|Botany 101 - How do seed function?|
Understanding how seed are formed and how they germinate will perhaps shed a little light on why certain steps are required to get your seed to grow, and this understanding can help you be more successful in your seed growing endeavors.
Plant propagation by seed is sexual reproduction, as opposed to asexual reproduction, which includes propagation methods such as growing from cuttings, division, layering, or grafting. Nearly all of the plants gardeners start from seed are Angiosperm, flowering plants which produce their seed within an ovary. The seed are produced by flowers and are the result of the fusion of male and female gametes. Gametes are the vehicles by which hereditary genetic material is passed from parent to offspring. The male gamete is called the sperm and the female gamete the ovule. Pollen grains, which contain the male gametes, are produced within the anther from pollen mother cells. The anthers and their stalks, known as filaments, constitute the male part of the flower, called the androecium or stamen. The ovules are formed within the ovary. The ovary, style, and stigma make up the female part of the flower, which is called the gynoecium or pistil.
Pollination and the subsequent fusion of gametes occur after pollen is transferred from the anther to the stigma by wind, animals, insects or hand. Once on the stigma, the pollen grain germinates, producing a pollen tube that grows through the style to the ovary. The male gamete or sperm travels through the pollen tube and enters the ovule through a small opening called the micropyle. At this point fertilization takes place, fusing the genetic material of the parents. The fertilized ovule matures into the seed.
Self-pollination occurs when male and female gametes of the same individual plant are fused, either in the same flower or different flowers. Cross-pollination is defined as the fusion of gametes from different individuals.
Varieties are naturally occurring selections within a species, while cultivars are selections resulting from man’s endeavors. In the strictest sense, hybrids are the product of cross-pollinated, genetically-unlike parents. Hybrids of the seed trade are crosses between highly selected individual strains and are completely repeatable year after year by using the same parental strains.
Hybrids retained in the trade are improvements over their parents and are of increasing importance in flower and vegetable gardens because of the high degree of uniformity of plant habit, leaf and flower color, flower form, productivity and other desirable characteristics. Hybrid seeds are more expensive because of the extensive breeding efforts that go into their production and their resulting superior traits, such as increased disease resistance, longer flowering period, stronger stems, etc. Seed from hybrid plants should not be saved for general garden use because the offspring will segregate, yielding many less desirable types. Since hybrids are superior, they are worth the added cost.
The primary parts of a typical seed are a seed coat, endosperm and embryo. The seed coat encloses the other parts and gives protection. The endosperm is stored energy, usually in the form of carbohydrates, and can be either developed or undeveloped. The embryo is the small dormant plant. The lower part of the embryo develops into the roots and is referred to as the radicle or hypocotyl. Towards the middle of the embryo are one (monocotyledon) or two (dicotyledon) cotyledons. The cotyledons or cotyledonary leaves are the first to appear after germination in many plants with two cotyledons. At the top of the axis above the attachment point of the one or two cotyledons, the shoot growing point appears, known as the plumule or epicotyl. Sometimes a small shoot is already present at seed maturity.
At the end or along the side of the seed is a scar, the hilum, which marks the point of attachment of the seed to the ovary wall, sort of like a navel marks where a baby was attached to its mother. On seeds of some species, like lima beans, the micropyle, a tiny hole that the pollen tube grew through at the time of pollination, may be seen at the end of the hilum.
For germination to take place, the seed must be viable, the internal conditions of the seed must be right, and there must be proper balance of the environmental conditions of moisture, temperature, light and air. When these environmental requirements are met, enzymes are activated. These enzymes convert the stored carbohydrates (mostly starch) into sugars usable by the embryo. As the embryo grows, the seed coat bursts and then the radicle emerges and anchors in the ground. Following that, the growing tip emerges.
And thus a new little plant is born!
NASA Seeds in Space
A garden, if it is to mirror your soul, needs to enrich your life on a daily basis. It should be a good friend you want to visit often and to know intimately."