Seed plants
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Scots pine ( Pinus sylvestris ) |
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The seed plants (Spermatophytina) are one of the groups in the plant kingdom . They form seeds as organs of expansion .
features
The seed plants are primarily woody plants with secondary growth , herbaceous growth forms are considered derived. They form seeds that contain the embryo protected in maternal tissue (seed coat) and represent a resting stage as well as the organs of expansion. In contrast to mosses and ferns, fertilization is completely independent of water.
The sporophylls are usually grouped into flowers or cones .
Reproduction
The seed plants are - like all plants - diplohaplons ; d. H. they go through a heterophasic generation change in which diploid and haploid generations alternate (cf. nuclear phase change ). In contrast to ferns and mosses , the haploid generation is very small and usually consists of only a few cells, so that it is hardly noticeable as a separate generation.
The diploid generation is represented by the actual plant, which appears as a tree, shrub or herb. It is called the sporophyte because it corresponds to the spore-forming generation of ferns and mosses. The reproductive organs of the sporophyte are the carpels and the stamens. In these the meiosis , the transition to the haploid core phase, takes place. After meiosis, the embryo sac is formed in the carpel , in which the egg cell is located and which is therefore called the female gametophyte . The pollen grains , inside of which the male gametophyte is located, are produced in the pollen sacs of the stamens . The gametophytes represent the haploid generation.
After pollination , the male gametophyte grows out of the pollen grain as a pollen tube , which grows through the stylus to the embryo sac and there fertilizes the egg , whereby the two sets of chromosomes unite. The diploid embryo emerges from the zygote and is spread as part of the seed.
Systematics
External system
The progymnosperms are now generally accepted as the forerunners of the seed plants . However, there are essentially two theories:
- Rothwell assumes that the seed plants descend monophyletically from an ancestor corresponding to the Aneurophytales .
- Beck assumes that the seed plants originated diphyletically: the seed ferns would therefore descend from the Aneurophytales, Cordaitales and conifers from the Archaeopteridales .
The two hypotheses cannot be verified by molecular genetic studies, since the precursor groups in question have all died out. Molecular genetic studies reveal the ferns as the next related group .
Internal system
There are four recent groups of seed plants. The first three are also summarized as Nacktsamer (Gymnospermae):
- Cycads (Cycadopsida)
- Ginkgo plants (Ginkgoopsida)
- Coniferopsida including Gnetales
- Bedecktsamer (Angiosperme) = flowering plants (Magnoliopsida)
There is also the group of seed ferns , which has only survived in fossil form, but which do not form a natural family group. Their Palaeozoic representatives rather mediate between the ferns and the other seed plants, the Mesozoic representatives are discussed as the forerunners of the Bedecktsamer.
Some groups of fossil naked-seed plants cannot be assigned to any larger group:
- Gigantopteridales
- Vojnovskyales
- Czekanowskiales
- Iraniales
- Pentoxylales
- Hermanophytales
- Dirhopalostachyaceae
- Cordaitales
There is still no agreement on the relationships within the seed plants. Essentially four possibilities are discussed, with the anthophyte hypothesis receiving the least amount of support from molecular biological studies. The hypotheses differ mainly in the different positions of the Gnetophyta in the family tree. Some of them even come to lie within the conifers, so that these would then be paraphyletic.
The anthophyte hypothesis:
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The Gnetales as a sister group of the Bedecktsamer are often the result of phylogenetic studies based on morphological features, which often also include fossil representatives. Molecular genetic investigations usually contradict this.
The Gnetales-Sister-Hypothesis (Gnetales as sister group of all other seed plants):
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The Gnetifer hypothesis (Gnetales as a sister group of the conifers):
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The Gnepine hypothesis (Gnetales as a sister group of the Pinaceae):
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The Gnetales as a sister group of the Pinaceae is a fairly common result of molecular genetic studies. This result is countered by another party that the Gnetales can hardly be derived from modern conifers and that the molecular genetic results are distorted by not including the many extinct groups.
More recent work shows the Gnetales as a sister group of the conifers (without Pinaceae) as a further possibility.
supporting documents
- Peter Sitte , Elmar Weiler , Joachim W. Kadereit , Andreas Bresinsky , Christian Körner : Textbook of botany for universities . Founded by Eduard Strasburger . 35th edition. Spektrum Akademischer Verlag, Heidelberg 2002, ISBN 3-8274-1010-X , p. 750-783 .
Individual evidence
- ↑ a b c Sarah Matthews: Phylogenetic relationships among seed plants: Persistent questions and the limits of molecular data . American Journal of Botany, Volume 96, 2009, pp. 228-236. doi : 10.3732 / ajb.0800178
- ^ A. Bresinsky, Ch. Körner, JW Kadereit, G. Neuhaus, U. Sonnewald: Strasburger - Textbook of Botany . 36th edition, Spektrum Akademischer Verlag, Heidelberg 2008. ISBN 978-3-8274-1455-7
- ^ Thomas N. Taylor, Edith L. Taylor, Michael Krings: Paleobotany. The Biology and Evolution of Fossil Plants . Second Edition, Academic Press 2009, ISBN 978-0-12-373972-8 . P. 757ff.
- ↑ Representation according to the overview in: J. Gordon Burleigh, Sarah Mathews: Phylogenetic signal in nucleotide data from seed plants: implications for resolving the seed plant tree of life . American Journal of Botany, Vol. 91, 2004, pp. 1599-1613. (Abstract and full text)
- ↑ Aljos Farjon: In defense of a conifer taxonomy which recognizes evolution . Taxon, Vol. 56, 2007, pp. 639-641.
