Fingerprint characters, Spot characters
When we consider how characters (=characteristics) are distributed across species:
- Some field characters are useful for dividing species up into large groups. Leaf arrangement, and exudates of cut bark (slash exudates) are field characters of this type. These characters are excellent for primary divisions of keys in guides that that cover many families.
- Some field characters, by contrast, may be of use for distinguishing one species from another, but are of little value for dividing species up into groups of plants that are similar overall. Flower colour and leaf harininess are characters like this.
- Some characters are specific to each species, across a wide range of species in a given set. In other words, each species is distinctive in respect of these characters. These can be called fingerprint characters for that set of species.
- Such taxon-specific characters are sometimes called spot characters, but this term is typically used for simply defined characters of any plant part. For instance, in a particular African context, thick yellow, custard-like latex might be a spot character for species of the family Guttiferae, and spirally recurved hooks might be a spot character for the genus Hugonia.
- We wish to emphasise with the term fingerprint character those cases where the same character, or plant part, has myriad, often species-specific shades of variation. Typically, spot characters can be simpy expressed with words, whereas fingerprint characters are often only useful in a picture of the relevant plant part.
- Typically, there may be many attributes or sub-characters defining a single fingerprint, but a fingerpint character should be a useful attribute in itself. 'Leaf venation pattern' usually defines a good fingerprint character within a given set of flowering plants, and although it can be broken down into numbers and curvature of laterals nerves, secondary nerves, degree of prominence, upper and lower surface, etc. it is sometimes more useful simply to show a picture and treat this as a single fingerprint character.
- Broader 'themes' of characters, like the 'general appearance of the tree' might be as distinctive as a fingerprint, but it would be too hard to describe without specifying smaller characters like bark type, crown shape and so on, so probably would not qualify as a useful fingerpint character.
Importantly, Police fingerprint records work, even though not all people are represented in the police’s databases. These sorts of characters are ideal for use in Modular and incomplete-set guides for the same reason.
Bark slash characters are about as good a fingerprint field character as one can hope for in a field key to large rainforest trees. Leaf form and shape with venation pattern a useful fingerprint within many groups of species.