Identification of plants is easier if the specimen is complete. At least two voucher specimens of each plant should be collected along with additional reproductive structures (flowers, cones, etc.); sometimes, extra vegetative parts, such as leaves, may be needed. Extra reproductive parts are particularly valuable in the initial identification process and for later research. These parts can be attached to the herbarium sheet or stored in small envelopes called fragment packets. Collection techniques for herbaceous versus woody plants are somewhat different.
For herbaceous plants, the entire plant is collected including roots, stems, leaves, flowers, and fruits, assuming that all of these parts are present. Soil on the roots should be removed. If the plant is too cumbersome or large to fit in a plant press, representative parts can be pressed. For example, a collection of a large plant could include: roots; part of the lower, mid, and upper stem with characteristic leaves; inflorescence or part of it; and other important morphological structures. When only representative parts are collected, the original plant height should be recorded on the label. Senescent specimens should be avoided because they deteriorate during and after the mounting process.
For woody plants, representative parts are chosen as follows: tips of a terminal branch (about 30-35 cm, 12-14 in) showing leaf arrangement; flowering and/or fruiting material; and other morphological structures unique to a species. As with herbaceous plants, extra reproductive parts and leaves are desirable. Leaves should be pressed open and flat.
The optimal time for plant collection varies with the species and can be determined by the examination of existing herbarium specimens. Some taxa, such as Pinus spp. or some Ulmus spp., have a short polleniferous period that occurs in the late winter to early spring. Whereas, some taxa, such as Castilleja spp., flower in south Texas from February to November.
For permanent preservation, vouchers are usually pressed and dried in a plant press (illustrated in Gould and Shaw 1968 or in other taxonomic texts). Fresh plant material once pressed makes better voucher specimens than material that is partially dried before pressing. The press prevents the plant from losing its original integrity by ensuring against shrinking, wrinkling, and shattering of the specimen.
The actual drying process requires a convectional or forced heat source that allows warm air to move through the plant press. Ideally, moisture should be extracted from the specimen quickly enough to prevent the growth of mold and mildew and to produce a dry but non-brittle specimen.
Wet specimens should not be pressed without utilizing extra precautions for three main reasons. First, chlorophyll and pigments in the flowers, are often removed with the water during the drying process. The result is a bleached or faded specimen. Second, wet specimens, like those dried too slowly, are more likely to produce a discolored, spotted, or generally deteriorated specimen. Finally, fungus and mold are more likely grow on and destroy a wet specimen. Therefore, remove excess water from wet specimens prior to pressing; and/or use blotters boards within the plant press to absorb excess moisture. If blotter boards are used, they should be changed several times during the drying process.
Each voucher specimen must have a label with basic information including, if available, the following information:
A properly identified site enables subsequent researchers to return to the exact
location. Today, GPS (Global Positioning System) can be obtained that give the precise
latitude and longitude of the collection site. A collection site description should read
similar to, "3.4 mi NE on US 45 from its jct. with TX 88, W of College Station. T12N,
R51W, Section 29." If available, the latitude and longitude from quadrangle sheets or
a GPS system should be included. To state that the plant was collected "southeast
(SE) of College Station, Texas" is unacceptable.
EXAMPLE:
| PLANTS OF BRAZOS COUNTY, TEXAS, U.S.A.
CYPERACEAE Cyperus croceus Vahl 3.4 mi NE on US 45 from its jct. with TX 88, W of College Station. T120N, R54W, Section 20. Open, mesic to submesic post oak savanna with deep sandy soil. Elevation ca. 310 feet. Associates: Quercus stellata, Ilex vomitoria, Schizachyrium scoparium, Sorghastrum nutans, and Cyperus retrorsus. Frequent S. & G. Jones 11456 15 June 1994 |
Once the voucher specimen is dried, identified, and a label placed with it, the specimen is ready to be placed in an herbarium. Proper identification by a competent plant taxonomist is essential for a research quality reference pollen collection. Arrangements can be made with the curator of most herbaria to identify plant specimens. Although some curators will identify the occasional specimen free of charge, there is often a fee for this service. Customarily, a specimen submitted for identification is donated to the herbarium.
Botanical nomenclature is based on Latinized names for plant taxa and follows rules established by the International Code of Botanical Nomenclature (Greuter et al. 1988). The purpose of the Code is to regulate and maintain the usage of scientific plant names and promote nomenclatural stability. The Code does not address common names, thus encouraging the use of scientific names in botanical research.
Each plant taxon has only one correct scientific name. This name consists of three or four parts: the genus, the specific epithet, and the full or abbreviated name of the author(s) who originally described the plant or who later made changes in the plant's classification. The genus and specific epithet together constitute the scientific name; both parts are italicized. The first letter of the genus is uppercase, but the specific epithet is entirely lowercase.
An author's name placed in parentheses represents the original author. The name following the parentheses is the person(s) who changed the taxon's classification [e.g., Castanea pumila (L.) P. Miller]. Carl Linnaeus was the original author for Castanea pumila; however, P. Miller changed the classification. Scientific names change for many reasons, but mainly because a taxon has been reclassified or an older, validly published name is rediscovered. However, the best sources of nomenclatural changes are recent botanical monographs. Summarized Checklist of Plants of Texas (Jones et al 1997) is the most up to date and best reference reflecting recent nomenclatural changes for Texas.
Polleniferous material can be collected at the same time as voucher plant specimens. Place them in a small coin envelope labeled with the same number as the voucher specimen. Ogden et al. (1974) suggested that pollen-bearing material be stored in a folded glassine paper and placed inside a coin envelope.
For some species field collection of pollen along with the voucher specimen are preferred to collection from a herbarium specimen. The collected field material may result in a greater quantity of pollen and less contamination. Many anemophilous taxa, such as those in the Cyperaceae and Poaceae, shed their pollen early in the flowering cycle (Faegri et al. 1989). Once the pollen is shed, the anthers fall from the flowers. Herbarium specimens of the Cyperaceae and Poaceae will most be collected in fruit not in flower, because the fruit is essential for identification. Therefore, a collector is more likely to obtain anthers with pollen from field collected specimens.
For either field or herbarium collections, partially-open buds are usually the best sources for polleniferous material because open flowers are often contaminated with pollen from other taxa or have already shed the pollen (Scrophulariaceae and Lamiaceae). Only the staminate (male) flowers from taxa that bear separate male and female flowers are necessary for pollen analyses.
Many flowers and/or partially-open buds may be needed for a pollen study, depending on the nature of the study and on the nature of the male reproductive structures (stamens) of the particular taxon. Thus, if the purpose of pollen collection is to assist in identification of a particular species, then fewer flowers may be required; but if the objective is to prepare pollen slides and to mount material for SEM studies, then more flowers may be needed. Plants vary greatly in the number of stamens per flower and in the number of pollen grains per anther. For example, taxa in the Ranunculaceae and Malvaceae typically have numerous stamens as opposed to taxa in the Rubiaceae and Scrophulariaceae. For gymnosperms, no more than a couple of microsporangiate strobili (pollen-producing cones) are needed to provide ample pollen.
Like plant specimens, fresh pollen-bearing material must be dried or preserved by some other means to prevent decomposition. Material can be dried separately or pressed and dried with the voucher specimen. When material is dried separately, one can use a regular oven or an improvised method.
When a regular oven is used, a temperature of about 150o F for two to three days is recommended. A fungicide may be added to the packets. In place of drying, flowers can be preserved in 75 - 95% ethyl alcohol (ETOH) or acetone. However, prolonged storage in ETOH or acetone can cause dehydration of the pollen, resulting in easily broken and ultimately destroyed grains. Traverse (1965) suggested storage of pollen-bearing material in labeled vials of glacial acetic acid.
Plant specimens preserved in herbaria are usually good sources for pollen collections. First, the specimens have usually been accurately identified. Second, the herbarium method of collection is fast and inexpensive as compared to field collection. Third, an herbarium usually contains a great number of taxa from various habitats and geographic areas. Fourth, herbarium specimens often contain many examples of the same taxon collected across the entire range of the plant.
Permission must be obtained from the herbarium curator before pollen-bearing material can be removed from specimens. Also, many curators will request duplicate slides, copies of micrographs, and require that a pollen annotation label be affixed to the herbarium specimen. Annotation labels should consist of high quality acid-free cotton paper, preferably with a buffer and should include the following information: name of person removing the material; company or university that the person represents, if applicable; date; project; and a statement of purpose about the pollen analysis.
Plants chosen for pollen collection, should have adequate flowers and/or pollen-bearing material, ensuring that some material will remain for future researchers. Moreover, if possible, select an annotated specimen, that is, a specimen that has been identified or verified by a taxonomist and bears a label to establish that fact. Additionally, where available, choose a specimen that contains an attached fragment packet with extra flowers, using the packet flowers for the pollen source.
For removing pollen-bearing material from an herbarium sheet, necessary tools and supplies include: several pair of fine-tipped forceps, razor blades, glassine paper and small coin envelopes or substitute pollen packets, SEM stubs (if necessary), annotation labels, paper towels, ETOH, and a pen with indelible ink. To guard against contamination any instrument used to remove the pollen (e.g., forceps, razor blades) should be cleaned after each use with fresh ETOH and a fresh paper towel.
To remove flowers or partially-open buds, hold down the specimen, grab the peduncle (stem) at the base of the flower with a pair of clean forceps, and gently pull. For thick peduncled specimens, such as Rosa spp., cut the peduncle at the base of the flower with a razor blade.
For many taxa, remove only the anthers from the flower. In such cases, hold down the flower, grab several anthers with the forceps, and gently pull with a twisting motion. Place the anthers or flowers into a labeled packet or onto a prepared SEM stub.
All of the polleniferous material removed from a specimen should be placed into a single pollen packet. Label the packet with the taxon's scientific name including author, the acronym for the herbarium, the collector's name and collection number, the date of collection, and the location of the specimen collection. Also, if desired, anthers or pollen-containing material can be placed directly onto a prepared SEM stub.