Pollen and Spore Morphology

The single most useful character for rapidly classifying an unknown pollen grain is its aperture, the thinned zone through which the pollen tube will eventually emerge. At the crudest level, a grain is inaperturate (no apertures), monoaperturate (one), or polyaperturate. Within those groups the form of the aperture matters enormously.

• Colpate: one or more elongated furrows (colpi) running toward the poles. Tricolpate pollen is the defining character of most eudicot flowering plants, an enormously species-rich group.
• Porate: rounded pores without associated furrows. Common in grasses (Poaceae), sedges, and many members of the daisy family (Asteraceae in some treatments).
• Colporate: a compound aperture combining a colpus with a central pore. Many oak (Quercus), birch (Betula), and hazel (Corylus) grains are tricolporate.
• Monosulcate: a single distal aperture typical of monocots (lilies, orchids, palms) and gymnosperms. The sulcus position is distal, unlike the polar colpus of eudicots, which is a structural difference visible at the microscope.

For forensic work these groupings matter because they reduce the identification problem quickly. A triporate grain from an outdoor scene will direct the palynologist toward grasses, plantain (Plantago), or nettles (Urtica), all common constituents of grassland and disturbed ground pollen spectra. A monosulcate grain suggests a different vegetation association altogether. Aperture typing is the first triage step at the microscope before finer characters are checked.

Once aperture type is established, the second major character is exine sculpture, the surface texture of the outer wall. Sculpture develops in the layer called the sexine, which sits atop the inner nexine layer. The sexine can be sculpted into a variety of elements, each with a standardised name in the Erdtman terminology.

In practice, sculpture types can combine. A grain can be echinate-reticulate (spiny with a background network) or granulate-psilate at the colpus region. The challenge under the light microscope is that some fine distinctions, between gemmate and verrucate, for instance, require careful focusing through the grain and can be ambiguous with degraded material. SEM bypasses this by giving a direct three-dimensional image of the surface at high magnification, which is why it is used when testimony will be contested.

Pollen grains are measured in micrometres (thousandths of a millimetre) with an eyepiece micrometer or, in modern labs, image-analysis software. The size classes recognised in palynological literature run from very small (under 10 micrometres, as in Myosotis) to very large (over 100 micrometres, as in certain Cucurbita and Hibiscus species). Most temperate wind-pollinated trees fall in the 20-40 micrometre range.

Shape is described relative to the grain's polarity. Pollen is polar: one end is the distal pole (facing outward from the parent anther, where many apertures are located in eudicots) and the other is the proximal pole. The ratio of the polar axis to the equatorial diameter gives the shape class:

• Prolate: longer than wide (polar axis exceeds equatorial). Typical of many birch and hazel grains.
• Oblate: wider than long. Characteristic of many grass genera.
• Spheroidal: polar and equatorial axes are roughly equal. Common in many herb pollen.
• Perprolate: very elongated, with polar axis more than twice the equatorial. Relatively rare but diagnostic where it occurs.

Size measurements are taken in both orientations (polar and equatorial) and reported as a mean from at least 20-25 grains, because individual grains shrink or swell depending on processing method and hydration. This variability is why comparison to a reference collection processed in the same way matters for forensic comparisons.

Forensic samples routinely contain plant spores alongside pollen, and the distinction between the two is visible at the microscope. Spores are the reproductive units of ferns (Pteridophytes), mosses (Bryophytes), and fungi (though fungal spores are grouped separately). They differ from pollen in origin and in aperture structure.

Instead of colpi or pores, fern spores typically bear a trilete mark, a Y-shaped ridge on the proximal face where three spores were attached in a tetrad. Moss spores often have a monolete mark, a single linear scar. The trilete mark is easily spotted even at 40x magnification and immediately signals a fern or lycophyte rather than a flowering plant.

The NPC system, devised by Gunnar Erdtman and Hans Straka and published in 1961, encodes aperture information into a three-character code. Each digit has a defined meaning, and the combined code can be applied to any grain before the analyst has a species in mind.

1. N: Number of apertures
   Counts the total number of apertures visible on the grain. Zero means inaperturate; higher digits encode the count directly (1, 2, 3, 4, 5, 6, and so on; codes above 6 are grouped).
2. P: Position of apertures
   Records where on the grain surface the apertures sit: polar (at one or both poles), equatorial (around the equator), or scattered (pantoporate). This reflects the underlying symmetry of the grain.
3. C: Character of apertures
   Describes the form of each aperture: colpus (furrow), porus (pore), or colporus (compound). This gives the third dimension of the classification and maps directly onto the colpate/porate/colporate vocabulary used in most keys.

An NPC code of 3-3-4, for example, describes a grain with three apertures, arranged at the equator, each of the colporate type, which is the description of the typical tricolporate grain common across most flowering plant families. A 1-2-6 code describes a monosulcate grain, typical of monocots. In forensic reports NPC codes can be listed for unknowns before a tentative identification is offered, making the evidence trail explicit.

The routine workhorse of forensic palynology is transmitted light microscopy at 400-1000x magnification. A well-prepared acetolysed slide of 300 grains can be counted and assessed in two to three hours. Light microscopy resolves aperture type, wall layering visible at high power, and most major sculpture classes well enough for identification to genus or occasionally species.

SEM is used selectively. It gives a three-dimensional surface image at magnifications of several thousand times, resolving sculpture details invisible under light. The costs are preparation time (grains must be sputter-coated with metal and examined in vacuum), inability to section or see internal structure easily, and expense. For routine identification SEM adds little over light microscopy for most taxa. SEM is appropriate for distinguishing closely similar species, preparing court exhibits for a jury without microscopy training, and documenting novel records for peer-reviewed publication.

In practice a forensic palynologist sorts and counts by light microscopy, isolates ambiguous grains or key grains needing strong visual documentation, and sends those selected specimens to SEM. The combination makes both the routine and the contested identifications defensible.