Why are angiosperms important
Flowers serve the so-called Seed or flowering plants (Spermatophyta, this includes the naked ones (Gymnospermae) in the broader sense and the more opaque (Angiosperms) in the narrower sense) for reproduction. It houses the generative parts of a plant, which - at least in most angiosperms - are surrounded by a conspicuous flower cover that is supposed to attract insects and other animals. These act as pollinators for the plant. The end product is seeds which, in angiosperms, are hidden within a fruit.
The flower of a plant consists of an unbranched, short shoot with limited growth, the leaves of which have been transformed to perform specific functions. The structure of a flower consists of a mostly strongly compressed flower axis (receptaculum), around which the former leaves are arranged. The usually double perianth consists of the calyx and the corolla. Outside are the sepals (sepals), inside are the mostly colored and differently sized petals (petals). The generative part, consisting of the male stamens (stamina, in angiosperms) and the female carpels (carpels, in angiosperms), follows within the inflorescence.
The Design of the flower varies greatly depending on the plant family. The sepals are usually green and surround the plant when it is budding or when it closes at night. In some plants the sepals are fused or free or they are completely absent. (Example: Sharp buttercup, Ranunculus acris). A simple flower envelope without sepals is called perigone, the petals are then called tepals.
The Number of petals usually varies between three and five in most plant families. Some plants, such as the composites, form a large pseudo-flower (pseudanthium or "flower"), consisting of the conspicuous ray-flowers sitting on the edge, which are sterile, as well as the central, inconspicuous tubular flowers, which contain the generative organs. A well-known example of this is the sunflower (Helianthus annuus). In some plant families, for example in the mint family (Lamiaceae), the flower is not round (or radial symmetry), but has only one axis of symmetry. That is, you can only mirror them along a single axis. Such flowers are called zygomorph. Depending on the plant family, the petals are here wholly or partially fused or free.
The Stamens or microsporophylls form that Androeceum, the entirety of the stamens of a flower. They presumably consist of transformed leaves, but this has not yet been confirmed. A stamen carries the male ovules. It consists of a sterile stamen (filament) and a fertile dust bag (anther), which carries two so-called counters. In each of these there are two pollen sacs that produce the pollen.
The Carpels or megasporophylls form that Gynoeceum, the entirety of the carpels of a flower.
In the gynoeceum there is one or the other Ovary (contain the ovules). In addition, there is a scar (the area where the pollen is conceived) and the stylus that connects the two. The carpels can be open (apocarp) or grown together (coenocarp) be. In the case of coenocarpic carpels, the totality of stylus and stigma is called a pistil.
So the carpels envelop them Ovulesfrom which the seed later emerges, hence the name "Bedecktsamer". Both Gmnospermae the ovules are exposed.
Positions of the ovary
The ovary can assume different positions, each of which is species-specific:
- Above: Above the flower base (or the flower axis, continuation of the flower stalk, carries the flower organs)
- medium: the ovary is half sunk into the flower base
- below: The ovary is completely sunk into the flower base and grown together with it.
Nectaries are another special feature of angiosperms. They are glands that can be found on the flower, stamens or carpels, but also on the flower base. They secrete a nutritious sugar solution that acts as an attractant and reward for the pollinating animals.
A distinction is made between hermaphroditic, mono-and dioecious flowers. The hermaphroditic flowers are the original form of the flower in angiosperms. Here there are male and female parts within a flower, with the stamens always sitting between the petals and carpels. Separate-sex flowers have either male stamens or female carpels. A plant is called monoecious if both flower forms are on one plant, as dioecious if they are on different plants.
Flowering induction (triggering of flowering)
The timing of a plant's flowering depends largely on its age, but also on certain external factors such as temperature. Some plants need a cold stimulus in order to bloom. But genetic factors also play a role, for example in short-day and long-day plants (photoperiodism). Here the length of the night (!) Is decisive. Based on the period of darkness, the plant can estimate the right season to bloom. Certain light impulses can stimulate the flowering of some plants, while others can be stimulated by fumigation with flowering-inducing hormones.
Pollination precedes fertilization. This involves the transfer of a pollen grain to the stigma of another plant of the same species. As soon as the pollen comes into contact with the stigma, the pollen in the angiosperms forms a so-called pollen tube (it “germinates”) into the Scar in and down the stylus grows.
Forms of pollination
First, a distinction is made between foreign and self-pollination. Self-pollination is used as a last resort for some plant species when cross-pollination has not occurred. However, since it restricts genetic diversity, it is deliberately avoided by many plant species, for example through structural measures on the flower (varying length of pistil and anthers, e.g. caper bush, Capparis spinosa) or through temporal differences in the ripeness of Androeceum and Gynoeceum. Some plant species are also incompatible with themselves.
The main types of cross-pollination are:
- Wind pollination (anemophilia)
- Animal pollination (zoochory)
For the Wind pollination the pollen often have appropriate air sacs in order to be carried as far as possible by the wind. Characteristic of wind pollination are, among other things, very elongated filaments that swing freely in the wind, an extremely high number of pollen, enlarged stigmas to better collect the pollen, inconspicuous flowers usually without a flower cover (which would only disturb the pollination) and above all unisexual flowers. Wind pollination is the original form of pollination and is found mainly in gymnosperms that developed before angiosperms. But also some angiosperm families show wind pollination, especially the grasses and many deciduous trees (therefore many deciduous trees bloom before the leaves shoot, so as not to hinder pollination).
The Animal pollination (Zoophilia) is an "invention" of angiosperms. Their most important features are the formation of conspicuous flowers, inflorescences or pseudo-flowers that attract the attention of animals, e.g. B. by color, shape or fragrance. The flowers are mostly hermaphroditic and often adapted to the shape of the pollinating animals. Examples: The long, narrow calyxes of tobacco (Nicotiana tabacum), ideal for hummingbirds, the protruding "lower lips" of the mint as a landing place for insects, the large, "cave-like" flowers of the banana (Musa spec.) for bats.
Fertilization refers to the fusion of a sperm cell with an egg cell. It creates a zygote. In angiosperms, what is known as double fertilization occurs: of the two spermatozoids in the pollen tube, one fuses with the egg cell, the other with the embryo sac core, from which the endosperm is created, which nourishes the later seedling. The plant embryo, which is enclosed in the seed, develops from the fertilized egg cell.
After fertilization, the flower develops into a fruit in which the seeds are enclosed. Important: Only angiosperms are referred to as a fruit, since carpels are only present here.
The term fruit refers to the flower in the state of seed ripening. It protects the seeds until they are released. In addition, it is used for spreading, either by being spread together with the seed (e.g. closing fruits) or by scattering the seeds when they are ripe (e.g. opening fruits). As the fruit ripens, the gynoeceum enlarges, and the stigma and style usually die off.
A fruit consists of one or more seeds taken from one Pericarp (pericarp) are surrounded. The pericarp is formed from the carpel (s) and consists of:
- the outer exocarp
- the middle mesocarp
- the inner endocarp
These three units can have different consistencies: fleshy, membranous and hardened.
Both Opening or scattering fruits the pericarp is usually dry when the seeds ripen. The fruit opens and the seeds are released. The most important forms are follicles (a carpel, star anise, Illicium verum), legumes (a carpel, pea, Pisum sativum), capsules (two to four carpels, poppy, Papaver rhoeas) and pods as a special form of capsules ( or pods, if the fruit is less than three times as long as it is wide. Example: silver leaf, Lunaria rediviva)
Both Decay fruits the dry fruit splits. Either along the carpels (fissure fruit, sycamore maple, Acer pseudoplatanus) or in solitary pieces (broken fruits, e.g. the Klausen from lavender (Lavandula angustifolia).
Both Closing fruits the fruit remains as a whole until it spreads. A distinction is made here between berries with a fleshy meso- and endocarp and membranous exocarp (banana, Musa spec., Tomato, Lycopersicon esculentum), the stone fruit with a fleshy mesocarp, a membranous exo- and a hardened endocarp (cherry, Prunus avium, plum Prunus domestica) and the nut fruit. In some plant species, the exo-, meso- and endocarp are hardened (hazelnut, Corylus avellana), in others the pericarp and seeds are fused (caryopsis (upper ovary) in sweet grasses or achenes (lower ovary) in sunflower).
Both Collective crops the fruit consists of many carpels, which all form their own unit (little fruit), but are spread together. There are common nuts (strawberry, Fragaria vesca), Aggregate drupes (raspberry, Rubus idaeus) and pods (delphinium, Delphinium spec.). The apple is a special form, Malus sylvestris: Here the bellows, which are actually opening fruits, are surrounded by the tissue of the flower axis and are therefore not open.
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