Pollen Morphology

Pollen morphology is one of the most important and fundamental branches of palynology. It will not be an exaggeration if pollen morphology is referred to as the mother of palynological studies. Proper identification of pollen and spores both of living and fossil plants is a prerequisite for exploiting their applications. Pollen morphology is the principle tool used for correct identification. The slightest error in identification leads to an erroneous conclusion.

A recent example in this context which can be cited is concerned with the identification of airborne pollen in Bangalore. Due to lack of proper background knowledge of pollen morphology, Cassia pollen were shown to be the second most abundant pollen in Bangalore's atmosphere in a publication by the Asthma Research Society (1979). However, a systematic aeropalynological survey (Agashe et al., 1994) carried out by sound pollen morphological knowledge, proved that Bangalore's atmosphere was rich in Casuarina equisetifolia pollen, which, since it looks superficially similar to Cassia pollen, was earlier identified incorrectly.

Various features of pollen are studied in pollen morphology. Some of these pollen morphological characters are: symmetry, size and shape, pollen wall, exine stratification, ornamentation, furrows/grooves and apertures. The last three characters seem to be the most important pollen morphological characters useful in basic identification and classification of pollen. A number of terms have been coined to describe various pollen morphological characters. At times, confusion arises as the literature abounds with respect to different terminologies used by different palynologists. However, in the present account the almost worldwide-accepted terminology proposed by Gunnar Erdtman, will be used. The pollen morphological descriptions will be adequately supported by suitable illustrations and examples.

MORPHOLOGY OF ANGIOSPERM POLLEN

Pollen grains, when mature and dehisced, show a range of size, shape and even colour. Colour requires the pollen to be seen en masse, for example the pollen of Willow (Salix) is yellow, Dandelion (Taraxacum) is orange and Willow Herb/Fireweed (Chamaenerion) is grey. Such a colour aspect is of value to beekeepers in identifying pollen source. Most pollen however are yellow when seen en masse on a white background. The range of colours represented en masse is well illustrated by Hodges (1975).

The majority of pollen grains range between 20-30 ^m (microns*). Some are very small, for example Forget me not (Myosotis) whose pollen measures 18 ^m. The largest category where size may exceed 100 ^m is exemplified by Rosebay Willow herb/Fireweed (Chamaenerion) whose large triangular pollen measures c. 150 ^m.

The shape of pollen grains is, like size, variable: round, oval (flattered or elongated), long or triangular, semicircular, boat shaped. Others may have several sides (flat or rounded). It is important to bear in mind when examining pollen grains with a view to their identification, that they are the products of a biological system, which is subject to variation, that a degree of care is sometimes needed to interpret what is seen.

The wall (exine) of pollen grains have apertures - Poplar (Populus) is exceptional in lacking them as shown in Fig. 4.16 -A-C which take the form of simple holes (pores) and furrows (colpi). Pores and furrows may merge or be irregular in appearance.

The exine surface can vary in its structure considerably. It can be smooth, have granules, be striped (striated), have a mesh or network, small holes or pits, or appear dotted (which are the bases of spines more fully seen in the side view.The exine when viewed from the edge, may appear thin or be composed of two or more layers, which may be separated by thick, thin or beaded rods.

Grains are usually simple, separating from the quartet during development in the pollen sac, each having its own complete exine. Some pollen grains are released during dehiscence as quarters - compound grains for example in the Ericaceae and Juncaceae, families. The appearance of the cytoplasm may be as glass (hyaline or granular). Some plant families, for example Asteraceae, show a great variety of forms: grains with large spaces (fenestrae) such as Dandelions (Taraxacum), Hawkweeds (Hieracium) and Sowthistles (Sonchus), whilst others have spines of varying number and length, for example (Senecio). Again, in the Asteraceae family, Mugwort (Artemisia vulgaris) has a three-lobed grain with long rods (tectum) within each lobe separating the exine layers - this grain can be confused with those of Privet (Ligustrum) and grains of Cruciferae, such as Oil Seed Rape (Brassica napus), which are similarly three-lobed. Pollen of the large family of grasses, Poaceae, have the simplest structure, which is more or less round, thin walled with one single pore. Pollen of cultivated cereal grasses

* Pollen grains are measured in microns. (1 micron = 0.001 mm) magnification for measuring size is usually x 400 or x 1000 under the light microscope.

are usually much larger than their wild relatives - being tetraploid or polyploid genetically.

It is unlikely that genetically modified pollen (GM) can be distinguished from the natural non-GM, morphologically. GM treatment is of no significance to the palynologist working with pollen of wild plants as opposed to pollen of cultivated crop plants.

Pollen grains are reproductive cells (male gametophytes). These specialized cells are provided with an extremely hard outer wall (exine) and an inner softer cellulose wall (intine) surrounding the cytoplasm with the vegetative and generative cells (nuclei), and organelles.

The pollen grains may be single (monads) which is most common, or two-pollen united (diads) for example Scheuchzeria palustris or united in fours (tetrads), or many, for example, multiples of fours (polyads). In Orchidaceae, Asclepiaceae and a few other families the grains are united in club-like masses (pollinia) (Fig. 4.1).

With tetrads as a starting point we may discern polarity and symmetry. There are different kinds of tetrads, tetrahedral, rhomboidal, tetragonal, linear, etc. In Dicolyledons, the tetrahedral type (two planes) dominates, in Monocotyledons the tetragonal or rhomboidal type occurs (one plane only) (Figs. 4.2 and 4.3). The tetrad configuration (different types of tetrads) is often linked to the microspore divisions, for example tetrahedral tetrads (simultaneous), and one-plane tetrads (successive). A polar axis, equatorial axis, polar diameter and equatorial diameter, distal pole, proximal pole, distal face and proximal face are distinguished in a pollen grain as shown in Figs. 4.4 and 4.5. With reference to this, it is possible to describe the location of various morphological features of the pollen grains and spores. Pollen grains or spores may have similar poles (isopolar), almost similar (subisopolar), or dissimilar poles (heteropolar), e.g. Lycopodium (Fig. 4.6).

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