Vegetative Reduction in Epiphytic Bromeliaceae and Orchidaceae: Its Origin and Significance

Abstract

Assays of the roots and leaves of nine leafy and three shootless orchids indicated that the roots of the latter, as well as those of leafy Kingidium taeniale, engaged in CAM metabolism and possessed relatively high chlorophyll contents. All leaves were similar on both counts. Except for those of K. taeuiiale, the roots of the leafy orchids assayed were pale green and incapable of exhibiting a net carbon gain during day or night runs. A labeling experiment with leafy Encyclia tampensis indicated that its green roots did fix small quantities of carbon dioxide and that some of the product was translocated to the shoot. This species, like epiphytic Tillandsia circinnata of Bromeliaceae, used proportionally large amounts of several of its mineral elements to support high fecundity. Given these and other findings discussed in this report, we propose that the reduced shootless' state in epiphytic Orchidaceae and the rootless condition in Tillandsia have evolved primarily to enhance nutrient economy. This economy is achieved by reducing the cost of the vegetative body, thus freeing material resources for the purpose of supporting a high regenerative capacity in the form of a strong sexual effort. Great nutrient economy and substantial fecundity are mandated by the combination of high abiotic stress (particularly scarcities of N, P, and K), habitat patchiness, and disturbance characteristic of the driest portions of the epiphytic biotope. THE FAMILY ORCHJDACEAE is unsurpassed for ecological diversity, tolerance for various kinds of environmental stress and numerous deviations from conventional patterns of flowering plant form and function. Taken as a group, the orchids range across a broad ecological spectrum: thousands of species endure severe drought as extreme epiphytes; many are adapted to wet soils, and Habezaria repeins can grow as a floating aquatic; as bog dwellers, they tolerate very low pH; as calceophiles, alkaline substrata. Although predominantly tropical, a few cryptophytic forms inhabit arctic tundra and alpine meadows. Unusual and specialized nutrition is also a hallmark of the orchids. Their small seeds, some of unparalleled reduction, often yield, upon germination, mycorrhizal seedlings with various physiological peculiarities (Burgeff 1961). Adults extend this precedent for trophic versatility and, depending on the species, run the gamut from complete independence of organic substrates to various degrees of sapro