Life cycle of the higher plants

In gymnosperms (Fig. 13.7) and angiosperms (Fig. 13.8) the gametophyte generation is reduced to a few cells represented by the ovum (or ovule; female) and the pollen grain (male). In gymnosperms the megaspo-rangium (ovule) produces an exposed egg that is fertilized by a free-swimming sperm or one introduced through a pollen tube. Modern angiosperm pollen grains contain a tube cell nucleus (which controls the development of the pollen tube) and a generative cell nucleus, which divides before fertilization. The two generative nuclei effect double fertilization, in which one nucleus unites with the ovum to produce the zygote and the second unites with two subsidiary sexual nuclei of the female gametophyte to produce a triploid (3n chromosomal compliment) endosperm nucleus. This develops into the endosperm that nourishes the zygote within the seed. The flowering plants with which we are all familiar are the sporo-phyte generation.

The vast array of form in flowers and pollen grains reflects adaptations to the many types of pollination mechanism used by the angiosperms. The most common of these is transfer of pollen by insects (entomophily). Wind pollination (anemophily) is important to the palynologist because of the large quantities of pollen produced by these plants. Much of

Anemophily

Growth

Fig. 13.7 Simplified life cycle of a coniferous gymnosperm.

Growth

Fig. 13.7 Simplified life cycle of a coniferous gymnosperm.

Transfer Pollen Grains
Fig. 13.8 Simplified life cycle of an angiosperm.

this contributes to 'pollen rain' and is preserved in sediments. Most wind-pollinated angiosperms produce small oval and smooth pollen grains ranging from 20 to 40 |im in diameter. The air sacs of many gym-nosperms are known to have functioned to increase buoyancy for long-distance wind transport. However, most anemophilous gymnosperm pollen (produced by the Taxaceae, Taxodiaceae, Cupressaceae, cycads and other groups) is oval to spherical, smooth or weakly sculptured, and lacks air sacs.

Pollen morphology

Gymnosperm pollen varies from small, simple, spherical and inaperturate (e.g. modern Juniperus and Cup-ressus pollen) to large bisaccate and ornamented grains (e.g. Abies (Fig. 13.11a) and Pinus (Fig. 13.11b)) and polyplicate forms (e.g. Ephedra; Fig. 13.12q). Saccate pollen is characteristic of the gymnosperms and grains can bear one (monosaccate, e.g. Tsuga; Fig. 13.11c), two (bisaccate, e.g. Abies, Picea, Pinus; Fig. 13.11b; Striatopodocarpites, Fig. 13.13d) or rarely three sacs (trisaccate, e.g. Podocarpus; Fig. 13.11d). Some modern and fossil cycadophytes and ginkgophytes have produced monosulcate pollen (Fig. 13.9). Ginkgo and cycad pollen grains are typically subspherical to ellipsoidal, have a single distal furrow (sulcus) and smooth to scabrate outer surface. The sulcus appears to facilitate size increase during hydration. 'Advanced' gymnosperms (e.g. Gnetum) have ellipsoidal, striate or polyplicate pollen, or spherical grains with short spines.

The range of morphological variation in angio-sperm pollen is considerable and more detailed studies of pollen morphology are available in Erdtman (1986), Traverse (1988), Faegri & Iversen (1989) and Jarzen & Nichols (in Jansonius & McGregor 1996, vol. 3, pp. 2261-293). Angiosperm pollen can be shed singly (monads), in pairs (dyads), in groups of four (tetrads) or in multiples of four (polyads). Individual grains can be inaperturate, or have one or more pores (mono-porate, diporate, triporate, etc.), or slit-like apertures or colpi (monocolpate, tricolpate, etc., Fig. 13.10), or these features can be equatorial (stephanoporate or colpate, Fig. 13.13f) or distributed over the whole grain (peri-). There are numerous variations and combinations of apertureal arrangement. Triprojectate pollen (e.g. the extinct Aquilapollenites Fig. 13.11e) have apertures on three projecting arms. Occulate grains ('Occulata'), typified by the Late Cretaceous to Palaeogene genus Woodhousia (Fig. 13.11f), and have an elongate disc-shaped central body surrounded by a spinose flange.

The wall of the pollen grain comprises two layers, the outer, highly resistant exine and the inner intine that surrounds the cytoplasm (Fig. 13.6c). The exine is divided into two sublayers, the inner endexine and the outer ektexine. The ektexine consists of a basal layer with projecting columellae, these may be free distally (intectate), partially connected by a tectum (semitec-tate) or completely covered (tectate). In pollen grains the clavate condition, by expansion of the tops, may

Proximal pole

Proximal surface

Tetrad

Tetrad

Fig. 13.9 Morphology and terminology of monosulcate and related pollen grains.

Distal pole

Distal -surface

Sulcus'

Distal -surface

Sulcus'

Distal surface

Monosulcate

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