Scanning Electron Microscope Studies on the Leaves of Hepaticae. I. Ptilidiaceae, Lepidoziaceae, Calypogeiaceae, Jungermanniaceae, and Marsupellaceae

Abstract

In the 29 species of hepatics examined, the scanning electron microscope revealed several distinct types of leaf surface topographies. These varied from smooth to with striae, indentations, irregular mounds of wall materials and well defined papillae of different sizes and distributions. In every instance these features were due to sculpturing of the cell walls themselves rather than to superficial wax deposits. Whilst some forms of wall ornamentation were species specific, several instances of striking similarities between closely related taxa were also discovered. Discrepancies between light and scanning electron microscope observations are thought to be related to internal features of wall construction. In the light of the structural information obtained from this study, a partial overhaul and simplification of the terminology relating to surface architecture is outlined. The scanning electron microscope (SEM) with its relatively high resolution and great depth of focus, far surpassing that of the light microscope, is an important recent addition to the range of techniques available for structural studies in biology. Thus far this instrument has produced its most impressive results on the fine architecture of the surfaces of hard tissues which do not require complex histological techniques (Carr, 1971). Although some results merely confirm previous light microscope studies, the SEM has made it possible to elucidate the real nature of many surface features lying at or beyond the limit of resolution of the light microscope and at the same time revealed many, completely unsuspected, new structures. To date, in botany, the SEM has been utilized in fields such as wood, seed, fruit, and spore anatomy, palynology, palaeobotany, and the study of the distribution and physical nature of the cuticular layers of leaves (Carr, 1971). Although detailed studies have been made on diverse groups ranging from the angiosperms to the algae and fungi, bryophytes by comparison have been rather neglected. The surface topography of a few spores has been examined (Mozingo et al., 1969; Taylor et al., 1972), but no systematic study has yet been published. Robinson (1972) investigated the peristome teeth of 13 genera of mosses and the leaf surfaces of 7 genera. Although this last study was in no way intended to be comprehensive it did reveal that these structures were particularly suitable for SEM investigations and that the instrument 1 We thank Miss Ruth Owen and Mr. Albert Rees for technical assistance on the SEM, and Mr. William Neville for preparing the plates. 2 School of Plant Biology, University College of North Wales, Bangor, Wales, U.K. This content downloaded from 207.46.13.149 on Wed, 28 Sep 2016 05:50:51 UTC All use subject to http://about.jstor.org/terms 1972] DUCKETT & SONI: SEM STUDIES OF LEAVES OF HEPATICAE 537 has considerable potential for revealing a wealth of new information important in interpreting overall interrelationships. A more detailed SEM study has been made on the leaf surfaces of 17 species of Scapania Dum. (Duckett & Soni, 1972a). Not only did this provide several new characters to supplement conventional taxonomy but also demonstrated, by means of a chloroform extraction technique, that the surface features, generally referred to as cuticular ornamentation, were not due to superficial waxes but to the structure of the wall itself. Subsequently these authors (Duckett & Soni, 1972b) showed that the papillae of Scapania nimbosa and S. ornithopodioides contain high concentrations of silica and are thus similar to the silica-containing papillae of grasses and Equisetum (Kaufman et al., 1969; 1971a; 1971b). Throughout the leafy hepaticae the surface characteristics of the leaves are extensively used by taxonomists. The majority of these superficial features lie at the limit of resolution of the light microscope and we have little idea of the structural details responsible for the various finely verruculose surfaces. Thus the present SEM study was undertaken to investigate the nature of leaf surfaces throughout the leafy hepaticae. It was of particular interest to see if any taxonomic trends would emerge at generic and higher levels of classification as well as at the specific rank as has already been found in Scapania (Duckett & Soni, 1972a). It was also important to find out whether papillae of different sizes and abundance similar to those seen in Scapania, or other structural features, are responsible for the varied surfaces observed by optical microscopy. At the same time it was felt that an attempt should be made to use the information revealed by the SEM as a basis for rationalisation of the terminology of surface ornamentation. At present discrepancies between standard texts (Table 1) are frequent and it is clear that the number of terms used exceeds the number of different surface topographies which are actually observable by optical microscopy. The position is further complicated by the fact that little or no attempt is made to define these terms precisely or indicate which are synonymous. For example in MacVicar's flora (1926) it is unclear how verrucose (defined as covered with wart like protuberances) differs from papillate (defined as covered with papillae), and what exactly is meant by granulate (not defined at all) or punctate (defined as dotted). This last term could equally well refer to the presence of abundant ectodesmata instead of to a feature of superficial ornamentation. More difficulties arise in attempts to define different degrees of verrucosity in precise terms. It is not known whether the same fundamental structural features are responsible for slightly sculptured surfaces variously referred to as nearly smooth, finely hyaline punctate, minutely verruculose, or slightly papillose. Since it is clearly not feasible to investigate all species of leafy hepatics by SEM, in this and subsequent papers observations are concentrated on groups of taxa which are already known to display a wide range of surface topographies under the light microscope. MATERIALS AND METHODS Dried herbarium specimens were mounted on polished aluminium stubs with celloidin, and coated with gold/palladium in a vacuum evaporator (JEOL, JEE-SS) equipped with a rotating device to coat the material uniformly. The material was examined in a Cambridge Stereoscan Mark II operated at 20 kev and a beam current of 150-200 vamps. In order to investigate whether or not cuticular waxes were present, further dried material of each species was soaked in chloroform prior to coating. Since the material was coated in a dried condition, the cells were all observed in a collapsed condition under the SEM. Thus the walls are highly concave over the lumina This content downloaded from 207.46.13.149 on Wed, 28 Sep 2016 05:50:51 UTC All use subject to http://about.jstor.org/terms 538 THE BRYOLOGIST [Volume 75 making the position of the individual cells clearly visible. In life the upper and lower cell walls are normally flat or convex. Even though the cells are in a collapsed condition there is no reason to believe that the finer topographical details of the leaf surfaces, as seen under the SEM, are in any way artifactual. Specimens of all materials utilized for the investigation (Table 1) are deposited in the herbarium of the senior author. Nomenclature in this and subsequent publications follows Paton (1965) and the general classification of the leafy hepatics (Jones, 1958).