Propagation of the conifers

Conifers (conifers) are seed plants . Their ovules are openly accessible, not enclosed in an ovary. They are therefore more naked (gymnosperms). Typical of the conifers are the cones that are lignified when they are ripe, with the seeds between the scales. In the yew family (Taxaceae) and head yew ( Cephalotaxus ), however, they are modified beyond recognition. The peculiarities of the propagation of the conifers from the flowering to the formation and spreading of the seeds areshown below usingthe example of the Scots pine ( Pinus sylvestris ); other softwoods are briefly referred to.

Construction of the male flowers

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  Branch of a pine with male flowers

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  Branch with male flowers, shown schematically

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  Schematic representation of a male flower

Like almost all conifers, the pines are separate sexes and monoecious , i. H. there are male flowers and female flowers on one plant. The cone-like male flowers are in large numbers at the base of new long shoots. They fall off after the flowering period. When this long shoot sprouts later, the branch has a bald spot there.

A male flower consists of a central axis around which the stamens are spirally arranged. Each stamen has two pollen sacs on its underside, in which large quantities of pollen grains are formed. The pollen grains develop from pollen cells, which in turn have arisen from pollen mother cells through meiosis . So you only have one set of chromosomes; they are haploid . The partial detachment of the outermost layer of the exine (resistant outer layer around the pollen grain) creates two air sacs on each of the pollen grains, which are used for spreading by the wind and probably also for correct attachment to the micropyle of the ovule. So much pollen is released during flowering that it often forms a yellow layer on puddles; the vernacular speaks of sulfur rain.

Structure of the female flowers and inflorescences

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  Female cones of a pine

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  Female cone, shown schematically

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  Female cone scale

Female cones stand individually upright at the tip of a new shoot. On the pin axis are cover scales, which are arranged spirally around the spindle in the jaw. In the axilla of each cover scale there is a smaller seed scale that bears two ovules and that represents a side shoot that has been transformed into a female flower. The whole cone is therefore unlike the male cone, an inflorescence with many flowers.

The position in the axilla of the covering scale (= bract) speaks for the homology of the seed scale with a side shoot, since side shoots always arise from leaf axils in seed plants (homology criterion of the location). On the other hand, the extinct Voltcials , who are considered the ancestors of the conifers, had basically similar female inflorescences, but instead of the seed scales, a side shoot with several scale leaves and some stalked ovules (homology criterion of continuity).

Since the ovules are freely accessible on the seed scales and are not enclosed in an ovary, the pines, like all conifers, are naked. (Opposite: covers more)

The pollination

Pollination occurs by the wind. The pines produce so much pollen that some pollen grains get between the scales of the female cones, which are wide open during flowering, and slide all the way down to the basal end of the ovules. With many conifers, certain surface structures on the cover and seed scales facilitate this process and the pollen grains slide on prefabricated tracks to the right place. There, an excreted droplet of pollination and pincer-like extensions of the ovules ensure that the pollen grain is held in place and is in the right position to allow a pollen tube to grow inside the ovule to form the egg. But this will not happen until next spring. With the accumulation of the pollen grain, pollination is complete. (In the case of angiosperms , however, the pollen to a specific organ outwardly, the scar on the stylus is transmitted.)

The fertilization

After pollination, the pollen grain grows into a pollen tube that penetrates the ovule. In the case of the pine, this does not happen until the following spring.

The pollen tube draws the nutrients necessary for growth from the tissue of the ovule and continues to grow until its front end has come close to an egg cell. It consists of several cells, including two sperm cells , but only one of them is fertilized. Each ovule is surrounded by a shell (integument) that encloses the so-called nucellus. Meiosis takes place in a recognizable larger, special cell inside the nucellus, the embryo sac mother cell; only one of the four resulting haploid cells remains and develops into a multicellular so-called embryo sac. In this, several archegonia with one egg cell each arise . If this fuses with a sperm cell from a pollen tube, this process represents fertilization . The fertilized egg cell (zygote) and the cells that develop from it have two sets of chromosomes , one from the egg and one from the sperm cell, they are diploid . Only one fertilized egg cell per ovule develops into an embryo.

The formation of the seeds and the ripening of the cones

The embryo , which has several cotyledons and a radicle, is created by dividing the fertilized egg cell . The haploid embryo sac becomes the nutrient tissue that surrounds the embryo in the semen (primary endosperm). Hardly anything remains of the nucellus tissue until the seeds mature, only the outermost layers together with the integument become a hard seed coat. The wings, which are important for the spread of the wind, are formed by the seed scale. After pollination, the cones close by placing the scales close together and sticking them together with resin. In this way, the seeds can develop completely protected until they are mature. During this time, the seed scales also grow to their final size, while the cover scales of the pines wither completely.

Open pine cone

The ripe cones with the finished, winged seeds open when dry and close when wet. This ensures that the winged seeds only fall out when the weather is favorable and are spread by the wind. Later the cones fall off. Some of the seeds are spread by animals, e.g. B. Woodpeckers and squirrels that open the cones, eat some of the seeds and drop the rest or hide them as food supplies and forget some. Conifers that are fully adapted to animal distribution have large, wingless seeds.

While many other conifers form ripe seeds within one growing season, the pine needs a surprisingly long time to do so: In the first year pollination takes place, a year later fertilization and in the third year the seeds are ripe.

The opening and closing of the tenons

Closed pine cone

Like many other conifer cones, the mature pine cones open when dry and close when wet. At this point in time the cone scales consist only of dead wood and the movements are purely passive. Only the basal part of the scales close to the pin axis is responsible for the movement, the rest of the scales are carried along.

Such hygroscopic movements generally come about because water molecules attach to polar or charged groups of the elongated macromolecules (lignin, cellulose, hemicellulose) contained therein . Since they are arranged in bundles, they are pushed apart laterally and they swell. In contrast, the fiber bundles do not enlarge in their longitudinal direction. Therefore, it depends on the amount and the orientation of these swellable components in which way the plant material expands through water retention.

The thick cell walls on the underside of the scales (= outside) expand much more when wet than the top of the scales (= inside). This results in an inward curvature of the scales and the pin closes. When it is dry, the reverse process takes place, the water is released again, the scales bend outwards and the pin opens. The process can still be observed on cones that have long since fallen away and are many years old and can be repeated as often as required. If you put dry cones in water, it takes about two hours for them to close.

But there are also types of pine whose ripe cones remain closed on the tree for years, e.g. B. Pitch Pine in New Jersey, USA. Their cones only open at temperatures that can only be reached in a forest fire. In this way, the seeds fall on a fertilized soil free of twigs and leaves and thus have good germination and growth conditions.

The cones of other conifers

Cones of a false cypress

Although the basic structure is always the same, the mature cones differ so much in size, shape, scale arrangement, presence and size of the cover scales that they can often be clearly assigned to their tree species and are definitely a good identification aid.

The cover scales can, as with the pine, completely recede during the ripening period or be fused with the seed scales, as with most cypress plants (Cupressaceae). Their scales are shield-shaped and gape permanently apart in the ripe cone.

Often the cover scales remain small and lie close to the seed scales so that they cannot be seen from the outside on the intact cone, e.g. B. in the spruce ( Picea ) and in our European larch ( Larix decidua ).

Douglas fir cones, long, pointed cover scales between the seed scales

In some species the cover scales are so large that they protrude between the seed scales, e.g. B. in the genera Douglas fir ( Pseudotsuga ).

Firs ( Abies ) and cedars ( Cedrus ) do not shed their cones, but the scales gradually detach from the cone spindle. This releases the seeds.

In the juniper ( Juniperus ), some scales become fleshy and a berry cone develops. The seeds are dispersed by birds that eat these fruits and excrete the seeds again.

In the case of the dioecious yew trees (Taxaceae), finally, there is no recognizable cone and an ovule is located at the end of a tiny side branch in a bud-like short shoot. This has most likely developed from a pin, as u. a. the somewhat more cone-like inflorescence of the related head yeast (Cephalotaxaceae) suggests. The male flowers are in cone-like inflorescences and have shield-shaped stamens with 6-8 anthers in the genus Taxus. The relatively large seed is covered by an aril, which is bright red in our native yew ( Taxus baccata ) and grows around the seed from below as a ring-shaped bulge. The aril is the only part of the yew that is not poisonous.

Notes on terminology

In the past, people did not hesitate to call the seed scales of the cones carpels. However, since the carpels that form the ovaries of the angiosperms are homologous to leaves, but the seed scales are homologous to side shoots, this term is no longer used. The cone is also not called a fruit.

However, it has not caught on to use the same technical terms as for the generation change of mosses and ferns .

The term cones is usually defined as follows or similarly: "Cones are the female inflorescences of alder and conifers. They are ears of wheat whose axis and bracts lignify when they ripen." While not falling under this definition, the male coniferous flowers are also described as cones, or at least cone-like. The juniper berries and the flowers and seeds with the associated side shoots on the yew trees are also increasingly called this.

General literature

• Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: Strasburger Textbook of Botany. 36th edition. Springer Spectrum, 2008, ISBN 978-3-8274-1455-7 .
• Jost Fitschen: Woody flora . 12th revised and expanded edition. Edited by Franz H. Meyer et al. Quelle and Meyer Verlag, Wiebelsheim 2007, ISBN 978-3-494-01422-7 .
• Alan Mitchell: The forest and park trees of Europe, an identification book for dendrologists and nature lovers. Translated and edited by Gerd Krüssmann. 2nd Edition. Paul Parey Publishing House, Hamburg / Berlin 1979, ISBN 3-490-05918-2 .

Web links

Commons : conifer cones  - collection of images, videos, and audio files

• Information about cones at kiefernspezi.npage.de

Individual evidence

1. ↑ Joachim W. Kadereit: Chapter 10.2.1. Fourth subdivision: spermatophytina, seed plants . In Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: Strasburger Textbook of Botany . 36th edition. Springer Spectrum, 2008, ISBN 978-3-8274-1455-7 , pp. 806-807.
2. ↑ Joachim W. Kadereit: Chapter 10.2.1. Fourth subdivision: spermatophytina, seed plants . In Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: Strasburger Textbook of Botany . 36th edition. Springer Spectrum, 2008, ISBN 978-3-8274-1455-7 , pp. 836-837.
3. ^ Gunther Neuhaus: Part I structure . In Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: Strasburger Textbook of Botany , 36th edition. Springer Spectrum, 2008, ISBN 978-3-8274-1455-7 , pp. 155-157.
4. ↑ Adolf Remane, Volker Storch, Ulrich Welsch: Evolution, facts and problems . 3rd revised and expanded edition. dtv, Munich 1976, ISBN 3-423-04234-6 , pp. 45-47.
5. ↑ HCD de Witt: Knaurs plant kingdom in color, the first Vol Higher Plants I. . Droemersche Verlagsanstalt, Zurich 1964, p. 18.
6. ↑ Joachim W. Kadereit: Chapter 10.2.1. Fourth subdivision: spermatophytina, seed plants . In Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: Strasburger Textbook of Botany . 36th edition. Springer Spectrum, 2008, ISBN 978-3-8274-1455-7 , p. 824.
7. ↑ DESCENT WITH MODIFICATION "Transformational SERIES, AND ANALYZES TO phylogenetic INFER THE EVOLUTION OF MODERN CONIFER FAMILIES. 2009 Portland GSA Annual Meeting, accessed January 31, 2013 . 
8. ^ Neil A. Campbell: Biology . German Translation ed. by Jürgen Markl. Spektrum, Akad. Verlag, Berlin / Heidelberg / Oxford 1997, ISBN 3-8274-0032-5 , p. 614.
9. ↑ Joachim W. Kadereit: Chapter 10.2.1. Fourth subdivision: spermatophytina, seed plants . In Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, Gunther Neuhaus, Uwe Sonnewald: Strasburger Textbook of Botany . 36th edition. Springer Spectrum, 2008, ISBN 978-3-8274-1455-7 , pp. 825 and 828.
10. ↑ Jost Fitschen (abbreviation): Wood flora . 12th edition. edited by Franz H. Meyer et al. Quelle & Meyer Verlag, Wiebelsheim 2007, ISBN 978-3-494-01422-7 , p. 648.
11. ↑ Water, diffusion, osmosis and intracellular motility. (PDF) University of Marburg, accessed on January 31, 2013 . 
12. ↑ Experiment 9: Opening and closing movements of everlasting flowers and cones. (No longer available online.) University of Tübingen - Botanical Institute - Plant Physiology, archived from the original on January 1, 2005 ; accessed on January 31, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
13. ↑ Conifer Cornes. (No longer available online.) Dr. T. Ombrello - UCC Biology Department, archived from the original on January 29, 2013 ; accessed on January 31, 2013 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.  
14. ^ Jost Fitschen: Woody flora . 12th revised and expanded edition. edited by Franz H. Meyer et al. Quelle and Meyer Verlag, Wiebelsheim 2007.
15. ^ Alan Mitchell: The forest and park trees of Europe, an identification book for dendrologists and nature lovers . translated and edited by Gerd Krüssmann, 2nd edition. Paul Parey Publishing House, Hamburg / Berlin 1979, ISBN 3-490-05918-2 .
16. ↑ http://delta-intkey.com/gymno/www/taxaceae.htm
17. ^ Wilhelm Nultsch: General botany . Short textbook for physicians and scientists. 3. Edition. Georg Thieme Verlag, Stuttgart 1968.
18. ^ Karlheinz Senghas and Siegmund Seybold: Schmeil-Fitschen flora of Germany and neighboring countries . 92nd revised edition. Quelle & Meyer Verlag, Wiebelsheim 2003, ISBN 3-494-01328-4 .