Why measure pollen?

Pollen grains come in all sizes. The size of an unidentified pollen grain can sometimes help to narrow down the affiliation to a certain group, family, genus, or even species. Therefore, measuring the size of pollen (grains) and applying the standard size ranges provided under the “Combined Search” option could lead to the appropriate taxon.

How to measure pollen?

It can be complex to measure pollen, therefore, basic knowledge in palynology is helpful. When pollen grains in "liquid" samples are observed with light microscopy, the determination of the dominant pollen orientation is useful. Prolate pollen is usually observed with a view on the equator (equatorial view) and oblate pollen with a view on the pole (polar view). Spheroidal pollen usually do not show a specific orientation and are mostly observed with a view on the pollen in a tilted position (oblique view). How pollen is oriented in a sample, mainly depends on the P/E-ratio, which describes the ratio of the length of the polar axis (P) to the equatorial diameter (E).

Usually, a number of pollen grains from a particular species are measured in optical cross-section to figure out the P/E-ratio. In isodiametric pollen, the polar axis is +/- equal to the equatorial diameter, and the longest and shortest axis are measured in optical cross-section. In prolate and oblate pollen, the polar axis is longer (prolate) or shorter (oblate) than the equatorial diameter and must be measured in both polar and equatorial views. However, the procedure to measure pollen can differ depending on the pollen type, if it is sulcate, saccate, or if it occurs in tetrads. In monocots, pollen can be observed from four different sides (distal polar, proximal polar, and two different equatorial views), due to the mostly distal position of the aperture (sulcus), and each view must therefore be considered when measuring sulcate pollen (Goldblatt et al. 1991; Halbritter et al. 2018, page 40). For saccate pollen and tetrads the measurements are even more complex (e.g., Sivak 1973; Grímsson & Zetter 2011; Halbritter et al. 2018, pages 89-90).

The size of pollen can vary due to natural variation within a single anther/flower or between anthers/flowers/populations etc. of a single taxon, but they are also depending on the preparation and methods used. For example, there is a considerable size difference between hydrated pollen observed with LM compared to SEM, as dehydration and critical point drying influence the state of pollen hydration.

How to measure pollen for PalDat

In order to differentiate pollen grains based on their size, the measurements for PalDat are simplified. For practical reasons, the measuring fields are named differently (see also “Instructions for authors”). To obtain uniform measured values for PalDat, a description "How to measure pollen for PalDat" is provided in the “Instructions for authors”.

References:

• Goldblatt P, Manning JC, Bari A. 1991. Sulcus and operculum structure in the pollen grains of Iridaceae subfamily Ixioideae. Annals of the Missouri Botanical Garden, 78: 950-961
• Halbritter H, Ulrich S, Grímsson F, Weber M, Zetter R, Hesse M, Buchner R, Svojtka M, Frosch-Radivo A. 2018. Illustrated pollen terminology. 2nd ed. Vienna, Austria: Springer.
• Grímsson F, Zetter R. 2011. Combined LM and SEM study of the middle Miocene (Sarmatian) palynoflora from the Lavanttal Basin: Part II. Pinophyta (Cupressaceae, Pinaceae and Sciadopityaceae). Grana, 50:262-310
• Sivak, J. 1973. Observations nouvelles sur les grains de pollen de Tsuga. Pollen et Spores, 15: 397–457.