Spring is here and it’s time to start seeds for the vegetable garden. I find seeds to be amazing little things. The seed is a dormant embryo of a plant, with everything it needs to grow packed inside a small package – a sometimes VERY small package.
The first stage of germination is water absorption, called imbibition. (I remember this term by thinking the seed is imbibing.) Obviously, water must be present for imbibition and that water must be the proper temperature to activate the enzymes dormant within the seed. Once the enzymes are activated by water, the cells in the embryo begin to expand.
There are some exceptions, but typically growth begins with the emergence of the radicle – the primary root. This is followed closely by plumule – the initial sprout. The root always grows down and the shoot up by the seedling’s response to gravity. This is called gravinsensing. There are two plant mechanisms responding to gravity. For one, at the very tips of the roots there are special cells called statoliths. These cells are just a tad denser than the other cells, thus being pulled down by gravity.
Meanwhile, the auxin hormone, which controls cell growth and is concentrated in the shoot, is also affected by gravity which concentrates it to the bottom of the shoot. A higher concentration of auxin on one side of the shoot will cause that side to elongate until the sprout is even on both sides. This elongation on one side explains how the shoot will turn toward the surface until the seedling is growing straight up.
Once the shoot breaks the soil surface into the light, a protein in the plant triggers phytochrome pigment. Phytochrome is a plant’s continuous response to light and in the seedling, it stimulates the growth of chlorophyll and leaf formation.
Until the seedling has its own true leaves, it is unable to create any energy of its own. It relies on the carbohydrate nutrition inside the seed called the cotyledon. In some cases, the cotyledon becomes a leaf, in other cases it is absorbed completely and remains under the soil surface.
Each seed will respond to just the right stimulus to begin growing and that stimulus varies from plant to plant. Some seeds have thick coats that must go through some sort of process as a preliminary step – such as being scratched or scarified, partially decomposed, or perhaps passed through the acid in the gut of an animal. Some seeds must endure a cold stratification, a period of cold temperatures before they can germinate. On the other hand, some seeds must go through a fire before they are triggered to germinate.
I hope you are as amazed by the complexity of a miniscule seed as I am! Now that I’ve over-analyzed the situation, I’m off to plant my pea seeds. Happy gardening!
For more information on starting seeds, see these CSU fact sheets:
