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Pollen Rain | Sources of Pollen Evidence | Forensic Palynological Research | Case Study

Palynology is the study of fossil and modern pollen grains, spores, and other acid-resistant microorganisms such as dinoflagellates, acritarchs, and chitinozoans (Bryant et al., 1990). The main forensic application of palynology is to provide associative evidence, i.e. to establish or disprove links among people, places, and objects (Horrocks et al., 1998). If one knows the composition of the pollen rain for a given area, then one will know what pollen assemblage should be found in samples collected from that area (Bryant et al., 1990).

wattle pollen micrograph, 3.6kb

Scanning electron micrograph of wattle pollen, Acacia sp. Scale bar = 10 microns. Photo courtesy of Lynne Milne.


(Bryant et al., 1990)

A plant's mode of pollination has consequences for the volume of pollen released. Autogamous (self-pollinating) plants produce as few as 100 pollen grains per anther. Such pollen is rarely encountered in forensic samples in volumes large enough to be useful. Zoogamous (animal-pollinated) plants release much more pollen than autogamous plants, but not nearly as much as anemophilous (wind-pollinated) plants, some species of which produce as many as 70 000 pollen grains per flower. Anemophilous plants include all of the gymnosperms, some angiosperms, and the spore-producing lower plants (e.g. ferns and mosses). Fungi are also anemophilous.

Zoogamous pollen is particularly useful in a forensic analysis, because actual contact between the plant and object or person of interest is necessary in order for the pollen to transfer (Nickell and Fischer, 1999). A relatively large amount of zoogamous pollen in a forensic sample is not likely to be an environmental contaminant, and can be diagnostic of a particular locale.

Anemophilous pollen is ubiquitous, dominating the pollen rain of any area. It is less effective for establishing direct links between people and places, as a person need only be in the general area of an anemophilous plant to acquire the pollen (Nickell and Fischer, 1999). In certain cases however, the presence of anemophilous pollen is informative. For example, marijuana (Cannabis spp.) is a prolific pollen producer. For marijuana crops grown in the open air, the marijuana pollen as well as the ambient non-marijuana pollen of the surrounding area will become incorporated into the packing material of the shipment. The pollen profile of the shipment may help to identify the regional origin of the crop. Crops grown indoors will not incorporate the pollen assemblage of the surrounding area, but will contaminate the interior surfaces of the enclosure with pollen. Even if all other physical evidence has been removed, pollen recovered from dust filters, air ducts, insulation, cracks in the floors and walls, etc. confirms that the drug was grown and/or processed on that site.

Knowledge of pollen sinking speed can be used to identify source areas. Some types of pollen are very light and sink to the ground relatively slowly (e.g. alder, juniper, and birch pollen; 2 cm per second). Heavy pollen types (e.g. fir, maize) fall much faster. As a consequence, heavy pollen has a more concentrated dispersal area than light pollen, and gives greater precision in defining the source region.

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SOURCES OF POLLEN EVIDENCE (Bryant et al., 1990)

  • Dirt, mud, or dust recovered from a person or object
  • Hair and fur
  • Woven cloth, bags, baskets, and ropes
  • Packing materials (especially straw or cardboard)
  • Human remains; from the soft tissues of the stomach and intestines of corpses or the nasal passages of skeletons
  • Imported/exported goods, often to verify country of origin, e.g. honey, fruit, tea, coffee, tobacco
  • Antique goods, to validate age and authenticity, e.g. coins and paper money, furniture
  • Air filters, to determine where private vehicles may have travelled
  • Money suspected to have been used in drug transactions may be sampled for pollen of narcotic species

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In order for palynological evidence to be accepted in a court of law, investigators must establish that correlation between the pollen profile of a forensic sample and its purported area of origin is causal and not merely coincidental. The laboratory of Mark Horrocks and associates at the University of Auckland has undertaken several studies that demonstrate the veracity of pollen evidence. They have shown, for instance, that samples of hash oil originating from the same marijuana crop but subjected to different common filtering materials will show significantly similar pollen assemblages, a result which is of great use to law enforcement agencies who often seek to determine a link between illicit drug samples found on different people or at different places (Horrocks et al., 1997).

It may be argued that the pollen assemblage of a particular scene, for example an open grassy area, could be similar to the pollen assemblage of any other similar open grassy area. Horrocks has also shown that this is not the case; while different soil samples collected from within a localized region (up to 15 m) show similar pollen assemblages, there were significant differences among soil samples collected from different localized regions of similar vegetation type (up to 1 km). Again, a demonstration that pollen evidence is a useful tool in associating suspects and objects with crime scenes (Horrocks et al., 1998).

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On December 2, 1989, a recreational pilot and his wife departed San Diego, California in a Beechcraft Super KingAir F90, headed for the Sierra Blanca Regional Airport in New Mexico. Near their destination, witnesses reported hearing the plane fly off course above the clouds, then watched it exit the clouds at low altitude and fly directly into rugged terrain at full power. Both occupants of the plane were killed (Brunk, 1997).

The wreckage was recovered and stored in an outdoor storage yard at Sierra Blanca Regional Airport throughout the spring and summer of 1990. At that time, attorneys for the couple's heirs brought a lawsuit against the respective manufacturers of the airplane, the engines, and the fuel control units. The plaintiffs claimed that during flight, the plane's engines ingested some sort of contamination which lodged in the fuel line, causing a power reduction to one of the engines and subsequent loss of control of the aircraft (Brunk, 1997).


The case revolved around a component of the fuel line called the B2 elbow, a tube approximately 4 mm in diameter. A mass of biological material completely blocked the elbow. The mass consisted of plant trichomes in an amorphous matrix. A clump of pollen adhered to one side of the mass (Rozen and Eickwort, 1997). In order to assess whether this "B2 mass" was a factor in the crash, investigators had to determine how and when it got into the fuel line.


There were three major aspects to the pollen analysis: (1) species identification, (2) germination experiments, and (3) charring experiments.

Identification. The vast majority of the pollen from the B2 elbow was either gumweed (Grindelia squarrosa (Pursh) Dun., family Compositae; 62.3%) or sweet clover (Melilotus officinalis (L.) Pall., family Leguminosae; 36.6%). The other 1.1% comprised globe mallow (Sphaeralcea sp., family Malvaceae), other Compositae, and a pine, Pinus edulis Englm. (Lewis, 1997). All species grew abundantly in the area adjacent to the storage yard, and with the exception of the pine, all are entomophilous (insect-pollinated). Other species in the area included juniper and cedar (Juniperus), various grasses (Gramineae), maple (Acer), alder (Alnus), ragweed (Ambrosia), sagebrush (Artemesia), hackberry (Celtis), ash (Fraxinus), mulberry (Morus), oak (Quercus), and willow (Salix), all anemophilous (Graham, 1997).

gumweed photo, 6.3kb

Curly-cup gumweed, Grindelia squarrosa (Pursh) Dun. Source: Mike Haddock


Germination. Samples of the gumweed and sweet clover pollen obtained directly from the B2 tubing were placed in a 10% sucrose solution overnight; light micrographs were taken. Germinating tubes emerged from the pollen grains of both species (Lewis, 1997).

Charring. Sweet clover pollen was collected from plants in the immediate vicinity of the storage yard and subjected to temperatures of 250 ºC for five minutes. Transmission electron micrographs of the charred grains were compared to pollen taken directly from the B2 elbow. The heat-treated pollen grains showed destruction of the cytoplasm and intine, and damage to the exine and organelles, while the B2 grains showed no such damage (Lewis, 1997).

Colour is an indicator of the temperature to which pollen grains have been exposed; the B2 grains were bright yellow, typical of fresh pollen for many species (Graham, 1997). To test the extent of colour change with heat, investigators heated fresh Malvaceae pollen at 80 ºC for four hours, a treatment which produced noticable darkening in the grains (Graham, 1997).


The plaintiffs claimed that the mass of biological material accumulated in the B2 elbow, either suddenly or during the operational life of the engine, ultimately causing the plane to crash. However, the pollen recovered from the B2 tubing was almost exclusively entomophilous. The investigators could think of no plausible scenario whereby entomophilous pollen could be filtered from the air to the virtual exclusion of wind-pollinated types, which were abundant in the ambient air (Lewis, 1997).

Moreover, the crashed plane burned for many hours at temperatures in excess of 1000 ºC, hot enough to distort aluminum parts and melt the resin around fibreglass components (Graham, 1997). Under no circumstance could pollen grains subjected to such temperatures germinate successfully (Lewis, 1997), nor would they retain the bright yellow colour indicative of fresh pollen (Graham, 1997).

Investigators concluded that the pollen could not have been present in the fuel line prior to the crash, and must have accumulated afterwards, while the wreckage was in storage. This conclusion was supported by examination of the trichomes that made up part of the B2 mass. So if the pollen was deposited in the B2 elbow after the crash, how did it get there?


The presence of pollen on only one side of the B2 mass led investigators to suspect that solitary nest-building bees were the architects of the blockage (Rozen and Eickwort, 1997). All bees feed pollen to their offspring, and species of the family Megachilidae (leaf-cutters) are known to build their nests in pre-existing tunnels with foraged debris. The hypothesis was confirmed with the recovery of individuals of the megachlilid genera Osmia and Ashmeadiella from junked parts stored with the wreckage, and with the recovery from the B2 mass itself of a single hair belonging to Ashmeadiella.

Osmia bee photo, 2.5kb

A leaf-cutter bee, Osmia sp. Source: USDA-ARS-Logan BBSL



A jury ruled 10-2 in favour of the defendants (Brunk, 1997).

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The Forensic Botany site was created in 2002 by Jennifer Van Dommelen as a project in the Web Literacy For the Natural Sciences class at Dalhousie University, Halifax, Canada. Terms in bold are defined within the body of the text. Highlighted terms and author citations are linked to a glossary and reference list, respectively, which open in new windows. All images have been used with permission. Header banners created by Jennifer Van Dommelen.

Last content update: April 2002
Last editorial/layout update: 18 August 2011


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