The poor condition of the material makes morphological approaches particularly difficult to conduct. Vanilla herbaria are therefore currently insufficient to resolve the genus taxonomy for technical and biological reasons. The difficulty of preparing suitable samples is a major handicap.
Tips for Preparing Good Herbarium Samples
Kill the Plant Quickly
Like many orchids, vanilla is particularly resistant to dehydration. The thick skin prevents water from escaping easily. Samples of Malagasy leafless vanilla have been seen to remain green for more than one year on paper. It is therefore necessary to kill samples rapidly. The most common method in the field is to immerse the plant in boiling water, then dry it and put it in a dryer. Sulfur dioxide fumes can also be used, with the advantage that they avoid oxidation, which deteriorates the pigments.
Prepare the Flowers
The flowers are often thick and fleshy. Failure to prepare them in the herbarium usually leads to the deterioration of structures. It is therefore necessary to dissect fresh flowers and to dry the perianth parts separately (Figure 4.1). Petals and sepals are detached from the flower; one of the two petals and one of the two lateral sepals are presented on the lower face. The lip, which is always fused lengthwise with the column, is cut on the side in order to be able to spread it out and to present the higher face with its scales and its gibbosities. The large inflorescences must imperatively be simplified, retaining only two or three flowers.
FIGURE 4.1 Dissection of flower and presentation of the leafy stem of Vanilla trigonocarpa Hoehne.
Monitor Dehydration
Drying should be monitored and in tropical conditions, the paper should be renewed.
Prioritize Collections in Alcohol
Collections in 70% alcohol are valuable additions to the herbarium. Higher levels that make the tissue brittle should be avoided. Ensuring that the structures remain flexible and unpressed makes them easier to study (Figure 4.2). Alcohol also makes it easier to observe vascularization. The alcohol collections are, however, difficult to maintain since large volumes of alcohol require strict safety conditions and the level of the liquid must be constantly monitored owing to evaporation. Furthermore, alcohol specimens cannot replace dry samples, but they make a very useful addition.
FIGURE 4.2 Dissection of a flower of Vanilla bahiana Hoehne preserved in 70% ethanol.
Photographs taken on the field and the fine details of flowers can also be very useful. However no picture, even the best possible one, can replace a collection specimen. Botanists observe many minute details, such as pilosity on the labellum, the structures on the column or organ vascularization, which photography cannot capture.
Phylogenetic analysis, phenetic, and then cladistic methods helped considerably with the comprehension of the plant groups. The use of molecular characters, today very widespread, made it possible in numerous cases to confirm the morphological hypothesis. A combination of molecular genetics and taxonomy, is a very powerful tool. Vanilla herbaria collections must therefore be accompanied by specimens in silica gel to allow further molecular studies, for example, pieces of leaf rapidly dehydrated in absorbent silica crystals.
The herbaria are used as tools in the identification of the species. For all the aforementioned reasons, this is not always easy to obtain. When a regional flora exists, or if a genus has been revised, species determination can be based on the keys of identification defined. But some characters nonetheless require certain knowledge of the group.
The specimen used for the reference to a species name is the holotype (Figure 4.3). In theory, any specimen should be compared with this holotype in order to be suitably identified. It does not, however, represent the variability of a species. Holotypes are used for the stability of names. But it is necessary to examine all the specimens to have an idea of the taxonomy, the number of species, and the morphological relationships.
FIGURE 4.3 Vanilla ochyrae Szlach. et Olsz. (holotype). [Reproduced from the Herbier National (MNHN) Web site. With permission.]
The duplicates of the holotypes are the isotypes (Figures 4.4 and 4.5). These are different parts of the same individual collected on the same day by the same botanist. They are often deposited in other institutions. Isotypes are important for ease of consultation and for the security of the material. An unhappy incident was observed after the Second World War, when the Berlin herbarium was largely destroyed, including the type collections. The isotypes conserved in other herbaria are the only reference material available to the scientific community. The International Code of Botanical Nomenclature (McNeill et al., 2006), which sets the rules for giving and using plant names, envisaged the replacement of a holotype that disappeared with an isotype. If there is no isotype, one of the other specimens (paratypes) quoted in the original publication will be used. This replacement specimen is called the lectotype. If all the original material is lacking, the botanist who revised a species can choose a specimen (neotype) that best corresponds to the description published.
FIGURE 4.4 Vanilla tahitiensis Moore (isotype). [Reproduced from the Herbier National (MNHN) Web site. With permission.]
FIGURE 4.5 Vanilla humblotii Rchb.f. (isotype). [Reproduced from the Herbier National (MNHN) Web site. With permission.]
Scarcity of Flowers in Nature
Vanillas are seldom observed as flowers in the field. In floristic inventories, it is much more common to observe stems and leaves than inflorescences or fruits.
For these reasons, ex situ collections must be created to make it possible to monitor the bloom and properly prepare the samples. If information on initial collections in the field is suitably preserved, the specimens will appear rich and ready to be studied.
Herbarium specimens are difficult to prepare, expensive to maintain, and require specific skills for analysis. They are, however, absolutely indispensable in order to revise the taxonomy of the genus as long as molecular tools, based on DNA analyses in particular, have not been established.
References
Anonymous, 2009. The International Plant Names Index (2009). Published on the Internet http://www.ipni.org [accessed October 28, 2009].
Hoehne, F.C. 1941. Arq. Bot. Estado Sao Paulo new ser. 1, fasc. 3: 61, tab. 81.
Hoehne, F.C. 1944. Arq. Bot. Estado Sao Paulo n. ser. form. maior, 1: 125, tab. 136. McNeill, J., Barrie, F. R., Burdet, H. M., Demoulin, V., Hawksworth, D. L., Marhold, K., Nicolson, D. H., et al. (eds.). 2006. International Code of Botanical Nomenclature (Vienna Code) Adopted by the Seventeenth International Botanical Congress Vienna, Austria, July 2005. Gantner Verlag, Ruggell, Liechtenstein.
Miller, P. 1768. Gard. Dict., ed. 8. n. 2.
Portères, R. 1954. Le genre Vanilla et ses espèces. In: G. Bouriquet, ed. Le vanillier et la vanille dans le monde. Editions Paul Lechevalier, Paris, 94–290.