Seeds are crucial to the survival of a plant species. No viable seeds and the genetic line will die out, although some plants (e.g. English elm) mainly asexually reproduce from suckers forming a genetically identical cluster of plants.
Seeds are a genetic mixture of DNA (genes) from the two parents, plus genetic mutations that throw in the ‘joker’ of new genes. Constant re-shuffling of the genes in a population (called the gene pool) is good, yet adding new genes is better longer-term.
The seed is an immature plant: stem + one or two seed leaves (cotyledons) + shoot (plumule) + root (radicle) + food store in the leaves or separately as an endosperm (in monocotyledonous plants such as the grasses or sweetcorn). Around all this is the seed coat, the testa.
Seeds start with an endosperm, then in the dicotyledonous plants (two seed leaves) the food store moves into the seed leaves which enlarge. Peas, beans and peanuts all have two seed leaves filled with stored resources.
(The white of a coconut is endosperm, as is white flour.)
Most books ignore the testa. Sadly, as it is the most interesting part! It is the seed’s protector and it controls germination. A seed germinating under the wrong conditions will be committing suicide. A waste of resources.
The seed coat is made up of several layers of sturdy, thick-walled cells, and in many plants it is impregnated with wax or varnish-like chemicals. Unchanged, this testa will stop water ingress and inhibit germination. The rate at which the seed coat breaks down controls germination. Seeds can occasionally remain viable for thousands of years (Egyptian tombs).
In arid locations, the seed coat is designed to take up water and rupture when water is sufficiently abundant. Sometimes, it breaks down when rubbed by water against sand grains. Gardeners can rub seeds against sandpaper to stimulate germination in a seed tray. With dodder (Cuscuta), I have treated the seeds with an acid (white vinegar) to encourage growth. (It didn’t work!) In germinating hardy orchid seeds, I have used bleach to break dormancy.
Oxygen can also be a stimulant to germination in some species. Presumably these seeds are in danger of being deposited deep underground, and the diggings of hogs allow them access to oxygen and growth when nearer the surface.
The range of germination stimulants is long, including light (lettuce), micro-organism attack, release of anti-inhibitors by other plants and time. Seeds of some plants germinate in phases and others need smoke to stimulate growth.
(Seeds come from a fertilized egg cells. As you can see at the end of the article, it is not as simple as human egg plus sperm! In reality, the plant’s egg cell is one of several cells that make up a female gametophyte, that has arisen from a spore.)
Above: life cycle of a fern. Note the two stages, that can live independent lives. Such cycles occur in all plants from mosses to flowering plants.
Below is a diagram of the ‘female’ parts of a flower. Stigma, style and ovary – containing the ovule (within yellow coloured part.) This flower has a single ovule in the ovary. That is not always the situation.
Below is a flower with many ovules with one ovary. The number varies. In the pea, the pod is the ovary wall and the peas (seeds) form from the ovules.
Seeds are contained within a modified ovary wall. Seeds + modified wall = a fruit. The starches that may be present in the fruit change to sugars when the seeds are ready for dispersal. (My yellow fruited viburnum and crab apple trees have fruit that never ripens to the red colour and are mostly ignored as inedible by the birds.)
Seeds eaten by birds mostly pass through the gut unchanged. If they are crushed they will be digested and die. (By putting chilli seeds in with a bird food mixture it will discourage rodents, who chew seeds, and not birds who usually just swallow. The birds do not taste the hot chilli seeds.)
Giberillins (gibberellins) are plant hormones release from the embryo when it becomes hydrated. The hormone works on the stored starch in either cotyledons or endosperm causing amylase production. Amylase is a starch digesting enzyme (Like salivary amylase in mammals). The sugars produced is the energy and raw material for growth and germination. However, it all is triggered by the testa allowing water into the seed.
Above, a diagram showing the complexity of ‘real’ flowering plant reproduction. Meiosis is cellular and nuclear division that halves the chromosome number to make gametes. Gametes (sex cells) combine to form a zygote and to grow to form the next generation via the seed.
In flowering plants the gamete-producing stages are held within the ‘normal’ green plant.