ABSORBING TRICHOMES IN THE PLEUROTHALLIDINAE (ORCHIDACEAE)

Glandular trichomes occur on both surfaces of leaves of all examined genera and species of the subtribe Pleurothallidinae (Orchidaceae). Trichome initiation is effected by one periclinal division of a protodermal cell, producing a thin-walled, globose apical cell with a relatively large nucleus and a subapical stalk cell with heavily cutinized lateral walls. In some species a second periclinal division produces a third small basal cell also having thick lateral walls but thin transverse walls. As leaf development proceeds, the trichome apparatus assumes a sunken position due to continued anticlinal divisions of protoderm. Prior to laminar expansion and guard-mother-cell division on the abaxial surface, the wall of the apical cell ruptures and is replaced by a brown opaque residue. Finally, after vascular tissue differentiation and the cessation of meristematic activity, two or more pitted foot cells develop at the base of the trichome and adjacent to the water-storing hypodermal layers. Preliminary investigations indicate that the trichome apparatus is absorptive throughout its development and similar in function to tillandsioid scales in Bromeliaceae. THE PLEUROTHALLIDINAE is a large, exclusively neotropical subtribe of Orchidaceae, containing over 3,000 species distributed among 30 genera. The largest genera are Pleurothallis R. Br. with at least 1,000 species, Stelis Sw. (250 species), Masdevallia Ruiz and Pavon (300 species), and Lepanthes Sw. (220 species). Most taxa are epiphytic with sympodial growth habit, abbreviated aerial stems, coriaceous leaves, and velamenous roots. Rather than a multiple epidermis, leaves of all genera, except Dracula Luer, possess a multiseriate true hypodermis resulting from divisions of ground tissue instead of from periclinal divisions of protoderm (Esau, 1960). Hypodermal cells frequently exhibit helical thickenings, not only in Pleurothallidinae but in other orchids as well (Solereder and Meyer, 1930; Williams, 1974). Hiinecke (1904) investigated the vegetative anatomy of the Pleurothallidinae, but considered only 50 species representing 17 genera. Solereder and Meyer (1930) examined the vegetative anatomy of Orchidaceae as a whole and treated pleurothallids in more detail than Hiinecke. Following earlier observations by Mobius (1887), both Hiinecke and Solereder and Meyer mentioned unicelReceived for publication 17 December 1979; revision accepted 29 April 1980. I thank Dr. Norris H. Williams, Dr. Margaret Y. Menzel, and Dr. Loran C. Anderson for their advice and critical comments during preparation of the manuscript. Helpful discussions with Dr. Shirley C. Tucker are gratefully acknowledged. Technical assistance with the scanning electron microscope by Mr. William B. Miller III is also appreciated. lular, sunken trichomes over both adaxial and abaxial surfaces of mature leaves. Hiinecke further noted pitted cells at the base of the trichome and in close association with hypodermal layers. Solereder and Meyer observed this same phenomenon, but only for orchid genera not included in the Pleurothallidinae. From my own anatomical studies of these glandular trichomes, a more complete course of trichome initiation, development, and possible functional significance of each stage will be described and discussed. MATERIALS AND METHODS-Living plants or herbarium specimens of all species examined are now housed at Florida State University (FSU) or The Marie Selby Botanical Gardens, Sarasota, Florida (SEL). A listing of species investigated is available upon request. Epidermal scrapes of leaves were made with a razor blade from either fresh or preserved material, mounted in glycerine:ethanol (3: 1), observed with a Wild M20 microscope under bright field, and photographed with a Nikon M-35 camera. Freehand transections were made approximately equidistant from the base and apex of mature leaves, stained in toluidine blue 0, observed and photographed as above. Rotary microtome-sectioned materials were first fixed in formalin-propionic acid-alcohol (FPA) for at least 24 hr, dehydrated in a tertiary butyl alcohol series, and embedded in Paraplast. Sections 7-12 ,um thick were then stained with safranin-fast green prior to photographing them. For scanning electron microscopy (SEM)