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Fruit anatomy

A fruitfruitfruit is the matured ovary enclosing the seedsseedseeds. It is produced by angiospermsangiospermsangiosperms. Despite the fact that fruits take on different forms, each consists of the same parts: pericarp and seeds.

The process of fruit development begins after the fertilisation – a sperm cell and an egg cell coming together inside the ovule located in the ovary. After fertilisation, the ovule transforms into a seed, and the ovary wall into a triple layer pericarp. The outer layer is called exocarp, often fitted with different outgrowths – wings, hooks, hair. They facilitate transporting fruits by animals or wind. The middle layer is called mesocarp, it can be thick and fleshy like that of a peach, or thin and dry like that of a pea. The inner layer (endocarp) surrounds the seeds. It is woody in the case of drupes, leathery in the case of apples, and in the case of peas – membranous. The pericarp’s function is to protect seeds and facilitate their propagation. A fruit can contain one or many seeds.

Fruit types

Fruits display different adaptations for seeds propagation, reflected in their structure.

Singular fruits develop from a single ovary of a single flower. When they ripen, they can be fleshy or dry. Berries are fruits with the pericarp made of soft tissue, usually containing many seeds, like for example the watermelon. Drupes are fleshy fruits with a stone, represented by the cherry. Abundant edible flesh developed from receptacle growing around the ovary is a trait of accessory fruits like apple. In their case, the pericarp is a small part of the fruit, covering leathery seed cases. The pear and chokeberry are both examples of similarly structured fruits.

Majority of plants produces dry fruits with hardened pericarps. Dehiscent fruits are an example of those. When they ripen, they dry and open violently, launching seeds over long distances. They are divided into three categories: capsules (poppy), pods (bean) and siliques (rapeseed). Indehiscent dry fruits don’t open when they’re ripe. They have a hard and thick pericarp that cracks only when the seed sprouts. An example of those is the caryopsis – a type of fruit found in the wheat and corn, which develops as a result of the seed coat and thin pericarp fusing, making it look like a seed, and not like a fruit.
Similar to caryopsis, achenes have the pericarp and seed unfused, making it easy to separate them, for example in the case of the sunflower. On many achenes’ pericarps (thistle, dandelion) structures for seed propagation develop. A schizocarp (hollyhock fruit for example) contains many seeds. After ripening, it breaks down into mericarps. A completely hardened pericarp of a nut usually contains only a single seed. Fruits of the hazelnut tree, oak and buckwheat are nuts.

Seed structure and types

Seeds perform survival function. They adapted to stay dormant until conditions are right for sprouting. Annual plants stay dormant through the winter (in our climate) or the dry season, which is the only way for them to survive the wait for the right conditions. In the mature seed the processes of growth and development cease and the respiration is heavily slowed down. The dormancy can last from several days to many years. A typical seed consists of the embryo, the nutrition storage called endosperm, and the seed coat.

The hard and difficult to crack episperm protects the seed from drying, damage and pathogens. Many species develop all kinds of outgrowths facilitating propagation. Nutrition storage contained in the seed is barely used during the dormancy, when life processes are very slow. It is mainly used during sprouting, serving the embryo as an energy source during the growth and transformation into a seedling. In the beginning, the seedling uses the nutrition storage until it develops leaves and is able to photosynthesise.

Because of the origin of the nutrient tissue, we divide the seeds into endospermous and non‑endospermous seeds. The former have a small embryo surrounded by the big endosperm, like in cereals. The latter, like beans and sunflowers, have embryo filling the whole seed. In this case the endosperm was used during the embryo’s development, which resulted in creating large and thick cotyledonscotyledonscotyledons serving as storage organs.

The nutrients accumulated in the seeds vary. Depending on which substance dominates, we differentiate oilseeds, containing a lot of fat such as those of the sunflower and rapeseed, starchy seeds typical for cereals, and protein seeds such as the bean or pea.

Fruits and seeds propagation

Plants lack the ability to move. They can spread only thanks to fruits and seeds. The propagation can occur without any external help or via carriers such as wind, water and animals. In the case of some plants, it’s the whole fruit that gets propagated, in the case of others – it’s just the seed.

The self‑propagating of seeds is possible thanks to the inner forces acting on the fruit. Dry bean pods twist and explode, launching the seeds into the air violently. Capsules of the snapweed and violet fruits open up in a similar way: barely touched, they shoot their seeds several meters away. A plant called exploding cucumber squirts a stream of mucilaginous liquid containing its seeds.

The wind carries seeds and fruits adjusted to such method of locomation: lightweight and equipped for air travel. Maple and ash fruits have wings; dandelion and poplar fruits have pappus that increases their surface area.

Animals play an important part in transporting seeds and fruits, which can stick to their fur and feathers thanks to hooked outgrowths or spikes. Such catching devices can be found in burdock, cleavers and some species of sorrel. On the other hand, fleshy, juicy and colourful fruits make alluring food for animals. Inside their digestive tract, the pericarp is digested and the seeds get excreted unharmed with faeces. Humans in particular play a huge role in propagating fruits and seeds, consciously sowing plants in selected places and importing exotic decorative plants seeds from distant countries.

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Fotografie przedstawiają rośliny, rozsiewające się samoczynnie. Na tej po lewej na pędzie znajdują się dwa pomarańczowe kwiaty. Po prawej trzy grube, kolczaste owoce. Niewielkie zielone i brązowe smugi w dół na obu fotografiach oznaczają „wystrzelenie” nasion.

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Fotografie przedstawiają nasiona, rozsiewane przez wiatr. Z lewej z kuli dmuchawca unoszą się ciemne nasionka, przymocowane do srebrzystych, delikatnych „parasolek” z włókienek. Z prawej z drzewa, znajdującego się w tle spadają brązowe nasiona zaopatrzone w „skrzydełka”.

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Fotografie przedstawiają nasiona, rozsiewane przez wodę. Z lewej szary, beczułkowaty owoc leży na słonecznej plaży. Wyrastają z niego w górę podłużne listki, a w dół korzonki. Po prawej zbliżenie zielonego, butelkowatego owocu. Znajduje się na „łódeczce” utworzonej przez płatki kwiatu, a wokół znajdują się liczne czarne pozostałości pręcików.

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Fotografie przedstawiają rozsiewnie owoców przez zwierzęta. Z lewej szary ptak z czarnymi nóżkami trzyma w dziobie zerwany z gałązki czerwony owoc. Po prawej zbliżenie koszyczków łopianu, zwanych „rzepami”. Mają one liczne beżowe łuski, zakończone haczykami. Przy ich pomocy przyczepiają się do sierści zwierząt.

Seed germination

When the conditions in the environment are favourable, seeds start to germinate. During the first phase of germinationgerminationgermination they draw water intensely and puff up, gaining mass and increasing their volume. As a result, the seed coat cracks open. The radicle is the first to appear, going down into the soil. Soon the stem appears, and cotyledons with it. Once they’re in light, they become green and can begin to photosynthesise. The embryo slowly turns into a seedling. When the endosperm nutrition storage is all used up, the cotyledons wither and fall off.

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Ilustracja przedstawia schematycznie pięć etapów kiełkowania nasion na tle podzielonym poziomo na część niebieską- atmosferę i brązową - glebę. Po lewej ciemno brązowe nasiono fasoli znajduje się w glebie. Dalej w lewo wyrasta z niego różowy korzonek, a do góry wznoszą sie liścienie. Między liścieniami pojawia się zielony liść. Ostatni rysunek prezentuje pęd z liśćmi i resztkami liścieni, a w glebie duży korzeń palowy.

During germination the seed wakes from the dormancy and life processes speed up inside the developing embryo. The presence of water is necessary for this process to commence. Water activates the enzymes that break down nutritious substances from the endosperm and cotyledons. The temperature of the environment is also important. Different plants seeds germinate in temperatures characteristic for the species, in the 0.5 – 40 °C range. Germinating seeds respire intensely, thus oxygen is also needed. Its deficiency or absence can be the cause of stopping the germination. In case of some plants, the process can be influenced by the light. It can both stimulate germination (lettuce) or inhibit it (cereals).

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Ilustracja przedstawia centralnie umieszczoną, młodą, jasnozieloną roślinę na brązowej glebie. Wokół niej w czterech zielonych kółkach znajdują się piktogramy, oznaczające czynniki, które mają wpływ na kiełkowanie nasion: temperature, water, light, oxygen

Summary

• Fruit is an organ typical of angiosperms only.

• Fruit protects the seeds and facilitates their spreadingspreadingspreading.

• Seeds have a survival function.

• Seeds contain the embryo of a new plant.

• Seeds and fruit have different adaptations to spreading.

Homework

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Opisz przystosowania owoców do wysiewu nasion.

Task 5.1

Classify the fruits shown in the illustrations and indicate their adaptation to seed spreading.

Task 5.2

Name the similarities and differences in the structure of endospermic and non‑endospermic seeds.

Task 5.3

Describe the germination stages and name the conditions necessary for its course.

Keywords

fruit, pericarp, seed

Glossary