In plants, cross- pollination or indirect , heteroclinic pollination refers to the transfer of pollen from one flower to the stigma of another flower of the same (neighboring fertilization, pollination, geitonogamy ) or another plant of the same species (cross-pollination, xenogamy), (from ancient Greek ξένος xénos " Guest; stranger "and γάμος gámos " marriage, marriage; marriage "). Pollen can be transported within plants by wind , water or animals (e.g. insects, bats, birds).
A distinction is made in cross-pollination (xenogamy):
- Cross between different individuals of the same variety; (isomorphic xenogamy)
- Cross between different individuals of unequal variety; (heteromorphic xenogamy) Blendling pollination (mongrel production, nothogamy ).
Cross-pollination as fertilization with the participation of pollen that is transferred to another flower is a sub-area of cross- fertilization or allogamy . This includes not only flowering plants, but also plants without flower formation, which reproduce, for example, via germ cells or spores, e.g. B. algae and ferns . In cross-fertilization, the genome of the mother plant and the father plant is recombined. The aim of cross-pollination is to increase the likelihood of such genetic recombination . Cross pollination was discovered in 1790 by the theologian and botanist Christian Konrad Sprengel, among other things on the narrow-leaved willowherb .
A distinction must be made between cross-pollination, cross-pollination of the second type (heteromorphic xenogamy) and gnesiogamy ( exogamy , heterogeneous pollination) and neighboring pollination ( geitonogamy ) and sibling pollination ( endogamy , autogenetic pollination). In neighboring pollination, pollen is transferred from the flower of one plant to the stigma of a flower of the same plant. Neighbor pollination is therefore genetically equivalent to self-pollination (autogamy), since there is no distribution and rearrangement of genetic material.
The cross pollination takes place in the opened flowers ( Chasmogamy ). If allogamy leads to fertilization, this is called allocarpy , in geitonogamy geito (e) nokarpy and in xenogamy xenocarpy .
Types of cross-pollination
- Abiotic pollination
- Biotic pollination
The animal pollination is further subdivided depending on the type of pollinating animal. The most common animal pollination in the area is that of insects. The insect pollination can continue to Fliegenblütigkeit , Bienenblütigkeit , Tagfalterblütigkeit and more differentiated. In the tropics, pollination by birds and bats is important.
The means by which plants attract their pollinators are varied. Many insect-pollinated plants are pollinated by nectar and / or pollen- collecting insects such as bees , bumblebees , butterflies or hover flies. These are usually attracted by a large and vividly colored flower envelope . Often the flower is designed dorsiventrally . If nectar and fragrances are present, one speaks of nectar flowers ; if they are missing, the plant is called pollen flower . In adaptation to insect pollination, the stamens are often shorter and the stigmas are not very divided. Plants have also developed certain characteristics of adaptation with regard to the main pollinators. In plants pollinated by butterflies, for example, the nectar is often at the bottom of long tubes, while in plants that are mainly pollinated by flies, shallow nectaries dominate. A characteristic mushroom or carrion odor is typical here . If moths are the main pollinators, the flowers often only open in the evening. The flowers are mostly inconspicuous in color, but have an intense fragrance.
Orchids in particular have developed special mechanisms to attract pollinator insects. Some species do not offer nectar, but rather imitate flowers of other plants that offer nectar through the shape and color of the bracts. Some orchid species attract the males of certain insect species with pheromones and induce them to copulate (for example the ragweed species) → deceptive flower .
Kettle trap flowers , for example , have aristolochia , yellow lady's slipper and arum . Due to the special structure of the flower, small insects get into a bowl-shaped extension of the flower or, in the case of arum, the spathe and can only leave this again through devices such as trap hairs when pollination has taken place.
Flowers that are pollinated by birds, so-called bird flowers , are often conspicuously red in color, as insects cannot see this color.
While the animals collect the nectar, they are powdered with pollen. If they fly to the next flower, the pollen will stick to their stigma.
With wind pollination , the pollen is transmitted by the wind and accidentally falls onto the stigma of another flower.
Wind-pollinated plants often have inconspicuous flower covers, or these are completely absent. Nectar and fragrances are not produced. The flowers are often grouped in multi-flowered, often unisexual inflorescences . The abundant pollen is produced on often long filaments that move in the wind. Pollen cement is usually missing. The scars are large and severely divided.
The rare water pollination occurs in some plants that grow submerged or on the surface of the water. The pollen can be transported above or below the water surface.
Mechanisms for promoting cross-pollination
In many types of plants, devices have been developed to prevent the flowers from self-pollinating . The most common are:
When Vormännlichkeit ( Proterandrie ) the empty anthers the pollen before the stigma of the same flower is ready to conceive. This occurs, for example, with the composites , sage , bellflower and corn .
In the case of pre-femininity ( proterogyny ), the scar is ready for conception some time before the anthers are emptied. During this time, the stigma can only be pollinated by pollen from other flowers, which increases the likelihood of cross-pollination. The femininity occurs, for example, in the plantain .
In some types of plants there are individuals in which the styles are long and the anthers are set deep, and individuals in which the styles are short and the anthers are high. So there are two different types of flowers. This is called different stylus or heterostyly . In other species with two stamen circles there are even three different types of flowers, depending on whether the style is on the lower, middle or upper level ( tristyly ).
Fertilization occurs only when the pollen passes from one level to a stigma on the same level. For example, pollen from short stamens must also get onto the stigma of a short stylus. However, this cannot happen within a flower.
The stamens and stigmas of a flower are spatially separated in this. An example of this is Iris .
Cross-pollination is also promoted by a physiologically caused incompatibility between stigma and pollen or stylus tissue and pollen of the same individual.
Formation of unisexual flowers
Single or dioecious distributed unisexual flowers ( dikline ) also support cross-pollination. With monoeye there are female and male flowers on one plant specimen. In the dioeciously distributed flowers, there are male and female individuals. The single individual consequently only has male or female flowers. This group of plants includes the sea buckthorn , the willow or the ringelkraut . Since male and female flowers are distributed among different individuals, self-pollination is impossible.
Function of cross-pollination
Cross-pollination supports the genetic diversity of a species, as genotypes with different alleles are mixed during fertilization . This reduces the likelihood that disadvantageous recessive inherited alleles will come together as a pair in an organism and that the harmful property will be expressed in the phenotype . Cross-pollination also increases the number of genetic combinations within a population. For example, if there are changes in environmental conditions, there is a greater likelihood that at least some members of the population will be able to successfully adapt to the new conditions. Cross-pollination can also lead to the emergence of hybrids , which can be more vital in terms of their properties than the respective parent plants.
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