- Email: [email protected]
A modified thermal convection drying technique for herbarium preservation
Botanical Journal of the Linnean Society (1995), 117: 29–38. With 6 figures
A modified thermal convection drying technique for herbarium preservation RAINER GREISSL, ANDREAS HORN Institut fu¨r Spezielle Botanik und Botanischer Garten, Johannes Gutenberg-Universita¨t Mainz, Saarstr. 21, D-55099 Mainz, Germany AND ALEXANDER VOGELEI Institut fu¨r Allgemeine Botanik, Johannes Gutenberg-Universita¨t Mainz, Saarstr. 21, D-55099 Mainz, Germany (c:o Dornier GmbH, Earth Observation Data Service, D-88039 Friedrichshafen) Received June 1993, accepted for publication November 1994
A simple and efficient installation for drying plants that is both rapid and preserves colour, and which works on the principle of autonomous thermal convection, is presented. The described apparatus offers significant advantages including: uniform heat distribution and short drying times; a system for applying pressure to prevent shrinkage, which uses polyurethane foam sheets and elastic straps; a compact, space-saving system suitable for field or laboratory applications; absorbent materials which can be re-used immediately; ability to run off differing power supplies (110:220 V) and to use infra-red lamps of varying specifications; low cost and ease of use. ADDITIONAL KEY WORDS:—colour preservation – desiccation.
CONTENTS Introduction . . . . . . . Principle, system design and drying process Results and discussion . . . . . Acknowledgements . . . . . . References . . . . . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
29 30 32 37 38
INTRODUCTION
Preserving plants by placing them between absorbent paper and drying under pressure is one of the oldest preservation methods used in herbaria. With this method the quality of the specimens produced largely depends on the drying conditions. Different methods of plant drying have been developed to optimize these conditions, including conventional plant presses or ‘polish’ presses, in combination with different artificial heat sources or sunlight ( Jenne, 1968; Tillett, 0024–4074/95/010029+10 $08.00/0
29
© 1995 The Linnean Society of London
30
RAINER GREISSL ET AL.
1977; Hicks & Hicks, 1978; Sinnott, 1983; Bacci et al., 1985; Forman & Bridson, 1989; Bridson & Forman, 1992). The natural colour of flowers is an important element in flower biology (Menzel, 1990) and plant systematics. Obtaining more attractive herbaria specimens is far more than a question of aesthetics; colour charts may help to describe flower appearance, but with human vision able to distinguish between more than 100,000 colour shades (Ku¨pper, 1981) they cannot be a substitute for a fresh or dried specimen in good condition. In order to maintain natural colour as far as possible, Napp-Zinn (1961) proposed a process which was further developed by Weber (1977). Even though the process described by Weber is straightforward and particularly suitable for field work, it does show some fundamental deficiencies in practice. In this paper we introduce modifications to improve the system. PRINCIPLE, SYSTEM DESIGN AND DRYING PROCESS
The method is based on the principle of autonomous thermal convection generated by an infra-red source. Infra-red radiation is absorbed by water and thus also by hydrated plant tissue. The main absorbance range of water is between 3.0 mm and 10 mm. Only short-wave radiation is able to penetrate the specimen and to pass through the water vapour layer which is produced during the drying process (Philips, 1991). Therefore, it is essential to use an infra-red source that mainly emits short-wave radiation. The absorbed energy causes uniform heating of the water within tissues with a resulting increase in evaporation. The longer waves which are also generated heat the air between the individual layers of the press and promote, via convection, a drying process by exchanging and dissipating the moisture-laden air. In order to remove the vapour-laden air rapidly, an electric fan may be used in addition. This method has the advantage that the entire specimen is heated from the outset and that autolytic processes within the plant tissue are inhibited. The system design and drying processes described here are based on those described by Weber (1977) and Krach (1981). Our drying apparatus consists of a trestle support, a plant press, two linked elastic straps with a tensioning device, an infra-red reflector with socket, electronic dimmer, thermo-control unit, and a connection to a power source (Fig. 1). The material inside the press consists of sheets made of: corrugated cardboard; 400 g:m2 felt board (obtainable from Fa. Astra, Germany, Art.Nr.8040); 15 mm medium density (say 30–35 kg:m3) polyurethane foam; absorbent paper (such as blotting paper—newspaper could be used as an inexpensive substitute), and, for delicate specimens, double flimsies (folded pieces of paper half way between tissue paper and grease-proof paper in thickness). The fresh plant material is placed between a folded sheet of absorbent paper (and double flimsies, if required) and arranged between the other supporting elements, as shown in Figure 1A. The press is held under pressure with elastic straps (Fig. 1B), the tension of which is maintained by means of a simple device such as a wooden block (Fig. 1C), and then placed on the trestle support. For this we use a collapsible camping chair with an aluminium crossframe; the two legs are easily adapted to the width of the press and held in place by means of an adjustable link chain as shown in Figure 1D. The infra-
THERMAL CONVECTION DRYING TECHNIQUE
31
Figure 1. Construction of drying apparatus. A, arrangement of materials within the press: corrugated cardboard, felt board, polyurethane foam sheet, absorbent paper, double flimsies, plants. B, link connection between elastic straps. C, increasing tension by rotating the wooden block used as a tensioning device. Infra-red reflector, E27 porcelain socket, electronic dimmer, thermo-control unit, and connection to power source.
red reflector is placed beneath the press. The electronic dimmer allows the operator to use infra-red lamps of differing wattages and maintain close control of the temperature ranges to be used. The thermal element of the control unit is located in the upper part of the press during the drying process, and is automatically controlled by the other constituent parts of the control unit—an electronic relay and a transformer. The drying process takes approximately 1– 2 days in our laboratory. The corrugated cardboard, felt board and polyurethane foam can be reused immediately, while the specimens, still within the double flimsies and:or absorbent paper, are removed and stored.
32
RAINER GREISSL ET AL. RESULTS AND DISCUSSION
Our system was initially tested in the laboratory and then tested in the field on expeditions to Europe and South and Central America. In comparison to other drying methods, the results obtained were much better. In particular, very delicate plants from the Liliaceae or Amarylidaceae showed excellent preserved morphological structure after drying. Good results were also obtained with problematic material. For example, the leaves, flowers, fruits and stem articulations of Loranthaceae tend to fall off easily and specimens are normally preserved in the field (Simon, 1962; Steyermark, 1968; Forman & Bridson, 1989), necessitating the transport and handling of toxic liquids such as formaldehyde. Figure 2 shows the results obtained with samples of Passiflora, Doronicum, Iris and Digitalis. In addition to the visual nectar guides on corollas (e.g. Digitalis), which were originally recognized by Sprengel (1973), ultraviolet absorbing patches, normally invisible to the naked eye, were well preserved. These patterns can be revealed using ultraviolet photography. The use of polyurethane foam in addition to other materials (Fig. 3) gives excellent results. Padding the cavities with felt-board to even out pressure as recommended by Krach (1981) was not necessary. The thicker parts of the plant, such as stem, are moulded by the flexible and elastic polyurethane foam, and the thinner parts, such as the leaves, are subjected to equal pressure (Fig. 3); in this way, wrinkling of the foliage is completely avoided, even at the base of delicate leaves. The foam also prevents damage to the corrugated cardboard, and the timeconsuming padding of each plant layer can be omitted. Furthermore, as the foam is porous, moist air can easily escape from the press. The advantages of using foam were discussed by Chmielewski & Ringius (1986). Pike (1964) had earlier described the use of plastic sponge layers applied directly to the specimen, and reported that the use of such sponges gave much better colour preservation than had been achieved previously by other methods. Our results agree with this observation. Sufficient pressure should be applied to the press, but it is not advisable to overtighten it, because specimens could either be partly crushed or the corrugated cardboard become damaged. Weber (1977) proposed using punched leather belts which could be re-tightened from time to time. As a rule, however, the drying process is so rapid that re-tightening with a leather belt, or with cotton web straps with spiked buckles, usually occurs too late and the plants have already become curly and wrinkled. The use of two or three elastic straps, of the type used on luggage carriers, is a substantial improvement. These elastic straps are pre-tensioned by hand and each one fixed by means of a link of a chain (Fig. 1B). Tension can be increased by inserting a simple tensioning device, such as a wooden block. This is inserted flat and then turned through 90° (Fig. 1C). In this way, shrinkage is continuously compensated for, and sufficient pressure is maintained throughout the entire drying process. The length of the drying process is directly related to the texture and water content of the various parts of the specimens, and also depends upon atmospheric conditions (relative humidity, temperature). For example, it may last for up to 3 days in wet climates, such as South American rainforest, and about a day
THERMAL CONVECTION DRYING TECHNIQUE
Figure 2. Fresh (a) and dried (b) flower of Passiflora caerulea (Passifloraceae) with longitudinal sectioned blossom and leaf: dried specimen of (c) Digitalis purpurea (Scrophulariaceae); dried flowers of (d) Iris pseudacorus (Iridaceae); (e) Doronicum pardalianches (Asteraceae) and (f) Digitalis purpurea.
33
34
RAINER GREISSL ET AL.
Figure 3. View of press from top. The polyurethane foam sheets allow efficient moisture removal (indicated by arrows) while applying continuous, even pressure, and prevent the flattening or distortion of thicker plant parts. This enhances the retention of structure and colour. Note that the corrugated cardboard is orientated in the direction of the radiation axis of the infra-red lamp.
in relatively dry climates, e.g. Chilean semidesert. Other methods claim shorter drying times due to temperatures of up to 60°C in the centre of the press (Sinnot, 1983), but temperatures which exceed the critical drying temperature (c. 45°C) destroy cells and pigments, and make the dried plants brittle and therefore delicate to handle. The aim must be to inhibit lysis by drying as fast as possible, but without overdrying, resulting in destruction of colour pigments or the plant structure itself. On how to collect and prepare specimens, see Bridson & Forman (1992). Other heating sources instead of the infra-red lamp may be substituted. Croat (1979) used a portable propane gas oven, while Hale (1976) preferred to use a portable electric herbarium drier. During expeditions in South America we lacked access to any power source but still achieved quite satisfactory results using the top of a wood-burning stove. Weber (1977) positioned the press horizontally above the heat source. Our tests showed that with this type of arrangement, a considerable area of the press is outside the cone of heat generated by the lamp. Depending on the reflector type, this may amount to 45–50% of the total cross-sectional area of the press (Fig. 4). As a result, the plants located at the periphery of the press do not dry fast enough with attendant dangers of artifacts. We therefore keep the press upright, so as to make optimum use of the light:heat cone. To permit convection of the damp air current, the corrugated cardboard must have the corrugations running lengthwise, i.e. in the direction of the radiation axis of the infra-red lamp. Aluminium corrugates may be substituted for cardboard (Stevens, 1926; Bridson & Forman, 1992). Aluminium has the advantage of being waterproof and has a long life-span, but is heavier than cardboard (1437.6 g:m2 when 0.5 mm thick as opposed to cardboard’s 496.7 g:m2 when 5.00 mm thick) and the latter is also more readily available, both at home and abroad. The temperature distribution in the press depends on the radiation angle of the infra-red reflector (Fig. 5). The temperature profile in the press when placed on its side showed a considerable decrease of temperature at the periphery. However, when the press was positioned vertically, temperature loss at the
THERMAL CONVECTION DRYING TECHNIQUE
35
Figure 4. Comparison of the drying zones in the press, with different orientation. In a vertical position, c. 80% of the complete cross-sectional area of the press is within the angle of radiation of the infra-red lamp, while it is c. 50% when it is positioned horizontally.
periphery was lower, and at the top only slightly higher. This ensures a more uniform and quicker drying time. Even originally wet plant material placed in the press can be completely dried within 3 days. The temperature profiles within and outside the press were measured with a digital electronic thermo:humidity-sensor (Therm 2281–8, thermal elements of NiCr-Ni type, accuracy 20.1°C; relative humidity sensor FH 9626, accuracy 0.1% Ahlborn Meß- und Regelungstechnik, Holzkirchen, Germany), until a constant value was reached. The temperature showed a typical time curve during the drying process (Fig. 6). At the beginning of the process, the temperature in the press increased very rapidly and reached a maximum after 1–2 hours. In parallel, the relative humidity in the press reached a maximum shortly after the maximum temperature. Due to the rapid increase in humidity, the temperature decreased for about 1 hour in the middle and for 5–6 hours at the top of the press (Fig. 6). This temperature curve corresponds to a cooling effect caused by the rapid water loss of the plant tissues. After most of the water vapour had escaped from the press, the temperature again increased and reached a maximum value at the end of the drying process. The end of the process is indicated by a constant temperature level combined with a constant relative humidity. It can, therefore, be monitored manually or controlled automatically by means of a thermo-control unit on the top of the press which switches the infra-red lamp off when the maximum temperature level is reached. It is important that the process is not over-extended and that
36
RAINER GREISSL ET AL.
Figure 5. Comparison of the angles of light radiation of two infra-red lamps, which are determined by the reflector. The differing lamp foci are attributable to the differing reflector geometries. A, Siccatherm PAR SL:r, 100 W, 230–240 V, sealed beam lamp with reflector, bulb bowl with red filter, Osram GmbH. B, Infrarubin IR2, 150 W, 230–240 V, mushroom bulb, bulb bowl with red filter, Tungsram GmbH.
Figure 6. Experimental determination of the temperature profile and moisture content during the entire drying process. Temperature in the middle (a) and at the top (b) of the press; relative humidity at the top (c) and in the middle (d) of the press. Infra-red source: Philips IR 100RPAR (100 W); distance between lamp and press: 28 cm; temperature at the bottom of the press: 3721°C; plant material: leaves (e.g. Saxifraga).
THERMAL CONVECTION DRYING TECHNIQUE
37
it is stopped after the maximum temperature is reached, to avoid specimens turning brown and becoming brittle. To prevent charring of the samples, the temperature at the lower press edge should not exceed 45°C. Using the Weber (1977) method, the choice of a suitable infra-red reflector is heavily restricted. Lamps which do not emit visible red light as well as infra-red may also be used, although they are usually not cheap. If the drying device is to be used primarily indoors irradiation could be an argument in favour. In our experience, red light is not inconvenient, as the visible area of the spectrum (380–780 nm) of the lamps we use only accounts for approximately 6%. Weber was emphatic that the power consumption of the bulb should not exceed 100 W. However, it should be noted that the ratings of commercial lamps vary greatly. New generation infra-red reflectors, for instance, show a uniform intensity of radiation combined with low power consumption. To ensure optimal functioning of the system, the angle of radiation and intensity of the lamp must be taken into consideration as well as power consumption (Fig. 5). We have found that the design of the reflector and face-plate of the commerically available ‘Siccatherm’ lamp manufactured by Osram has narrowbeam radiation and uniform distribution of radiation, combined with energy savings of 30% (Osram, 1987). Similar characteristics are shown by the IR 100R-PAR reflector produced by Philips (Philips, 1991). With a power consumption of 100 W, the electromagnetic thermal radiation of the Siccatherm corresponds to that of a normal 150 Watt infrared reflector with a mushroom bulb (Fig. 5B). The resulting higher temperatures have to be taken into account. We therefore propose a modification of the electrical subsystem by connecting it to the power supply via a standard electronic dimmer. In this way, all commercially available infra-red reflectors with a power consumption of 60 to max. 400 W can be used. Calibration and a dial on the control knob allow the optimal heat radiation to be set for each infra-red lamp. The system is compatible with different supply voltages. In addition to the usual 235 volt supply voltage, the system can also be connected to 110 V via an adapter (available in electrical shops). However the energy loss of 220 V reflectors is c. 55% when they are used on a 110 V system. Depending on the cross-sectional area of the reflector type, the extent of the light:heat cone (Fig. 4), and on the thickness and size of the inserted plants, up to 50 layers can be arranged in the press. If preferred, Weber’s horizontal positioning of the press can be used, provided that the corrugations of the cardboard follow the vertical direction of the radiation axis of the lamp. However, to obtain uniform drying over the total area of the press, the use of two lamps will be necessary.
ACKNOWLEDGEMENTS
The authors wish to thank Prof. Renner for her kind support, Mrs Anke Berg for the drawings, Mr A. Rutloff for taking some of the photographs, and Mr C. Lenz for the construction of the thermo-control unit.
38
RAINER GREISSL ET AL. REFERENCES
Bacci M, Checcucci A, Checcucci G, Palandri MR. 1985. Microwave drying of herbarium specimens. Taxon 34: 649–653. Bridson D, Forman L. 1992. The Herbarium Handbook. Royal Botanic Gardens, Kew. Revised Edition. Chmielewski, JG, Ringius GS. 1986. Polyurethane foam: an alternative plant pressing material especially suitable for population-based studies. Taxon 35: 106–109. Croat TB. 1979. Use of a portable propane gas oven for field drying plants. Taxon 28: 573–580. Forman L, Bridson D. 1989. The Herbarium Handbook. Royal Botanic Gardens, Kew. Hale AM. 1976. A portable electric herbarium drier. Rhodora 78: 135–140. Hicks HA, Hicks PM. 1978. A selected bibliography of plant collection and herbarium curation. Taxon 27: 63–99. Jenne G. 1968. A portable forced air plant dryer. Taxon 17: 184–185. Krach JE. 1981. Sammeln von Herbarbelegen. Go¨ttinger Floristische Rundbriefe 15: 17–24. Ku¨pper H. 1981. DuMont’s Farbenatlas. DuMont Buchverlag, Ko¨ln. Menzel R. 1990. Color vision in flower visiting insects. Internationales Bu¨ro Forschungszentrum Ju¨lich GmbH, Bundesministerium fu¨r Forschung und Technologie, 16 pp. Napp-Zinn K. 1961. Eine neue Methode des Pflanzenpressens. Kosmos 57: 88–89. Osram Product Information. 1987. Licht fu¨r Innen und Außen. Osram GmbH, Berlin and Mu¨nchen. Philips Product Information. 1991. Philips Lighting. IRK-Halogen-Infrarotstrahler, IRZ-Reflektoren, IRP: IRE-Einzelelement, IMR-Fla¨chenmodul. Pike RB. 1964. Plant pressing with plastic sponges. Rhodora 66: 172–176. Simon C. 1962. Erfahrungen mit wenig bekannten Methoden der Herbartechnik. Bauhinia 2: 63–69. Sinnott QP. 1983. A solar thermoconvective plant drier. Taxon 32: 611–613. Sprengel CK. 1973. Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen. Vieweg, Berlin. Stevens FL. 1926. Corrugated aluminum sheets for the botanist’s press. Botanical Gazette 82: 104–106. Steyermark JA. 1968. Notes on the use of formaldehyde for the preparation of herbarium specimens. Taxon 17: 61–64. Tillett SS. 1977. Technical aids for systematic botany: new models of plant-press driers. Taxon 26: 553– 556. Weber HE. 1977. Eine Methode zum raschen und farbkonservierenden Trocknen von Herbarbelegen. Go¨ttinger Floristische Rundbriefe 11: 85–88.
A modified thermal convection drying technique for herbarium preservation RAINER GREISSL, ANDREAS HORN Institut fu¨r Spezielle Botanik und Botanischer Garten, Johannes Gutenberg-Universita¨t Mainz, Saarstr. 21, D-55099 Mainz, Germany AND ALEXANDER VOGELEI Institut fu¨r Allgemeine Botanik, Johannes Gutenberg-Universita¨t Mainz, Saarstr. 21, D-55099 Mainz, Germany (c:o Dornier GmbH, Earth Observation Data Service, D-88039 Friedrichshafen) Received June 1993, accepted for publication November 1994
A simple and efficient installation for drying plants that is both rapid and preserves colour, and which works on the principle of autonomous thermal convection, is presented. The described apparatus offers significant advantages including: uniform heat distribution and short drying times; a system for applying pressure to prevent shrinkage, which uses polyurethane foam sheets and elastic straps; a compact, space-saving system suitable for field or laboratory applications; absorbent materials which can be re-used immediately; ability to run off differing power supplies (110:220 V) and to use infra-red lamps of varying specifications; low cost and ease of use. ADDITIONAL KEY WORDS:—colour preservation – desiccation.
CONTENTS Introduction . . . . . . . Principle, system design and drying process Results and discussion . . . . . Acknowledgements . . . . . . References . . . . . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
. . . . .
29 30 32 37 38
INTRODUCTION
Preserving plants by placing them between absorbent paper and drying under pressure is one of the oldest preservation methods used in herbaria. With this method the quality of the specimens produced largely depends on the drying conditions. Different methods of plant drying have been developed to optimize these conditions, including conventional plant presses or ‘polish’ presses, in combination with different artificial heat sources or sunlight ( Jenne, 1968; Tillett, 0024–4074/95/010029+10 $08.00/0
29
© 1995 The Linnean Society of London
30
RAINER GREISSL ET AL.
1977; Hicks & Hicks, 1978; Sinnott, 1983; Bacci et al., 1985; Forman & Bridson, 1989; Bridson & Forman, 1992). The natural colour of flowers is an important element in flower biology (Menzel, 1990) and plant systematics. Obtaining more attractive herbaria specimens is far more than a question of aesthetics; colour charts may help to describe flower appearance, but with human vision able to distinguish between more than 100,000 colour shades (Ku¨pper, 1981) they cannot be a substitute for a fresh or dried specimen in good condition. In order to maintain natural colour as far as possible, Napp-Zinn (1961) proposed a process which was further developed by Weber (1977). Even though the process described by Weber is straightforward and particularly suitable for field work, it does show some fundamental deficiencies in practice. In this paper we introduce modifications to improve the system. PRINCIPLE, SYSTEM DESIGN AND DRYING PROCESS
The method is based on the principle of autonomous thermal convection generated by an infra-red source. Infra-red radiation is absorbed by water and thus also by hydrated plant tissue. The main absorbance range of water is between 3.0 mm and 10 mm. Only short-wave radiation is able to penetrate the specimen and to pass through the water vapour layer which is produced during the drying process (Philips, 1991). Therefore, it is essential to use an infra-red source that mainly emits short-wave radiation. The absorbed energy causes uniform heating of the water within tissues with a resulting increase in evaporation. The longer waves which are also generated heat the air between the individual layers of the press and promote, via convection, a drying process by exchanging and dissipating the moisture-laden air. In order to remove the vapour-laden air rapidly, an electric fan may be used in addition. This method has the advantage that the entire specimen is heated from the outset and that autolytic processes within the plant tissue are inhibited. The system design and drying processes described here are based on those described by Weber (1977) and Krach (1981). Our drying apparatus consists of a trestle support, a plant press, two linked elastic straps with a tensioning device, an infra-red reflector with socket, electronic dimmer, thermo-control unit, and a connection to a power source (Fig. 1). The material inside the press consists of sheets made of: corrugated cardboard; 400 g:m2 felt board (obtainable from Fa. Astra, Germany, Art.Nr.8040); 15 mm medium density (say 30–35 kg:m3) polyurethane foam; absorbent paper (such as blotting paper—newspaper could be used as an inexpensive substitute), and, for delicate specimens, double flimsies (folded pieces of paper half way between tissue paper and grease-proof paper in thickness). The fresh plant material is placed between a folded sheet of absorbent paper (and double flimsies, if required) and arranged between the other supporting elements, as shown in Figure 1A. The press is held under pressure with elastic straps (Fig. 1B), the tension of which is maintained by means of a simple device such as a wooden block (Fig. 1C), and then placed on the trestle support. For this we use a collapsible camping chair with an aluminium crossframe; the two legs are easily adapted to the width of the press and held in place by means of an adjustable link chain as shown in Figure 1D. The infra-
THERMAL CONVECTION DRYING TECHNIQUE
31
Figure 1. Construction of drying apparatus. A, arrangement of materials within the press: corrugated cardboard, felt board, polyurethane foam sheet, absorbent paper, double flimsies, plants. B, link connection between elastic straps. C, increasing tension by rotating the wooden block used as a tensioning device. Infra-red reflector, E27 porcelain socket, electronic dimmer, thermo-control unit, and connection to power source.
red reflector is placed beneath the press. The electronic dimmer allows the operator to use infra-red lamps of differing wattages and maintain close control of the temperature ranges to be used. The thermal element of the control unit is located in the upper part of the press during the drying process, and is automatically controlled by the other constituent parts of the control unit—an electronic relay and a transformer. The drying process takes approximately 1– 2 days in our laboratory. The corrugated cardboard, felt board and polyurethane foam can be reused immediately, while the specimens, still within the double flimsies and:or absorbent paper, are removed and stored.
32
RAINER GREISSL ET AL. RESULTS AND DISCUSSION
Our system was initially tested in the laboratory and then tested in the field on expeditions to Europe and South and Central America. In comparison to other drying methods, the results obtained were much better. In particular, very delicate plants from the Liliaceae or Amarylidaceae showed excellent preserved morphological structure after drying. Good results were also obtained with problematic material. For example, the leaves, flowers, fruits and stem articulations of Loranthaceae tend to fall off easily and specimens are normally preserved in the field (Simon, 1962; Steyermark, 1968; Forman & Bridson, 1989), necessitating the transport and handling of toxic liquids such as formaldehyde. Figure 2 shows the results obtained with samples of Passiflora, Doronicum, Iris and Digitalis. In addition to the visual nectar guides on corollas (e.g. Digitalis), which were originally recognized by Sprengel (1973), ultraviolet absorbing patches, normally invisible to the naked eye, were well preserved. These patterns can be revealed using ultraviolet photography. The use of polyurethane foam in addition to other materials (Fig. 3) gives excellent results. Padding the cavities with felt-board to even out pressure as recommended by Krach (1981) was not necessary. The thicker parts of the plant, such as stem, are moulded by the flexible and elastic polyurethane foam, and the thinner parts, such as the leaves, are subjected to equal pressure (Fig. 3); in this way, wrinkling of the foliage is completely avoided, even at the base of delicate leaves. The foam also prevents damage to the corrugated cardboard, and the timeconsuming padding of each plant layer can be omitted. Furthermore, as the foam is porous, moist air can easily escape from the press. The advantages of using foam were discussed by Chmielewski & Ringius (1986). Pike (1964) had earlier described the use of plastic sponge layers applied directly to the specimen, and reported that the use of such sponges gave much better colour preservation than had been achieved previously by other methods. Our results agree with this observation. Sufficient pressure should be applied to the press, but it is not advisable to overtighten it, because specimens could either be partly crushed or the corrugated cardboard become damaged. Weber (1977) proposed using punched leather belts which could be re-tightened from time to time. As a rule, however, the drying process is so rapid that re-tightening with a leather belt, or with cotton web straps with spiked buckles, usually occurs too late and the plants have already become curly and wrinkled. The use of two or three elastic straps, of the type used on luggage carriers, is a substantial improvement. These elastic straps are pre-tensioned by hand and each one fixed by means of a link of a chain (Fig. 1B). Tension can be increased by inserting a simple tensioning device, such as a wooden block. This is inserted flat and then turned through 90° (Fig. 1C). In this way, shrinkage is continuously compensated for, and sufficient pressure is maintained throughout the entire drying process. The length of the drying process is directly related to the texture and water content of the various parts of the specimens, and also depends upon atmospheric conditions (relative humidity, temperature). For example, it may last for up to 3 days in wet climates, such as South American rainforest, and about a day
THERMAL CONVECTION DRYING TECHNIQUE
Figure 2. Fresh (a) and dried (b) flower of Passiflora caerulea (Passifloraceae) with longitudinal sectioned blossom and leaf: dried specimen of (c) Digitalis purpurea (Scrophulariaceae); dried flowers of (d) Iris pseudacorus (Iridaceae); (e) Doronicum pardalianches (Asteraceae) and (f) Digitalis purpurea.
33
34
RAINER GREISSL ET AL.
Figure 3. View of press from top. The polyurethane foam sheets allow efficient moisture removal (indicated by arrows) while applying continuous, even pressure, and prevent the flattening or distortion of thicker plant parts. This enhances the retention of structure and colour. Note that the corrugated cardboard is orientated in the direction of the radiation axis of the infra-red lamp.
in relatively dry climates, e.g. Chilean semidesert. Other methods claim shorter drying times due to temperatures of up to 60°C in the centre of the press (Sinnot, 1983), but temperatures which exceed the critical drying temperature (c. 45°C) destroy cells and pigments, and make the dried plants brittle and therefore delicate to handle. The aim must be to inhibit lysis by drying as fast as possible, but without overdrying, resulting in destruction of colour pigments or the plant structure itself. On how to collect and prepare specimens, see Bridson & Forman (1992). Other heating sources instead of the infra-red lamp may be substituted. Croat (1979) used a portable propane gas oven, while Hale (1976) preferred to use a portable electric herbarium drier. During expeditions in South America we lacked access to any power source but still achieved quite satisfactory results using the top of a wood-burning stove. Weber (1977) positioned the press horizontally above the heat source. Our tests showed that with this type of arrangement, a considerable area of the press is outside the cone of heat generated by the lamp. Depending on the reflector type, this may amount to 45–50% of the total cross-sectional area of the press (Fig. 4). As a result, the plants located at the periphery of the press do not dry fast enough with attendant dangers of artifacts. We therefore keep the press upright, so as to make optimum use of the light:heat cone. To permit convection of the damp air current, the corrugated cardboard must have the corrugations running lengthwise, i.e. in the direction of the radiation axis of the infra-red lamp. Aluminium corrugates may be substituted for cardboard (Stevens, 1926; Bridson & Forman, 1992). Aluminium has the advantage of being waterproof and has a long life-span, but is heavier than cardboard (1437.6 g:m2 when 0.5 mm thick as opposed to cardboard’s 496.7 g:m2 when 5.00 mm thick) and the latter is also more readily available, both at home and abroad. The temperature distribution in the press depends on the radiation angle of the infra-red reflector (Fig. 5). The temperature profile in the press when placed on its side showed a considerable decrease of temperature at the periphery. However, when the press was positioned vertically, temperature loss at the
THERMAL CONVECTION DRYING TECHNIQUE
35
Figure 4. Comparison of the drying zones in the press, with different orientation. In a vertical position, c. 80% of the complete cross-sectional area of the press is within the angle of radiation of the infra-red lamp, while it is c. 50% when it is positioned horizontally.
periphery was lower, and at the top only slightly higher. This ensures a more uniform and quicker drying time. Even originally wet plant material placed in the press can be completely dried within 3 days. The temperature profiles within and outside the press were measured with a digital electronic thermo:humidity-sensor (Therm 2281–8, thermal elements of NiCr-Ni type, accuracy 20.1°C; relative humidity sensor FH 9626, accuracy 0.1% Ahlborn Meß- und Regelungstechnik, Holzkirchen, Germany), until a constant value was reached. The temperature showed a typical time curve during the drying process (Fig. 6). At the beginning of the process, the temperature in the press increased very rapidly and reached a maximum after 1–2 hours. In parallel, the relative humidity in the press reached a maximum shortly after the maximum temperature. Due to the rapid increase in humidity, the temperature decreased for about 1 hour in the middle and for 5–6 hours at the top of the press (Fig. 6). This temperature curve corresponds to a cooling effect caused by the rapid water loss of the plant tissues. After most of the water vapour had escaped from the press, the temperature again increased and reached a maximum value at the end of the drying process. The end of the process is indicated by a constant temperature level combined with a constant relative humidity. It can, therefore, be monitored manually or controlled automatically by means of a thermo-control unit on the top of the press which switches the infra-red lamp off when the maximum temperature level is reached. It is important that the process is not over-extended and that
36
RAINER GREISSL ET AL.
Figure 5. Comparison of the angles of light radiation of two infra-red lamps, which are determined by the reflector. The differing lamp foci are attributable to the differing reflector geometries. A, Siccatherm PAR SL:r, 100 W, 230–240 V, sealed beam lamp with reflector, bulb bowl with red filter, Osram GmbH. B, Infrarubin IR2, 150 W, 230–240 V, mushroom bulb, bulb bowl with red filter, Tungsram GmbH.
Figure 6. Experimental determination of the temperature profile and moisture content during the entire drying process. Temperature in the middle (a) and at the top (b) of the press; relative humidity at the top (c) and in the middle (d) of the press. Infra-red source: Philips IR 100RPAR (100 W); distance between lamp and press: 28 cm; temperature at the bottom of the press: 3721°C; plant material: leaves (e.g. Saxifraga).
THERMAL CONVECTION DRYING TECHNIQUE
37
it is stopped after the maximum temperature is reached, to avoid specimens turning brown and becoming brittle. To prevent charring of the samples, the temperature at the lower press edge should not exceed 45°C. Using the Weber (1977) method, the choice of a suitable infra-red reflector is heavily restricted. Lamps which do not emit visible red light as well as infra-red may also be used, although they are usually not cheap. If the drying device is to be used primarily indoors irradiation could be an argument in favour. In our experience, red light is not inconvenient, as the visible area of the spectrum (380–780 nm) of the lamps we use only accounts for approximately 6%. Weber was emphatic that the power consumption of the bulb should not exceed 100 W. However, it should be noted that the ratings of commercial lamps vary greatly. New generation infra-red reflectors, for instance, show a uniform intensity of radiation combined with low power consumption. To ensure optimal functioning of the system, the angle of radiation and intensity of the lamp must be taken into consideration as well as power consumption (Fig. 5). We have found that the design of the reflector and face-plate of the commerically available ‘Siccatherm’ lamp manufactured by Osram has narrowbeam radiation and uniform distribution of radiation, combined with energy savings of 30% (Osram, 1987). Similar characteristics are shown by the IR 100R-PAR reflector produced by Philips (Philips, 1991). With a power consumption of 100 W, the electromagnetic thermal radiation of the Siccatherm corresponds to that of a normal 150 Watt infrared reflector with a mushroom bulb (Fig. 5B). The resulting higher temperatures have to be taken into account. We therefore propose a modification of the electrical subsystem by connecting it to the power supply via a standard electronic dimmer. In this way, all commercially available infra-red reflectors with a power consumption of 60 to max. 400 W can be used. Calibration and a dial on the control knob allow the optimal heat radiation to be set for each infra-red lamp. The system is compatible with different supply voltages. In addition to the usual 235 volt supply voltage, the system can also be connected to 110 V via an adapter (available in electrical shops). However the energy loss of 220 V reflectors is c. 55% when they are used on a 110 V system. Depending on the cross-sectional area of the reflector type, the extent of the light:heat cone (Fig. 4), and on the thickness and size of the inserted plants, up to 50 layers can be arranged in the press. If preferred, Weber’s horizontal positioning of the press can be used, provided that the corrugations of the cardboard follow the vertical direction of the radiation axis of the lamp. However, to obtain uniform drying over the total area of the press, the use of two lamps will be necessary.
ACKNOWLEDGEMENTS
The authors wish to thank Prof. Renner for her kind support, Mrs Anke Berg for the drawings, Mr A. Rutloff for taking some of the photographs, and Mr C. Lenz for the construction of the thermo-control unit.
38
RAINER GREISSL ET AL. REFERENCES
Bacci M, Checcucci A, Checcucci G, Palandri MR. 1985. Microwave drying of herbarium specimens. Taxon 34: 649–653. Bridson D, Forman L. 1992. The Herbarium Handbook. Royal Botanic Gardens, Kew. Revised Edition. Chmielewski, JG, Ringius GS. 1986. Polyurethane foam: an alternative plant pressing material especially suitable for population-based studies. Taxon 35: 106–109. Croat TB. 1979. Use of a portable propane gas oven for field drying plants. Taxon 28: 573–580. Forman L, Bridson D. 1989. The Herbarium Handbook. Royal Botanic Gardens, Kew. Hale AM. 1976. A portable electric herbarium drier. Rhodora 78: 135–140. Hicks HA, Hicks PM. 1978. A selected bibliography of plant collection and herbarium curation. Taxon 27: 63–99. Jenne G. 1968. A portable forced air plant dryer. Taxon 17: 184–185. Krach JE. 1981. Sammeln von Herbarbelegen. Go¨ttinger Floristische Rundbriefe 15: 17–24. Ku¨pper H. 1981. DuMont’s Farbenatlas. DuMont Buchverlag, Ko¨ln. Menzel R. 1990. Color vision in flower visiting insects. Internationales Bu¨ro Forschungszentrum Ju¨lich GmbH, Bundesministerium fu¨r Forschung und Technologie, 16 pp. Napp-Zinn K. 1961. Eine neue Methode des Pflanzenpressens. Kosmos 57: 88–89. Osram Product Information. 1987. Licht fu¨r Innen und Außen. Osram GmbH, Berlin and Mu¨nchen. Philips Product Information. 1991. Philips Lighting. IRK-Halogen-Infrarotstrahler, IRZ-Reflektoren, IRP: IRE-Einzelelement, IMR-Fla¨chenmodul. Pike RB. 1964. Plant pressing with plastic sponges. Rhodora 66: 172–176. Simon C. 1962. Erfahrungen mit wenig bekannten Methoden der Herbartechnik. Bauhinia 2: 63–69. Sinnott QP. 1983. A solar thermoconvective plant drier. Taxon 32: 611–613. Sprengel CK. 1973. Das entdeckte Geheimnis der Natur im Bau und in der Befruchtung der Blumen. Vieweg, Berlin. Stevens FL. 1926. Corrugated aluminum sheets for the botanist’s press. Botanical Gazette 82: 104–106. Steyermark JA. 1968. Notes on the use of formaldehyde for the preparation of herbarium specimens. Taxon 17: 61–64. Tillett SS. 1977. Technical aids for systematic botany: new models of plant-press driers. Taxon 26: 553– 556. Weber HE. 1977. Eine Methode zum raschen und farbkonservierenden Trocknen von Herbarbelegen. Go¨ttinger Floristische Rundbriefe 11: 85–88.