Step-by-Step Guide to Building a Morphology‑Based Identification Key for Angraecum Species

At a glance

RECORDED PROFILE DATA

Most diagnostic characters

Spur length, ovary length, leaf shape, pseudobulb spacing, inflorescence arrangement

Standard measurement form

AOS SITF Species Identification Task Force Information Form (see PDF)

Digital key tool

R package monographaR for interactive or printable keys

Why a Morphology‑Based Key?

Angraecum species often differ by subtle vegetative and floral traits – spur length, ovary dimensions, leaf shape, and pseudobulb spacing are among the most reliable characters. A formal key lets growers, curators, and taxonomists consistently separate species, especially when molecular data are unavailable.^[1][3][5]

Scope and Preparation

This protocol is intended for

• Angraecum species native to tropical Africa, Madagascar, or the Mascarenes (the most widely cultivated groups).
• Herbarium specimens, living plants in cultivation, or field collections where flowers are present.

Before starting, decide which taxonomic section you will treat (e.g., sect. Conchoglossum or sect. Pectinaria) and gather a reference list of the species you expect to encounter. The recent dichotomous key for four Central African Conchoglossum species illustrates how a limited regional set can be handled.^[5]

Step 1 – Record Standardized Morphological Data

Use the AOS Species Identification Task Force (SITF) measurement form as a checklist. It forces all measurements into centimetres or millimetres and specifies which parts to measure (overall growth, leaf dimensions, pseudobulb spacing, inflorescence length, floral parts, ovary, spur, etc.).^[8]

Key fields to complete for Angraecum:

• Overall growth length (cane or ramicaul).
• Leaf number, length, width, margin, shape – note whether leaves are conduplicate, twisted, or iridiform.^[3]
• Pseudobulb/growth distance – distance between successive pseudobulbs or stems.
• Inflorescence – length, arrangement (single‑flowered, racemose, paniculate), pedicel length.
• Floral measurements – dorsal sepal, lateral sepals, petals, lip, spur, ovary, column. Record shape, colour, texture, and any calli or keels.

When possible, photograph each view with a scale bar (e.g., a ruler or a known‑size leaf) to verify later measurements.

Step 2 – Choose Variable (Diagnostic) Characters

After you have a data set for all target taxa, look for characters that show non‑overlapping ranges. In Angraecum, the most frequently used characters are:

• Spur length and shape – e.g., A. aporoides has the longest spur (≈0.7 cm) compared with A. bancoense (0.25–0.5 cm).^[1]
• Ovary length – 0.7–0.8 cm in A. aporoides versus 0.5–0.6 cm in several congeners.^[1]
• Leaf morphology – lanceolate, twisted, or bilobed apex; leaf thickness (thin‑textured vs. coriaceous).^[3][5]
• Pseudobulb spacing – tightly clustered versus widely spaced.
• Inflorescence arrangement – single‑flowered versus multiple‑flowered racemes.

Discard characters that vary within a species (e.g., slight colour shade) unless you have a large sample size that confirms distinct ranges.

Step 3 – Draft a Dichotomous Key

Begin with the character that splits the group most evenly. A simple example for four Central African species (based on the key in Droissart et al. 2016) is shown below. Replace the placeholder species with those you are treating.

1a. Spur longer than 0.6 cm …………………………… A. aporoides
1b. Spur 0.25–0.6 cm ………………………………………… go to 2

2a. Ovary longer than 0.7 cm …………………………… A. distichum
2b. Ovary 0.5–0.6 cm ……………………………………… go to 3

3a. Leaf lanceolate, twisted at base ………………… A. lanceolatum
3b. Leaf broadly ovate, not twisted ………………… A. bancoense

Each couplet should present a clear, mutually exclusive choice, using the exact measurement ranges you recorded. Cite the original source of any character if it is not derived from your own data.^[1][5]

Step 4 – Test and Refine the Key

1. Apply the key to specimens of known identity (herbarium sheets, living plants, or high‑resolution photos).
2. Note any mis‑identifications; check whether the character ranges overlap or if measurement error is likely.
3. Adjust thresholds (e.g., change “0.6 cm” to “0.55 cm”) or substitute a more stable character (e.g., pseudobulb spacing) if needed.
4. Repeat until the key correctly identifies all test specimens.

Document which characters proved most reliable; this information is valuable for future revisions.

Step 5 – Digitise (Optional)

If you want an interactive or printable key, the R package monographaR can import a spreadsheet of characters and generate a dichotomous key or an interactive web‑based key.^[11] The workflow is:

1. Create a CSV with the first column = species name, subsequent columns = characters (numeric or categorical).
2. Load the data in R and run dataKey() to build the dichotomous key.
3. Export the key as HTML or PDF for distribution.

Because the package handles polymorphic characters (entered as “/”) and missing data (“NA”), you can keep the key flexible as new specimens are added.

Practical Tips & Common Pitfalls

• Measure fresh flowers. Spur length can shrink after drying; always record in the field or from a living specimen.
• Use consistent units. The SITF form requires cm or mm; mixing units creates errors in the key.
• Beware of intraspecific variation. Some species (e.g., A. aporoides) show a narrow range of spur length, but populations at different elevations may differ slightly.
• Document the source of each character. Cite herbarium vouchers or published descriptions when you borrow ranges.
• Include a “unknown” or “cannot determine” option. If a character is missing, the key should allow the user to skip to the next couplet.

Conclusion

By following the standardized measurement protocol, selecting non‑overlapping morphological characters, and constructing a clear dichotomous key, growers and researchers can reliably identify Angraecum species without molecular data. Digitising the key with tools like monographaR further enhances accessibility for orchid societies and conservation programs.

FAQ

Which measurements are absolutely required for an Angraecum identification key?

At minimum you should record overall growth length, leaf length/width, pseudobulb spacing, inflorescence length, and detailed flower measurements (dorsal sepal, lateral sepals, petals, lip, spur, ovary, column). The AOS SITF form lists all required fields and ensures consistency.^[8]

Can I rely on leaf colour or flower colour for key characters?

Colour is highly variable with light exposure and age, so it is generally considered a secondary character. Use it only when supported by a narrow, documented range and when other morphological traits overlap.^[3]

Is a digital key necessary?

A digital key is optional but useful for large species sets or for sharing with a wider audience. The open‑source R package monographaR can convert a spreadsheet of characters into a printable dichotomous key or an interactive web key.^[11]

Related terminology

Spur

A tubular extension of the lip, often containing nectar; length is a key diagnostic trait in many Angraecum species.

Ovary

The basal part of the gynoecium; its length and shape (sessile or pedicellate) are frequently used in species delimitation.

Pseudobulb

A swollen stem segment that stores water and nutrients; spacing between pseudobulbs can differentiate taxa.

FOLLOW THE EVIDENCE

Sources & references

1. American Orchid Society. Angraecum aporoides 'M & B' CBR 2025-04-27. https://www.aos.org/awards-judging/sitf-findings/Angraecum-aporoides-MB-CBR-2025-04-27
2. American Orchid Society. Terminology PDF (International Code of Nomenclature for Cultivated Plants). https://op.aos.org/docs/Terminology.pdf
3. Kew Science. Angraecum Bory – Plants of the World Online. https://powo.science.kew.org/taxon/urn:lsid:ipni.org:names:325979-2/general-information
4. Droissart, V.; Ječmenica, V.; Noret, N.; Stévart, T. Taxonomy of Atlantic Central African orchids 5. A new species of Angraecum sect. Conchoglossum (Orchidaceae, Angraecinae) from Gabon and Cameroon. PhytoKeys 61: 61–71 (2016). https://www.biodiversitylibrary.org/part/216307
5. American Orchid Society. SITF – Species Identification Task Force – Information Form (PDF). https://images.aos.org/images/sitf/uploaded/new-sitf-7780-2.pdf
6. R Core Team. monographaR: Identification Keys vignette. https://cran.r-project.org/web/packages/monographaR/vignettes/keys.html