GNS Science NZ Bee Pollen database

New Zealand Bee Pollen Database

J.I. Raine, X. Li, L. Newstrom-Lloyd

Pollen morphology and descriptive features

Introduction

Key features for identification

Introduction

When shed from a flower or conifer pollen cone, a pollen grain is an independent organism, albeit short-lived. Within an outer protective wall the grain contains protoplasm and 2 or 3 nuclei, as well as food supplies in the form of starch granules. If the grain arrives on the receptive stigma of a compatible flower or the equivalent location in a conifer seed cone, it germinates to form a pollen tube down which nuclei pass to fuse with nuclei in the flower or conifer cone ovary and produce a seed.

Details of pollination, including the morphology of pollen grains, are consistent within individual species, families, and higher orders of plants, presumably because they are critical to the survival of a species. So, for example, the size and shape of pollen of manuka (Leptospermum scoparium complex) varies within narrow limits no matter the size of the parent plant or which part of the country it derives from. Manuka pollen is quite similar but recognisably different to the smaller and more compact pollen of kanuka (Kunzea species), but both share features such as flattened shape and united furrows with other members of the myrtle family (Myrtaceae), which includes rata and pohutukawa (Metrosideros species), ramarama (Lophomyrtus bullata), and eucalypts. Other plant families have their own characteristic pollen, some being very uniform among different genera, others less so. This means that an unknown pollen grain may frequently be identified to family, often to genus, but only sometimes to species, although if it is known to originate from a limited geographic area this may greatly restrict the range of possibilities.

For identification using a light microscope, we are mainly involved with details of the pollen grain wall, although the presence of abundant starch granules in the protoplasm, and the colour of pollen in fresh bee loads are also features that can be used in some cases. The wall consists of two main layers, the intine, an inner layer which encloses the protoplasm and which does not stain in our preparations, and the exine, an outer layer which does take stain and which displays characteristic internal and surface structural features. The intine is generally thin but may be thickened under apertures; along with the protoplasm it is quickly destroyed by acetolysis treatment. The exine is formed from a durable polymer, sporopollenin, which resists decay and only darkens gradually with acetolysis treatment.

Pollen structure
Pollen grain structure: an optical section of Pittosporum crassifolium pollen.

Because the exine walls of pollen grains are preserved in peats and other sediments, identification of its parent plants has been the subject of intense study by paleontologists engaged in climate and vegetation history. Pollen morphology is also very relevant to the study of plant evolution. These endeavours have resulted in numerous publications surveying the pollen flora of plant groups and geographic regions, and an extensive terminology for details of exine structure, mostly derived from analysis of acetolysed material (a good glossary is provided by Punt et al., 2007). The descriptions of morphological features used here are based on a scheme devised by the British melissopalynologist Sawyer (1981, 1988, with some modifications. This is designed for use with unacetolysed pollen and uses a simplified terminology.

The exine in flowering plants typically has several constituent sub-layers, although the upper ones may be absent in some species:

Where present, the arrangement of rods, tectum and ornamentation, along with systematic thickness variations, provide features for recognition of particular species.

Apertures are formed by the regular thinning of the exine, either in elongated areas called furrows or colpi, or of more limited extent, called pores. These serve as exit points for germination of the pollen, and also allow expansion and contraction of the protoplasm according to its degree of hydration. Often, furrows have pores in their mid-points, developed in a different (lower) level of the exine. The regions of a pollen grain are described using terms applied to the Earth. In dicotyledonous flowering plants, apertures are commonly arranged at equidistant meridians, with pores (if present) at the equator. In some species, pores or furrows are scattered evenly over the whole surface. An equatorial view looks perpendicularly upon the equator of the pollen grain, while a polar view is one centred on the polar region. In monocotyledonous flowering plants (grasses, lilies, palms, etc.) there is usually only one aperture (pore or furrow), which is located at a pole.

Pollen morphological regions
Morphological regions of a pollen grain with 3 furrows and pores:
(a) equatorial view, (b) polar view.
At right, Alectryon excelsus pollen grains in equivalent views.

Key features for identification

The online key enables the user to “home in” on a possible identification by entering observed features one at a time (and in any order). At each step, a range of best matches is displayed, and detail pages with illustrations and full descriptions can be brought forward. Each page also has botanical classification, plant common names, and remarks on the degree of discrimination which can be achieved among similar or related species. Where a number of species cannot easily be distinguished by their pollen morphology, a “pollen type” name is recommended. The images of pollen have been taken at high magnification (using a 100x oil immersion objective), usually of both acetolysed and unacetolysed material.

In the key, alternative interpretations or variations are included. For example, a species usually with 3 furrows but reasonably frequently with 4 will be recovered as a possible match by specifying either 3 or 4–6 furrows. It can be difficult, and perhaps unprofitable, to identify single or rare pollen specimens. Try to gain an understanding of an unknown pollen type’s features by examining several specimens in different orientations.

1. Aggregation

Most pollen is released as single units. Pine and podocarp pollen (e.g., Monterey pine, Pinus radiata; and kahikatea, Dacrycarpus dacrydioides) may appear to comprise aggregates of grains, but only the central body contains the living protoplasm. Two or three air sacs are attached to this: these have a spongy internal structure, the character of which may itself be diagnostic for different species. In a few flowering plant families, pollen is typically produced as multiple units, the morphology of the individual grains being affected by this close adherence. For example, groups of 4 pollen (tetrads) in tetrahedral arrangement are characteristic of the heath family (Ericaceae). The grains of the tetrad may be equal in size (Erica, Gaultheria) or some may be reduced in size leaving only one functional grain (Leucopogon). Tetrads with equal-sized grains arranged in one plane occur in kaiwhiri (Hedycarya arborea). Pollen of wattles (Acacia spp.) is released in polyads, groups of many individual grains. This initial key feature separates the following categories:

Pollen unit aggregation
Diagrams of basic pollen forms (P = position of pole).

2. Size

This is the maximum dimension measured using an eyepiece graticule, as described in a previous section. The dimension should include any protruding spines or other surface ornamentation (but not protruding protoplasm). For tetrads and polyads, measure the overall dimension of the aggregate. Size classes are:

3. Shape in equatorial view

This is the general shape of the outline of the pollen, as seen with the equator crossing the grain approximately midway. The categories are:

Pollen shape in equatorial view
Equatorial view shape categories (P = position of pole).

4. Shape in polar view

This is the general outline shape of the pollen, as seen looking down on one pole. The arrangement of apertures commonly affects the outline. Prolate and especially perprolate pollen grains tend to lie on their side, so polar views of these species may be difficult to find. Categories are:

Pollen shape in polar view
Polar view shape categories (P = position of pole).

5. Number of apertures

Furrows containing pores are counted as one aperture. In compound grains (tetrads or polyads), count the number for a single subunit. The individual grains of regular Ericaceae tetrads generally have 3 apertures (combined furrows with pores), but those of irregular tetrads (some Leucopogon species) may have more. The individual grains of Acacia polyads display a single rectangular to circular furrow. The key separates the following categories:

6. Aperture types

These are illustrated in the Introduction.

7. Surface pattern

Surface textures of pollen grains are best seen at high magnification, using 40x or a 100x oil immersion objective and focussing slowly down on to the top of the pollen. Each of the categories includes a range of variations.

Pollen surface pattern
Surface pattern categories. (a) tawari, (Ixerba brexioides);
(b) bidibid, Acaena profundeincisa; (c) ling heather, Calluna vulgaris;
(d) peach, Prunus persica, with detail magnification;
(e) rosemary, Rosmarinus officinalis; (f) wharariki, Phormium cookianum;
(g) coastal astelia, Astelia fragrans; (h) buttercup, Ranunculus sp., with tiny spines
on a granular to smooth background.

8. Exine section

This feature describes the exine as seen in optical section: both the thickness and internal structure, and surface projections such as spines. These two aspects may both be scored.

Exine structure
Exine section categories.

9. Other features

These include aspects of pollen apertures, and of the intine and protoplasm:

Exine structure
Other features (OS = optical section, SV = surface view).

10. Pollen pellet colour

Fresh dry bulk pollen in bee loads from many plants is a shade of yellow to brown and these are difficult to reliably identify on colour alone. However, some is distinctive; for example, the violet pollen of Phacelia (Newstrom-Lloyd et al., 2012), the red of horse chestnut (Aesculus hippocastanum), or the blue of Fuchsia. The colour can change with age of the pollen pellet, gradually becoming browner as the pollen decays. Where known, either from our own research or from Kirk (2006), Sawyer (1981) or Walsh (1978), the fresh bee load colour has been classed in a basic scale:

Cite this publication as: "J.I. Raine, X. Li, L. Newstrom-Lloyd (2022). New Zealand bee pollen database. "
© GNS Science, 2022

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