and Lowry 1975). Cultivated in better exposure, both species exhibit much less irridescence, and electron
Leaf variegation and shoot architecture
microscopy shows the adaxial cuticle of S. willdenovii to lack the lamination characteristic of shade-grown material.
Shade tolerance could be enhanced in some aroids by velvety leaf surfaces (e.g., Anthurium pallidiflorum) which feature domed, adaxial, epidermal cells thought to gather shade-light. On a larger scale, prominent, lenslike, adaxial, hypodermal tissues in certain Peperomia and gesneriad leaves (Fig. 3.1E) could do likewise and store water as well. Leaves of various Peperomia species range from typical thin bifacial types to the succulent subunifacial forms (Kaul 1977). Light piping through a clear, internal, multilayered hypodermis may preserve photosynthetic capacity in P. dolabriformis while water loss is reduced. Growing conditions affect hypodermal development, thereby substantially altering leaf anatomy. Leaf thickness in Codonanthe paula and Peperomia camptotricha varies several fold, depending on the season. Shifts in gross shoot form that affect photon interception can be pronounced in Bromeliaceae (Figs. 2.11, 2.12). Additional study is needed to confirm the beneficial effects of these properties on photosynthesis. Consideration of other leaf parameters could be equally rewarding.
Co-occurring taxa often possess leaves with differing forms and life histories, surely a signal that there are alternate solutions to common problems of carbon balance. For instance, secondary hemiepiphyhtic Peperomia tro-paeolifolia (Fig. 2.8) and juvenile Marcgravia coriacea climbing side by side in wet Ecuadoran forest possess foliage that is thick and deep green in the former but pale and delicate in the latter. Longevity also differs. Robust Peperomia tropaeolifolia leaves are more durable, a pattern consistent with a life spent no more than 2 m above the dark forest floor. Mature Marcgravia coriacea eventually grows into tree crowns where its broader, thicker leaves presumably outperform its less productive, older, shade-adapted foliage below. Use of a portable gas exchange system could reveal much more about how epiphytes with dissimilar foliage share the same type of microsite.
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