Placenta (botany)

from Wikipedia, the free encyclopedia
a Parietal (cross-section)
b Central angular (cross-section)
c Central placentation (longitudinal section)

The placenta is in the flowering plant , the point at which the ovule with its stem ( funiculus ) at the carpel has grown. The position of the placenta in the ovary or in the carpel is very important for systematic differentiation between plants.

Tomato cut lengthways and crossways. The placenta stands out in light pink.

The arrangement of the ovules on the carpel or in the ovary is called placentation . The ovules are always on the adaxial side of the carpel. In the original species, the point of attachment is not or barely differentiated from the rest of the carpel, the placenta is then simply a point. In most species, however, the point of attachment is a raised cushion or a longitudinal rib, or a complex tissue structure that can fill the entire ovary with the ovules.

Position to the carpels or in the ovary

There are several types of placentation in individual carpels and fused (coenocarpic) ovaries:

  • Laminal , laminar or superficial, planar placentation (with and without septum): here the ovules sit on the inner surface of the carpel, i.e. on the ovary and / or septum. Laminal placentation is considered a primordial feature and occurs in Butomus , Hydrocharis, and Nymphaea .
  • Marginal , marginal placentation (with and without partitions): here the ovules sit in rows on the edge of the carpel, near the seam of the carpels. Since the ovules are never completely on the edge, their position is actually submarginal , but the term marginal is widely used. The representatives of the Degeneriaceae and Winteraceae have transitional forms from laminal to marginal placentation.
  • Parietal , parietal placentation there in ovary with (paretial-septate, some aizoaceae ) and without septa (partitions). The edges of the carpels with the placentas extend more or less far into this cavity. The shape occurs, for example, in the pumpkin family .
  • Ovules medium (central)
    • Central angled , ( axial ) (axile) placentation (with partitions) occurs when several syncarpic carpels have a central placentation. The ovules are then, with more than two chambers , in the corners, in the central corners of the septum of the ovary. If there are only two chambers , they are on a central column in the partition. It is considered an original form of placentation and is widely used.
      • A false central-angular placentation is also possible, as in the Cucurbitaceae , here the parietal placentas grow into the middle and sometimes meet in the middle.
    • Central (axillary, free-central) placentation (without partitions), here the placentas are located on an ingrowth or a central column that protrudes from below into the ovary. It occurs mainly in two clans, but where it arises differently: in the primrose family , the central column arises from the center of the flower apex, independently of the carpels. In the clove family , the septa are still present in young ovaries as in the central-angled placentation, but they dissolve in the course of growth, so that only the central column of the septa is left in the mature ovary. One can also speak of an off- axis position here , especially if the column is continuous.
  • Basal , basilar: In species with a reduced number of ovules, the individual ovule is often at the lower end of the carpel and appears to arise from the axis, but in reality from the carpel. Examples are anemone or potentilla as well as the Polygonaceae .
  • A single ovule can seldom hang at the upper end: apical or subapical of the carpel; some gourds .

The partitions can each be formed by real or false, as well as incomplete.

Mixed forms are also possible, such as the superimposed ovules of the pomegranate , as well as paretial-axile ; in the corners of the partitions to the ovary wall of a multi-chambered ovary ( cruciferous ) or apical-axile ; above in the fan corners of a multi-chambered ovary ( umbelliferae ).

The Cactaceae usually have a special “hypanthial placentation”. Because here the septa are only briefly ridge-like at the top of the ovary. The usual (basal) paretial placentation is then cup-shaped at the bottom, distributed in the ovary and usually has no connection with the septa.

Some authors still differentiate whether the ovules in a placenta are attached in one or more longitudinal series or in an unclear order (linear or diffuse).

The terms are sometimes used slightly differently by different authors.

literature

  • Arthur J. Eames: Morphology of the Angiosperms. McGraw Hill, New York 1961, pp. 204-216, 234-239, archive.org .
  • Peter Leins: blossom and fruit. Morphology, history of development, phylogeny, function, ecology. E. Schweizerbart'sche Verlagsbuchhandlung, Stuttgart 2000, ISBN 3-510-65194-4 , p. 92 f.
  • G. Czihak, H. Langer, H. Ziegler: Biology. Springer, 1976, ISBN 3-642-96096-0 , p. 218, limited preview in the Google book search.
  • Richard Wettstein : Handbook of Systematic Botany. 3. Edition. Deuticke, 1924, p. 485 f, online at biolib.de, accessed on May 20, 2018.

Individual evidence

  1. Gurcharan Singh: Plant Systematics. Third Edition, Science Publishers, CRC Press, 2010, ISBN 978-1-4398-4363-5 , p. 78.
  2. Reinhard Lieberei, Christoph Reisdorff: Useful plants. 8th edition. Thieme, 2012, ISBN 978-3-13-530408-3 , p. 35.
  3. ^ Michael G. Simpson: Plant Systematics. Academic Press, 2006, ISBN 0-12-644460-9 , p. 378, limited preview in Google Book Search.
  4. ^ HD Behnke, TJ Mabry: Caryophyllales. Springer, 2012, ISBN 978-3-642-78222-0 , p. 309.
  5. Peter K. Endress: Diversity and Evolutionary Biology of Tropical Flowers. Cambridge Univ. Press, 1994, 1998, ISBN 0-521-42088-1 , pp. 68 ff.