Plants

Spores and pollen

The study of spores and pollen used to reconstruct long-term vegetative changes is called palynology. Spores and pollen are part of the plant reproductive system. As they are very resistant and vast numbers are dispersed over wide areas, they are important in biostratigraphy. They can also be useful paleo-environmental indicators, particularly for the Quaternary.

Morphology

Spores and pollen grains are very distinctive. In general, pollen grains are smaller than spores: 25-35 |lm compared with 100-200 |m in diameter. Large spores, megaspores, can be up to 4 mm in diameter. Most spores and pollens have a double-wall structure and the outer wall is very robust in order to resist desiccation and prevent microbial attack. Wall sculpture is variable. The surface can be granulated, pitted, or ornamented with rod-like projections. Apertures within the wall allow for germination of the pollen or spore and also accommodate size changes in response to variations in humidity.

Shape and aperture type form the basis of pollen and spore identification. Four main types can be identified on the basis of aperture shape (Table 13.3).

Pollen analysis

Examination of the pollen content of layered sediments, particularly from lakes and peat deposits, shows changes in regional vegetation through time. Pollen diagrams are used to quantify this information and to record the number or percentage of grains at each selected level (Fig. 13.16). Through

Table 13.3 Important groups of spores and pollen.

Abundance Low High

Abundance Low High

Fig. 13.16 Pollen record from Grand Pile, France. The white area represents the relative abundance of tree pollen, and the black bars show oak pollen (Quercus). The presence of oak pollen identifies the last interglacial period.

this method, regional and local pollen assemblage zones are established. These can then be used to recognize and date particular events within an area: for example, interglacial periods in the Quaternary and the effects of early man on the environment.

Evolutionary history

True spores first appeared in the early Silurian. At this time land plants were probably starting to colonize marginal marine environments. During late Silurian and early Devonian times there was an explosive radiation of land plants and diversification of spore types. Small herbaceous, seedless plants dominated the flora, some with distinctive megaspores. The first seed-bearing ferns appeared in the mid-Carboniferous and a range of pollen types was established. Classic Carboniferous swamp vegetation is known from extensive coal deposits of the period. Spores are much less abundant and pollen more common in the Permian. The decrease in spore-bearing plants has been linked with drier conditions and possible global cooling during late Palaeozoic times.

The gymnosperms dominated the Triassic and Jurassic flora. Angiosperms, flowering plants, first appeared in the early Cretaceous and gradually displaced the gymnosperms, becoming the dominant pollen producers by late Cretaceous times. The end-Cretaceous flora is similar to modern plant life although grasslands did not appear until Cenozoic times. For more information on plant evolution see Chapter 12.

Table 13.3 Important groups of spores and pollen.

Spore/pollen

Description

Appearance

Trilete spore, e.g., ferns

Tetrahedral spores with Y-shaped aperture (trilete mark)

Monoporate pollen, e.g., grasses

Spherical grain with single, rounded aperture

o

Triporate pollen, e.g., birch

Spherical to tetrahedral grain with three, equally spaced equatorial apertures

0

Saccate pollen, e.g., pines

Distinctive elongate grain with at least one spherical vesicle

cÄ)

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