Life cycles of lower land plants

Primitive vascular land plants differ from their algal ancestors in their development of special conducting vascular tissues. Nonetheless, the alternation of generations found in the life cycle of the algae was inherited by the vascular plants. This comprises a life cycle alternating between a spore-producing sporophyte generation (reproducing asexually with spores) and a gamete-producing gametophyte generation that reproduces sexually with male and female gametes (Fig. 13.1).

Bryophyta (mosses, liverworts and hornworts) appear to have an organization intermediate between the green algae and vascular plants, the Tracheophyta. The sporophyte generation is small and totally dependent upon the much larger leaf-bearing gametophyte. The haploid gametophyte contains half the chromoso mal compliment (1n) of the diploid (2n) sporophyte, and is the typical moss or thallose liverwort that is commonly found in damp habitats; it bears apical male (antheridia) and female (archegonia) reproductive organs. The motile biflagellate sperm swims through a film of water to the archegonia to fertilize the egg. The zygote is the first stage in the sporophyte generation and grows by mitosis into a slender stalk capped by a terminal fruiting body, the sporangium. Spore mother cells produced in the sporangium divide by meiosis to produce tetrads of four spores; thus, each spore is again haploid. The spores are ejected explosively from the ripe sporangium and germinate in damp habitats, growing to form the prostrate pro-tonema, thus completing the life cycle. Bryophytes adapted to dry environments have spores with thick walls capable of long periods of dormancy.

The term pteridophyta has no natural classificatory significance but is used here to define the ferns (the Pterophyta) and fern allies (the Psilophyta, Lycopod-ophyta and Sphenophyta). In these plants, and also the higher pollen and seed-producing Tracheophyta, the sporophyte is much larger and predominates over the gametophyte. Some pteridophytes (e.g. most ferns and some lycopsids) are homosporous, producing one type of spore. Other pteridophytes are hetero-sporous producing a male microspore and a much larger female megaspore (e.g. Tuberculatisporites, Fig. 13.13c). In fossil assemblages it can sometimes be difficult to distinguish the two spore types, particularly in Devonian assemblages (e.g. Scott & Hemsley in Jansonius & McGregor 1996, vol. 2, pp. 629-641). The term miospore is used to include all spores less than 200 |im in diameter. Heterosporous plants (Fig. 13.2) include the extant fern orders Marsileales and Salvinales

Sporangium

Spore tetrad

Microspore

Fig. 13.1 Reconstructed life cycle of a homosporous plant, the Devonian psilopsid Rhynia.

Sporangium

Spore tetrad

Microspore

Fig. 13.1 Reconstructed life cycle of a homosporous plant, the Devonian psilopsid Rhynia.

Tetrads Spore

Fertilization

Gametophyte Egg Ç cf Sperm

Fertilization

Cone

Sporophyte

Cone

Spore mother cells

Tip of cone

Gametophyte j Sperm

Spore mother cells

Tip of cone

Moisture

Gametophyte j Sperm

To fossil record

Ç Gametophyte

Moisture

Growth

Fig. 13.2 Reconstructed life cycle of a heterosporous plant, the Carboniferous lycopsid Lepidodendron.

Growth

Fig. 13.2 Reconstructed life cycle of a heterosporous plant, the Carboniferous lycopsid Lepidodendron.

and four orders of the Division Lycopodophyta (including the extinct Lepidodendrales and the living 'club-mosses', the Isoetales and Selaginellales).

Spore morphology

The morphology of spores can be described according to their shape, apertures, wall structure and size. The shape of a spore owes much to the nature of the meiotic divisions of the spore mother cell. In simultaneous meiosis, the mother cell splits into a tetrad consisting of four smaller cells. In tetrahedral tetrads each of the four spores is in contact with all three of its neighbours on the proximal face (Figs 13.3-13.5). The proximal face is characterized by three contact areas that are defined by a Y-mark or trilete mark centred on the proximal pole. The arms of the trilete mark may extend to the equator and can take the form of raised ridges or fissures in the surface, laesurae (Fig. 13.4). The exterior surface of the spore in the tetrad is the distal polar face. In successive stages of meiosis, the mother cell divides at first into two cells, these subdivide further along a single plane at right angles to the first division, or along two planes at right angles (Fig. 13.3). The tetrads here are tetragonal and may resemble the segments of an orange in shape; each spore is only in contact with two of its neighbours and only has two contact areas and a single scar. These spores are often bean-shaped. Spores are most commonly compressed proximo-distally in fossil material. The equatorial contour is called the amb.

The spores of vascular plants are characterized by well-formed and consistently placed germinal apertures. These allow ready germination of the prothallus and accommodate size changes caused by fluctuations in humidity. The form and position of these apertures are important in describing and classifying fossil spores (and pollen).

Trilete spores have three laesurae, which radiate 120 degrees from the proximal pole (Fig. 13.4). The symmetry of trilete spores is therefore radial, but heteropolar, i.e. with differently formed polar faces. Monolete spores tend to be less common, although they may be abundant in Palaeogene-Recent assemblages and only have one proximal laesura (the monolete mark) which separates the contact areas (Fig. 13.5). The symmetry of monolete spores is therefore bilateral and heteropolar. Some spores that bear tetrad scars but lack laesurae possess a hilum. This can be developed on either the proximal or distal faces and

Bilateral symmetry

- Contact face

Radial symmetry

Monolete mark -

Contact face'

| Dispersal

| Dispersal

Tetragonal tetrad

Contact face'

Trilete mark

Dispersal

Trilete mark

Dispersal

Tetrahedral tetrad

Spore mother cell

Fig. 13.3 Meiosis and the production of bilaterally or radially symmetrical spores.

Equatorial view

Proximal pole Proximal surface,.

Proximal view

Distal surface'

Equator

Distal pole t

Polar axis

Equator

Distal pole t

Polar axis

Contact area on proximal surface

Equator (amb)

Circular

Circular

Convex triangular

Straight triangular

Concave triangular

Contact area on proximal surface

Equator (amb)

Laesurae

Triangular polysaccate

Corona

Kyrtome

Corona

Valvae

Kyrtome

Concave triangular

Triangular polysaccate

Valvae

Auriculae

Auriculae

Clngulum

Clngulum

Triangular, with equatorial zone

Fig. 13.4 Morphology and terminology of trilete spores.

Triangular, with equatorial zone

Clngullzonate

Clngullzonate

Fig. 13.4 Morphology and terminology of trilete spores.

Equatorial view

Proximal pole

Equatorial axis-1 Equator'

Distal pole

Polar axis

Proximal view

Equator (amb) Equatorial plane

Laesura

Contact area on proximal surface

Fig. 13.5 Morphology and terminology of monolete spores.

Reniform

Reniform

Shape

Circular

Circular

Ovate

Ovate

Contact area on proximal surface

Boat-shaped

Lycopodium

Anemia (Anemia)

Exine

Ektexine-Endexine-

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