A seed consists of an embryo, stored food and a seed coat. The seed habit is the most complex and evolutionary successful method of sexual reproduction. It is found in vascular pIants. Today, seed plants, gymnosperms and angiosperms flowering plants are the most diverse lineage within the vascular plants. Most of this diversity in angiosperms occurred during Cretaceous time. The seed plants have an adaptive advantage. They occur in a wide variety of habitats and dominate today’s flora. This evolutionary success is due to the seed. It is one of the most dramatic innovations during land plant evolution. The origin and evolution of the seed habit was started in late Devonian times about 385 M.
History of seed habit
a) The earliest seed plants, “progymnosperms”, emerged in the late Devonian . Progymnoperm fossils show vegetative morphologies to seed plants. But all proaymnosperms did not have seeds or seed-like structures (ovules or ore-ovules). Archaeopteris spp. was the first modern tree. But it produced spores rather than seeds. However, it exhibited an advanced system of spore production called heterospory. Heterosporous plants produce two sets of specialized spores: megaspores (haploid female-like megaspores) and microspores (haploid male-like microspores). Heterospory, has been evolved independently in several lineages. It is believed to be a precursor to seed reproduction. The progymnosperms are regarded as the ancestors of the seed plants.
b) Fossils of seed-bearing seed ferns (Lyginopteridopsida) exhibit a variety of seed and seed-like structures. The seed’. might have evolved once or several times during evolution.
Conditions for evolution of Seed
Three major evolutionary trends were important for the transition from the seed ferns to the gymnosperms, from the spore to the gymnosperm seed:
1. The evolution from homospory to beterospory. It is connected with this from megasporangia with many spores to Megasporangia. But it has just one functional megaspore.
2. The evolution of the integument. It is a maternal tissue that protects the ovule. Ovule form the integument forms the seed coat.
3. The evolution of pollen-receiving structures. This includes
the transition to water-independence of the pollination/fertilization process (water is required for fern fertilization)
Lineages of seed producing plants
Today there are four major lineages of extant gymnosperm seed plants:
1. Cycadopsida (cycads, “Palmfarne”),
2. Ginkgopsida (ginkgos),
3. Pinopsida/Coniferopsida (conifers, “NadelbAume”),
4. Gnetopsida (gnetophytes).
Extinct gymnosperm groups include the
1. Lyginopteridopsida (seed ferns, Pteridosperms, “Samenfarne”, paraphyletic group including Devonian/Carboniferous Lyginopterids and Carboniferous/Permian Medul losans),
2. Bennettitales (cycadeoids),
3. Gigantopteridales (gigantopterids),
4. Pentoxylales (e.g.Pentoxylon),
5. Caytoniales (e.g. Caytonia),
6. Glossopteridales, Voltziales (e.g. Emporia),
7. Cordaitales (see figure below).
Hypothesis about different lineages
The evolutionary connections between gymnosperm groups are uncertain. Especially the position of the Gnetales is not clear. The different gymnosperm groups are either monophyletic or paraphyletic. Their evolutionary relationships to the angiosperms are unclear. There are two hypotheses about the evolution of seed plants.
a) Anthophyte hypothesis: According to this hypothesis, monophyletic group consisting of angiosperms and gymnosperms. This implies that the Gnetales are sisters of angiosperms. This hypothesis is not supported by molecular data. It places the Gnetales as sister to all remaining seed plants.
b) Gigantopterid hypothesis: According to this hypothesis, Bennettitales (cycadoeoids), are morphologically similar to the Cycadopsida (cycads). They are highly similar to angiosperms. But not to other gymnosperm groups. The gigantopterid hypothesis claims that the angiosperms derived from seed ferns via cycadoeoid-related gymnosperms (gigantopterids). Note that at the present time the issue of seed plant evolution remains unsolved.
Evolution of seed in different groups
The seed replaces the spore of the seed-less fern plants as propagation, dispersal and deposit/outlast/storage unit. Ferns and seed plants both exhibit a life cycle in which two heteromorph generations alternate:
a) Dominant diploid sporophyte, which is the fern or spermatophyte plant (2)
b) Haploid gametophyte: It is the small-sized for ferns. In the seed plants the haploid gametophytes became a hidden generation. It completely depend on the sporophyte. In seed plants the male gametophyte (microgametophyte) is hidden in the pollen grain. The female gametophyte (megagametophyte) is hidden in the ovule. After pollination and fertilization the ovule develops into the seed. Angiosperm and gymnosperm gametophytes and seeds are distinct.
The earliest seeds (Devonian seed ferns)
The oldest fossils of ovules or seeds are from the late Divonian (365 M). The earliest seed plants with seeds or seed-like structures are Devonian seed ferns (Lyginopteridposida. Pteridosperms). Several different types of preovules or preovule-like structures are known. The oldest seed-bearing seed fem (Middle Devonian, 385 M) had a small, radially symmetrical, integumented megasporangium. It was surrounded by a cupule. The megasporangium bears an unopened distal extension. It protruded out above the multilobed integument. This extension was involved in wind pollination (anemophily). In general, a seed is simply a mature ovule containing an embryo.
An immature °yule consists of a diploid megasporangium (nucellus). It contains a singlet functional megaspore that develops into a haploid megagametophyte. The megasporangium is surrounded by diploid covering layers, the integuments (which evolve into the seed coat). An integumentary opening at the apical end is important for pollination, wind pollination in seed ferns. In modern gymnosperms, this opening evolved into the micropyle.
Late Devonian seed
The Late Devonian and Early Carniferous seeds ferns are sometimes collectively called Lyginopterids. These earliest seed-bearing seed plants produced their preovules or ovules on dichotomously branched, sterile structures called cupules. Cupules are cup-like structures that partially enclose the ovule. In these early ovules the nucellus was surrounded by integumentary tissue consisting. The integumentary lobes curved inward at their tips. They form a ring around the apical end. The integuments of the ovules evolved through gradual fusion of the integumentary lobes. The integuments later evolved into the seed coat. An opening that was left at the apicil end, evolved into the mieropyle. In several cases a specialized lagenostome was used for pollen capture. A lagenostome is a funnel-like structure of the nucellus. It projects from the top of the megasporangium. It functions as a trumpet-like pollen-trapping device. The important issue was that pollination and/or fertilization became more water-independent during evolution. It facilitated the diversification of seed plants from Carboniferous through to the present day. So far, embryos have not been found in Devonian seed fern fossils.
Late Permian seed ferns
Seed ferns with angiosperm-like features are known from the late Permin (Paleozoic seed ferns) and early Triassic (Mesozoic seed ferns). It has been proposed the leaf of this series becomes much reduced in size. The seeds attached to the leaf of the plants in later stages are subsequently much smaller. The morphological leaf characters of gigantopterids are more similar to dicotyledonous angiosperms than those of glossopterids. Among several other possibilities, the glossopterids, gigantopterids, Caytoniales, Pentoxylales and Bennettitales have been proposed as ancestors of the angiosperms.
Seed in Gymnosperms
The gymnosperms have ‘naked seeds’, i.e. their ovules and seeds (fertilized ovules) are exposed on the surface of sporophylls. The megagametophyte (female gametophyte, n) develops from the functional megaspore (n) within the nucellus (megasporangium, 2n). The megagametophyte of the gymnosperms is homologous to the megaprothallium (n) of the ferns. It is sometimes called primary endosperm (n). The megagametophyte of seed plants is retained and nourished by the parent plant within the ovule.
Ovule = megagametophyte + megasporangium (nucellus) + integument (seed coat). The megagametophytes of the gymnosperms produce several archegonia (n) with egg cells (n). Fertilization by the sperm from the pollen grain (microgametophyte, male gametophye, n) often leads to the development of several embryos (2n) within a single ovule. Polyembryony of gymnosperm seeds is a known phenomenon. In most cases only one embryo survives. Therefore relatively few fully developed gymnosperm seeds contain more than one embryo.
Seed in Angiosperms
The:r ovules and seeds are enclosed inside the ovary. Ovary is the base of a modified leaf and is called carpel. Another very important difference to gymnosperms is the angiosperm double fertilization. This leads to an additional novel tissue with maternal protuberance, the triploid endosperm. In mature seeds of most angiosperm species, the embryo is enclosed by endosperm tissue. In addition, angiosperm seeds can be dispersed as fruits. The Triassic and Jurassic age was dominated by gymnosperms. The first angiosperms also evolved during at this time. The rapid rise and early diversification of the angiosperms occurred during the Cretaceous time.