Terrestrial palynomorphs -- spores and pollen

UNDER CONSTRUCTION

Introduction

Spores and pollen are produced during the reproduction of land plants. They are the small, mobile cells that enable plants to disperse and interact over long distances. Because they are dispersed in the surrounding environment, they need to be resistant to dessication and consumption by other organisms. To accomplish this, their external wall is composed of sporopollenin, a durable organic polymer, which preserves in the fossil record.

Spores are present in the more ancient plants, like ferns and mosses, while pollen is present in later seed plants like conifers (spruce, pine, cypress) and angiosperms (flowering plants). Spores are usually an asexual form of reproduction for plants, and are released to germinate a new plant when they encounter appropriate conditions. The only exception is large (up to several millimetres) female megaspores and small male microspores in certain types of ancient plants (with a few living examples). Megaspores and microspores foreshadow the development of seeds, where one reproductive cell (the female) is larger, is less mobile, and has nutrient supplies, while the male microspore travels longer distances to fertilize the female and begin the development of a new plant. In pollen-bearing seed plants, the dichotomy is taken further. Pollen is the transported male gamete which fertilizes the attached female portion of a plant to eventually form a seed. This has the advantage of allowing the female and eventually the fertilized zygote to gain nutrition from the originating plant.

Spores and pollen are produced by a particular type of cell division in the originating plant, and as a result they usually have a geometry related to contact in the original cluster of cells. For example, spores often have three-fold markings on their surface, reflecting contact with three other cells within a tetrahedral arrangement of four cells.

Trilete spores

Trilete spores have a tree-fold scar, defining their proximal surface.

All from the Albian (Cretaceous) age Kiowa Formation, Kansas, U.S.A.

?Cyathidites sp.

Note the trilete mark (the three-fold radiating ridges) and the slightly scalloped ornamentation on the proximal surface (same side as the trilete mark).

Cicatricosisporites sp.

Note the striate ornamentation and trilete mark.

Hamulatisporites sp.

This specimen is cracked at lower right.

Monolete spores

Monolete spores have a single linear scar on their proximal surface, and form in groups of four arranged about an axis (versus the tetrahedral arrangement of trilete spores).

Monosulcate pollen

Monosulcate pollen has a linear, thinned, furrow-like area on the distal surface of the grain. Sometimes the margins of the grain fold over the sulcal area, as in this specimen.

monosulcate pollen

Bisaccate pollen

Bisaccate pollen is a form of monosulcate pollen which two large "sacci" filled with spongey material. The sacci protect the sulcus and assist floatation.

Angiosperm pollen

From Maastrichtian (Late Cretaceous) sediments in the Hansen Point Volcanics, near Emma Fiord, Ellesmere Island, Canadian Arctic Islands.

triporate pollen

Angiosperm pollen with three pores arranged equatorially at 120 degree intervals. The wall around the pores is slightly thicknened with respect to the rest of the pollen wall.

Triprojectus unicus enlarged

Pseudointegricorpus sp. Pseudointegricorpus sp.

An unusual type of tricolpate pollen with a projection at each of three apertures/colpii (two are visible at left and right, the third is behind) and two inflated polar projections (top and bottom) with coarse reticulate ornamentation. This extinct group of pollen is called triprojectates and is common in the Late Cretaceous in northern polar areas ("Boreal province").

Wodehouseia gracile

A quadriporate pollen grain with a distinctive ellipsoidal shape, surface spines and a wrinkled flange. This genus is restricted to the Maastrichtian (the last stage of the Cretaceous) and slightly into the Tertiary. It is also found exclusively in northern polar areas.

Authorship

Andrew MacRae, email: macrae@geo.ucalgary.ca

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