Seeds

In this post I will be discussing about seeds, which is the structure of the seed, how do seeds germinated, why certain seeds germinate quicker than others, why some seeds need to be chilled, or put in acids or even burnt in order to stimulate germination, why certain seeds lose their viability.

What is a seed?

A seed is a small embryonic plant enclosed in a covering called the seed coat, usually with some stored food. It is the product of the ripened ovule of gymnosperm (coniferous) and angiosperm (flowering plants) plants which occurs after fertilization and some growth within the mother plant. The formation of the seed completes the process of reproduction in seed plants (started with the development of flowers and pollination), with the embryo developed from the zygote and the seed coat from the integuments of the ovule.

Which type of seed exist?

Some seeds are closed in a fruit like those of apples or pears, other are open like those from pine cones.
Although there various types of seeds we will be talking about seeds which have two cotyledons and one cotyledon. From this point of view there is a division among plants: those who produce seeds with two cotyledons (like beans or peanuts) or those who produce seeds with one cotyledon (corn, wheat). On the evolution scale we know that plants with one cotyledon appeared more recently than plants with two cotyledons.

What is a seed made of? Which are the parts of the seed?

A seed is a plant in miniature. It has a larger storage area (cotyledon, endosperm) where it keeps its food reserve for germination, it has also an embryo (the miniature plant) and a coat (for protection). Less than 2% of a seed is water. Compare this with 95% which is the quantity of water in a mature herbaceous plant. It is the low water content that protects the seed from frost.

How does germination occur?

When the seeds find the ideal condition the environment, they begin to germinate. First of all their coat is moisten with water and it becomes softer. As soon as the water begins to penetrate the coat and it reaches the embryo, if the seed is viable, the embryo reacts and begins to develop into a plant. First the seed will grow roots that which continue to absorb more water.

Which are the steps of the germination?

Once the coat has been broken, because of the contact with, water the seed will grow roots. A seed is like a sponge, is capable of retaining water. The roots have very fine hairs which help in water absorption. Once the small root is grown then the main stem begins to develop. At this point plants are not capable of photosynthesis and they take their food from the energy stored in their seed (in endosperm and cotyledons). In contact with water the substances (starch, proteins and fats) saved in these storage parts of the seed begin to break down into more simple compounds necessary for the nutrition of the new plant. Once the main stem reaches the surfaces of the soil they, under the influence of light they begin photosynthesis and develop further. They become greener and the leaves will become bigger. The roots continue to absorb water and nutrients from the soil and these are transported to the leaves, where under the influence of light they process this absorbed resources into food necessary for further development of the plant.

What happens with seed once the plant has grown above the ground and begins photosynthesis?

By that time, probably most of the energy reserves are depleted. Certain parts of the seeds may transform into leaves. This is obvious at plants with two cotyledons. This metamorphose ensure that the plants can begin earlier the process of photosynthesis. This first leaves are often named embryonal leaves or false leaves. In the left photos you can see how certains parts a seed metamorphosed.

The photo on the right shows you a healthy seedling of a pumpkin (Curcubita maxima). The cotyledons are transformed into two parallel large leaves (embryonal leaves), capable of photosynthesis, while in the center of the plant the first real leave begins its development.
Why my seedlings are growing long and frail? Why do my seedlings collapse after a while?

This happens when the young seedlings do not have enough light to sustain reliable photosynthesis. If you start a plant from seed in the dark, it will grow using the energy resources. It will grow high looking for light but it will be very fragile. Once the energy source from the seed is depleted the plants will die because they are not able to sustain themselves.
These very youg seedlings of Aster look frail because they dont receive enough light. They are not growing upwards because they twist after the light. The window pane inside a house might be the ideal place to start seeds, but once the seeds germinated need to be moved to a lighter and a colder place.

 Do all plants need light in order to germinate?

No. Germination of certain plants is not related to light. Some plants need complete darkness in order to germinate. Sweet pea (Lathyrus odoratus) is this type of plant.

Why certain seeds do not germinate at all?

Certain seeds have a short viability. This means that embryo inside stays alive only for a short while. This is a way in which plants determine that most viable embryos will produce the most viable plants. Certain seeds, like those from cocoa (Theobroma cacao) remain viable only for a few days, while other like those from wild poppies remain viable for decennia. Some other seeds simply do not have an embryo to germinate. This is mostly the case of some modern hybrids of vegetables and decorative plants.

How do the seeds lose their viability?

The seeds lose the capacity to germinate if they become completely dry. This is seeds are kept in lower temperature conditions in the seed banks all over the world.

Why the collected seeds from a certain plant refuse to germinate or take a longer time to germinate?

Plants like any other living organism are determined to conserve their species and survival. This is why certain seeds refuse to germinate immediately. If an acorn would germinate immediately then the winter frost would destroy the young plants. This is the case with the seeds of most deciduous trees. So certain plants purposely delay the germination of their seeds. This is done by providing the seed with a thicker coat, or certain chemicals etc.

Why would plants delay their germination?

Although it may annoying for horticulturist, plants seek the best condition for their flourishing. At the same time plants look after themselves, in the sense that desire to conserve their species. Staggered germination occurs only because of these reasons.

How do plants stagger germination?

One interesting scientific observation is the case of allelopathy (in Greek, "mutual suffering"). Certain plants, like the walnut tree eliminate competition by saturating the neighbouring region with substances than impede germination of its own seeds or even other plants. It is supposed that the roots of the plants release allelochemicals, and this is why farmers cannot grow other crops under walnut trees.

Certain plants produce seeds that can germinate only under specific wavelengths of light. Sunlight is composed of various colours (wavelenghts) Certain seeds desire only red light in order to germinate. In a forest were the canopy is dense, the red light is filtered by the leaves of other plants. A seed that rests on the floor of that forest may wait for years until one of the trees dies, creating thus an open space. When it receives the necessary light will start to germinate. This is mostly the case of evergreen rain forests.

Certain seeds need to be ingested by animals and birds and carried away for miles and miles. In this way plants ensure a wider propagation. However, in order to protect the embryo inside, the coat of the seed is thicker and has to withstand the action of gastric acids from the digestive systems of those animals. Once the seeds are eliminated, and the coat is scarified by the action of these acids, the seed is ready to germinate.

Another unusual requirement for the germination of some seeds is that to be scarified by fire. This treatment applies only to seeds with a thick coat and it is common only among the species living in areas where periodic lightning-fires are part of the balance of the nature. The plants known under the generic name of chapparal are known to germinate under these conditions. After the fire has passed the coat of the seed is scorched and as soon as the first rain comes in they begin to germinate, taking the place of their burnt to death mother plants.

When attempted to grow species of desert flowers, it has been shown that freshly collected seeds germinated the best when they were put for a week in an oven at about 50 degrees Celsius. This actually simulated their native environment. This happens during the summer months in certain desserts. During the winter months, the rains come in and the plants have better conditions for developing and then the seeds would germinate.

The horticulturists often mimic the general conditions of the native geographical areas of the in order to ensure a good germination. Sometimes this proves to be a real challenge. However, in a future article I will discuss about various methods of simulation.