Some fruits have a seed
Inside the fruit is the seed of the plant. The plant embryo is enclosed in it, along with a nutrient tissue that is supposed to supply the embryo as soon as it begins to germinate. The nutrient tissue arises either from the embryo sac, which receives the second spermatozoid during fertilization (double fertilization), it is then called the endosperm. Or it is formed from the tissue of the ovules (Nucellus) and is called perisperm. In some plant families both nutrient tissues occur (water lily, Nymphaea alba), in many only one of each.
The seed is covered by a seed coat (Testa) included, which are made up of the envelope layers (Integuments) the ovule exists and serves as protection against external influences and water loss.
One of the most important factors for the survival of plant species is the spread of the seeds. It is important that the seeds come to an area where they find favorable conditions for germination and later to exist. Another factor is the ’conquest’ of new territory where the plants can spread unhindered.
A distinction is made between the mechanisms of propagation that are carried out by the plant alone (autochorus) and those where ’aids’ such as wind, water, animals are required (allochor).
With the autochoric distribution, the seeds are either thrown up to several meters away (glandular balsam, Impatiens glandulifera) or the dried fruit opens and the seeds fall out (corn poppy, Papaver rhoeas).
Allochore distribution takes place, for example, when an animal eats a fruit together with the seed and then excretes it again (rowan, Sorbus aucuparia) or if the fruits or seeds attach themselves to the fur of animals (or the socks of humans) (burdock weed, Galium aparine). This form of distribution is called zoochory.
Propagation by the wind (anemochory) is one of the most common forms of seed dissemination. Either whole fruits are distributed (linden, Tilia spec.) or just the seeds (dandelion, Taraxacum officinale). The fruits / seeds either have a flight device (maple, Acer pseudoplatanus) or they are very light (birch, Betula pendula). Often times, a large amount of seed is produced because it greatly increases the chance that a seed will reach its destination and begin germinating. The spread of wind is therefore often found in so-called ’pioneer plants’ such as the birch, which can also colonize inhospitable areas and thus expand their territory. Sprouting birch trees can occasionally be found in gutters, disused chimneys, on walls.
Water dispersal (hydrochory) is rarely found in angiosperms. Here the seeds are transported by the water until they land in a suitable place and germinate. A well-known example here is the coconut (Cocos nucifera).
In order to be able to germinate, various requirements must be met. The most important are heat, water, light (or absence of light) and oxygen. Heat ensures that outside the germination environment the right temperatures are already in place, which the seedling needs for its further development. The seeds of many plant species are not yet capable of germination after they have spread (so-called dormancy or dormancy), but rather "wait" for certain environmental stimuli that first enable germination. These include, for example, cold stimuli (vernalization, the temperature must not exceed a certain level over a certain period of time), which ensure that germination does not start until a favorable season (after winter).
Some plants can be classified according to the required lighting conditions. So-called light germs usually have small seeds, so they do not have enough energy to penetrate a dense soil layer and need the light or heat to germinate. Examples are e.g. B basil (Ocimum basilicum) and grasses like rye (Secale cereale). Dark germs, on the other hand, do not germinate in light, they need complete darkness. Examples are maize (Zea mays) and monkshood (Aconitum napellus).
Water is another indispensable prerequisite for germination: many seeds are very poor in water (persistence state) and must first absorb (swell) a large amount of water in order to then begin with germination.
Oxygen is required for plant metabolism. The breakdown of the reserve materials (and thus the gain of the energy required for germination) takes place via dissimilative processes. Here, oxygen is required as an electron acceptor.
Structure of the seedling
The plant embryo or seedling consists of
- the hypocotyl (forerunner of the stem axis)
- the cotyledons (cotyledons, one or more, depending on the type of plant)
- the radicula (root system)
- the plumula (meristematic tissue that already bears the plant of the first leaves)
At the beginning of germination, the first thing to break through the seed coat is the root system (radicula). It begins immediately with the formation of the roots. This is important for the continued water supply, and it also anchors the seedling in the ground. Then the extension of the stem axis begins. A distinction is made between two forms: epigeic (above-ground) germination and hypogean (underground) germination. During epigeic germination, the hypocotyl stretches and pierces the surface of the earth in the form of a hook. With this he pushes up the cotyledons, which begin to unfold. Example: beech (Fagus sylvatica).
With hypogean germination, the cotyledons remain below the surface of the earth. This is where the epicotyls stretch (the area of the stem axis between the cotyledons and the first leaves (also called primary leaves). The first thing that emerges from the earth are the primary leaves, which are also immediately photosynthetically active. Example: Pedunculate oak (Quercus robur).
The main function of the cotyledons is to nourish the seedling as long as it is not yet able to photosynthesize. For this purpose, the necessary nutrients are formed from storage substances. During epigeic germination, they also carry out photosynthesis themselves (recognizable by their green color). They feed the seedling until the primary leaves have started their work. If they are fully developed, the cotyledons die.
After the development of the first leaves, the internodes (the areas between the nodes where the leaf bases are located) begin to elongate and the plant grows in length. At the same time, leaves begin to form, while the roots branch out in the earth in order to extract further nutrients from the soil. The seedling has developed into a complete plant. Depending on the type of plant, however, it can take a few years before it sets flowers for the first time and is therefore "grown up".
In contrast to sexual reproduction, vegetative reproduction does not add any new genetic material. The reproduction is based exclusively on cell division (mitosis). The disadvantage is that the plant cannot adapt to changed environmental influences because the genetic material remains identical. Seed plants often use vegetative reproduction to spread in a favorable location. The more flexible sexual reproduction, on the other hand, is often used to open up new locations. Typical forms of vegetative reproduction are the formation of runners (stolons), brood buds, tubers or depressions.
See also: flowering, fertilization, pollination, fruit.
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