Roots of Sobralia macrantha (Orchidaceae): structure and function of the velamen-exodermis complex.

Roots of Sobralia macrantha, a largely terrestrial orchid, bear a typical multilayered velamen underlaid by a cortical exodermis. The latter consists of densely cytoplasmic passage cells regularly interspersed among much larger, elongate, vacuolate cells. A structure which may be peculiar to Sobralia, or at least unusually well developed there-the fibrous body-consists of numerous intermeshed and divided wall outgrowths arising from two or three velamen cells positioned directly over the passage cell. This body is well designed to retard root transpiration. Should the fibrillar components be appropriately hygroscopic, it may act as a valve, promoting movements of water into the cortex while the velamen is engorged. The general mode of moisture and mineral procurement proposed for the Sobralia macrantha root probably applies to many other soiland canopy-dwelling orchids with a velamen-exodermis complex. ORCHIDACEAE is especially noteworthy for its capacity to exploit diverse, physically stressful, habitats. A large number of tropical forms prefer droughty, infertile sites, growing there as terrestrials and epiphytes (Benzing, 1973, 1978; Benzing and Ott, 1981). A very conspicuous feature of these taxa is a root equipped with a multilayered epidermis, the velamen, which is underlaid by a distinct cortical exodermis. Aspects of the structure and function of a variety of orchid roots have been described in some detail, yet controversy continues over how these organs operate to procure minerals and moisture and avoid desiccation. In the following report, we describe the root of Sobralia macrantha and suggest how it functions under the constraints imposed by epiphytism, saxicoly, or life in seasonally dry soil. MATERIALS AND METHODs-Roots were obtained from greenhouse-grown plants potted in bark chips or Osmunda fiber. Freshly cut material was immediately placed in 0.1 M cacodylate buffer containing 3% glutaraldehyde (pH 6.9) for 3 hr at 2 C. Samples were subsequently washed in four changes of the same buffer and then post-fixed for 3 hr in 2% OSO4 buffered as above. After a thorough washing in water, specimens were dehydrated at 2-3 1 Received for publication 9 January 1981; revision accepted 20 March 1981. This work was supported by a grant from NSF to the first author. We wish also to acknowledge the receipt of plant material and helpful advice from D. W. Stoutamire. 2 Present address: Department of Biology, University of Akron, Akron, OH 44325. C overnight (10-12 hr) in an acetone desiccator, allowing a slow exchange of water by acetone. Complete dehydration was assured by several additional changes of 100% acetone. Specimens destined for scanning electron microscopy (SEM) were critical point dried with liquid CO2 and sputter coated with gold. Mounted specimens were examined using a JEOL JSM-U3 scanning electron microscope at an accelerating voltage of 25 kv. Samples were prepared for light microscopy and transmission electron microscopy (TEM) by embedding in Epon. A mixture of 90% acetone and 10% Epon was added to vials containing root sections; the vials were then covered with perforated foil and maintained in a fume hood for 12 hr. Several changes of 100% plastic followed. Polymerization was induced at 60 C for 36 hr. Alternate thick and thin sections were cut for light microscopy and TEM respectively. Thick sections were stained with 1% toluidine blue-I% borax. Thin sections were secured on unsupported copper grids and stained for 5 min in 1% methanolic uranyl acetate followed by 10 min in Reynold's lead citrate. Grids were examined with a JEOL JEM120U TEM at an accelerating voltage of 100 kv. Figure 1 indicates where photographs of fine structure were taken. OBSERVATIONS AND RESULTS-General habit-Neotropical Sobralia macrant ha is most often a terrestrial, inhabiting rocky soils, cliffsides, and occasionally accumulations of humus in tree crowns. Its monopodial, serially produced, determinate shoots achieve maturity with the production of a terminal inflores-