Why do fruits have seeds
Fruiting occurs only in angiosperms in which the seeds are enclosed in an ovary. In the gymnosperms, on the other hand, the seeds develop freely on the carpels.
In angiosperms, the term fruit is used to describe the state of the flower during seed ripening. Perianth and stamens are usually dried up and fallen off at this point. During fruit formation (see also) the gynoeceum is largely remodeled, the ovary enlarges greatly, the scars dry up and the style usually degenerates or falls off, but can also be further developed if it is to serve the fruit spread.
In the simplest case, the seeds are released from the ovary after ripening, as the fruit opens in very different ways (opening fruits, see below). Very often, however, it is integrated into the process of seed dispersal (dispersion) (closing fruits, see below), whereby the ovary tissue is very differentiated, the pericarp is divided into three layers: exocarp, mesocarp and endocarp. In the course of fruit formation, these can become membranous and leathery, fleshy or woody (sclerotic). The changes create devices to protect and spread the seeds. Not infrequently, other parts of the flower can also contribute to fruit formation, e.g. the flower axis (with choricarpic gynoeceum: strawberry, etc.) or the calyx.
Depending on how the Gynoeceum was organized, single or collective fruits are produced. Individual fruits arise from a monomeric (coenocarpic) gynoeceum or from individual carpels of a choricarpic (apocarpic) gynoeceum. Collective fruits arise in flowers with choricarpic gynoeceum.
If fruits open and scatter ripe seeds, opening fruits are present, with them the pericarp is consistently dry. This contrasts with the closing fruits, in which the seeds are spread together with the fruit or parts of the fruit. These include the winged split fruits of the maple
Follicle: It is considered the most primitive form of fruit and is common in the buttercup family (Ranunculaceae) (further examples: Anemone pavonia and Aquilegia vulgaris (Columbine) and Caltha palustris (Marsh marigold), Ranunculus auricomus (Gold cockfoot)). The fruit consists of a single carpel. The opening is usually made at the abdominal seam, e.g. with the fruits of the delphinium (Delphinium). In rare cases, the opening is made at the back seam, e.g. at the Magnolia.
Legume: This is the legume type of fruit. In contrast to the bellows, it opens at the stomach and back seams at the same time (examples: pea pods (often incorrectly called pea pods), Swainsona galegifolia or acacia). When opening, the two halves of the amniotic sac often roll up in a screw-like manner. The process can be explosive, with the seeds being thrown away with great force (autochory, see below).
The pod, typical of the cruciferous family (Cruciferae, Brassicaceae), emerges from a paracarpic ovary; if the length does not reach three times the width, one speaks of pods (examples: Capsella, Hellerkraut and Silberblatt). The ovary comprises only two carpels and is divided by a false partition. When it ripens, two flaps become detached, leaving behind the false septum with the placentas (replum) to which the seeds can be attached for a while.
Capsules are a very common type of fruit (see also example: Veronica). They arise from coenocarpic gynoeces, the pericarp is dry. The fruit is opened in a variety of ways, e.g. through longitudinal cracks, pores or by lifting a lid.
Closing fruits do not spread their seeds. Depending on the nature of the pericarp, one differentiates between berries, stone fruits and nuts.
The pericarp of the berries becomes fleshy and juicy in all its parts, and often colored. A berry - e.g. tomato - usually contains numerous seeds. Both Stone fruits (see also example: Sambucus (Lilac)) the endocarp differentiates into a stone core, which usually only includes one seed. The outer layers of the fruit (mesocarp and exocarp), on the other hand, are like the berry fleshy. Well-known examples are cherry, plum, peach, olive, tomato, orange, mistletoe, but also the walnut. Berries and stone fruits are endozoochoric-adapted fruits, i.e. they are eaten by animals (mammals and birds) together with their seeds. The seeds or the stone cores with the seeds are excreted undigested and thus spread. At Nuts the entire pericarp usually develops into a hard, thick-walled housing, which usually only encloses a single seed (hazelnut, acorn, lotus, etc., but not: walnut, see above).
The nuts are also attributed to those of the sweet grasses, which are characterized by a relatively thin seed shell that is fused with the pericarp. Then also the achenes of the composites (Compositae, Asteraceae), in which the calyx develops into a pappus (a flight organ). A particularly interesting case is the coconut (Cocos nucifera), in which the outer layer is fibrous, making the fruit buoyant because of the air enclosed in it.
Collective fruits arise from flowers with apocarpic (chorikarpem) gynoeceum. The individual fruits can have a dry pericarp (single-seeded "nuts" or low-seeded follicles), or the outer layer can be fleshy like a stone fruit. The individual fruits are usually combined into a functional unit through an axis structure, either through a central journal (strawberry = collective nut fruit, blackberry = collective stone fruit) or an axis cup (rose hip and, for example, cinquefoil) (Potentilla) = Aggregate stone fruit, apple = aggregate fruit, usually referred to as apple fruit).
In some species, the numerous fruits that develop from an inflorescence remain connected to a functional unit (infructescence). Well-known examples are the mulberry (More) and the fig (Ficus).
As a rule, the seeds are surrounded by a solid seed coat (testa), which is derived from the integument (or integuments) and which protects a largely developed, nutrient-supplied embryo, protects it from water loss and withstands adverse weather conditions. The nutrients are either located in a special nutrient tissue, the endosperm, or in the perisperm (from nucellus tissue, e.g. in pepper). They must be opened up and absorbed by the embryo during germination, or they are already in the cotyledons (such as beans, peas and almonds). The monocotyledons usually have one cotyledon, the dicotyledons two and the gymnosperms usually several.
In many species, the seeds are not ready to germinate immediately after they have been detached from the mother plant (seed dormancy, dormancy). This dormant seed can be ended e.g. by a cold period (stratification, not to be confused with vernalization), so that the seeds only germinate at the beginning of the next vegetation period.
The number of seeds that a plant produces in the course of its life shows a wide range of variation. Some species (e.g. the coconut palm) produce only a few, but very well-endowed offspring, while other species produce up to a few million, but inevitably poorly endowed. A variety of devices promote the spread of the offspring, because a seed can only develop into a new viable plant in suitable locations. If the spread occurs exclusively through mechanisms that the plant itself develops, one speaks of autochoric spread (autochory). If external factors (so-called vectors: wind, water, animals, etc.) contribute to the spread, one is dealing with an allochoric spread (allochorie). Often the seeds containing fruits are spread, rarely the seeds alone. The centrifugal and splash movements are one of the autochoric mechanisms of propagation.
Wind propagation (anemochory) is common in plants. The expansion units (fruits, partial fruits or seeds) are inevitably either very small and light and / or often provided with specially built flying or hovering devices. These include, for example, air-containing cavities in the seeds of orchids, seeds that are hairy on all sides (cotton), cardboard-bearing seeds (composites - well-known example: Taraxacum and Tragopogon) and the units with differently constructed wings. There are also fruits with highly specialized propagation facilities.
Trample burdock: lignified pericarp with spines and barbs that can dig into the soft parts of the hooves of mammals and are thus spread out. (Dicerocaryum senecioides, Pedaliaceae)
Wind spread: flat, very light, dry pods with a relatively large surface and only one to three seeds per fruit (Lunaria annua, Silver leaf; Brassicaceae).
Velcro spread: dry fruits, the surface of which is provided with barbs that hook into the fur of the mammal. Relatively firm structure of the fruits, which, however, disintegrate into partial fruits over time, i.e. effective spread not by depositing the entire fruit in one place but distributing it to different (Xanthium riparium, Compositae)
Macro recordings, taken with a Color View digital camera, processed with Analysis (Soft Imaging System, Corp.) - Photo: A. ANDRES, Institute for Applied Botany, University of Hamburg
The spread through water (hydrochory) takes a back seat compared to the wind spread. Seeds spread by water usually have an unwettable outer layer, or they are hairy, with air bubbles being held between the hairs. Some large fruits, such as coconuts (see above), spread by the ocean currents, are surrounded by loosely structured, air-containing layers of tissue.
We already encountered the zoochory, i.e. the spread through animals, when we discussed berries, stone fruits and collective fruits. If seeds are eaten and excreted through excrement, one speaks of endozoochoric spread. This is opposed to the epizoochoric, in which seeds or fruits either attach to the animal body by means of slimy secretions that are excreted by the glandular hair on the seed / fruit surface, or have variously designed barbs with which they get caught in the hair of the animals .
A special case of animal spread is the spread by ants (myrmecochory). Such seeds or fruits have special appendages, elaiosomes, which contain attractants and nutrients. Myrmecochory is particularly widespread in plants from the forest floor, a well-known example being violets (Viola).
As the presented cases have shown, there is an abundance of different mechanisms of propagation for seeds and fruits in the zoochory. However, the variability is nowhere near as great as we encounter in the construction of flowers. This is mainly due to the fact that specialization and adaptation to seed- or fruit-dispersing animals is less advanced than specialization of the pollination process.
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