Theory: Seeds and Germination

A seed may be defined as an embryonic plant in a state of arrested development, supplied with food materials, and protected by one or more seed coats.

It is able to remain alive, although dormant, through conditions which may be unfavourable for immediate growth. When suitable conditions occur a seed will begin to germinate.

Sleeping Seeds

 

Relatively few seeds will sprout as soon as they mature. Even under ideal conditions most seeds remain dormant for what is called a "rest period", which varies in duration in different plant groups. Rest periods are thought to be necessary for certain chemical changes related to the "ripening" of the foods stored in the seed.

Seeds, though dormant, are living organisms and need favourable conditions to grow.

How Long Do They Live?

Several factors contribute to the life expectancy of a seed. The length of time a seed will remain viable (that is, able to sprout strongly and produce sturdy seedlings) varies widely with the species and the care taken in harvesting and storing the seeds. Some seeds (e.g., chervil), rarely retain their germinating power longer than one year. Other seeds, such as celery, cabbage, cucumber, and various other vegetables and flowers, may sprout well when ten years old or even more. Seeds not fully mature when gathered or stored before fully dry, or kept at too high a temperature, may sprout very poorly or not at all.

Travelling Seeds

Seeds vary widely in shape and size. The seeds of the begonia, are almost dust-like. Seeds of the "double coconut" may be 30 to 45 centimetres long and have masses up to 20 kilograms. Seeds also exhibit a marvelous diversity of form. In many cases the shape and size of the seed is a special adaptation to assist in its distribution. Some seeds, such as the "wings" of the maple tree and the "parachutes" of dandelions and milkweeds, are adapted for wind transportation. Other seeds use burrs and hooks, to attach themselves to animals. Some seeds, such as those of the tomato and blackberry, have impervious coats which enable them to survive digestion in animals' stomachs; while others are suited for transportation by water, by being buoyant and waterproof for long periods of time.

Conditions Necessary for Germination

1. Preconditioning

Some seeds require specialized "pre-conditioning" before they will germinate. Some examples include:

  • alternate freezing and thawing may be required to break down and crack hard shells
  • a dormant period of "rest" in cool dry conditions may be required by some seeds
  • ponderosa pine seeds must be "singed" by the heat of forest fires before they are able to germinate;
  • the seeds of the Calvaria tree (native to Madagascar) must be passed through the intestinal tract of turkeys before they are able to germinate
  • in the presence of red light and infrared light some seeds, such as tomato seeds, are known to produce bioactive chemicals called gibberellins (GAs) which promote seed germination. In dicotyledenous plants (that is, plants whose seeds have two embryonic leaves), dicots for short , such as the tomato, the GA promotes the growth potential of the embryo and weakens the structures surrounding the embryo.

     

2. Moisture

Seeds tend to remain dormant as long as they are dry. Water softens the seed coat and expands the protoplasm.

3. Oxygen

All green plants need oxygen to "breathe" or respire. Like humans, they need oxygen to live and grow. However, dormant seeds need little oxygen.

4. Warmth

  • Minimum: Tomato plants will not germinate at temperatures below 10°C.
  • Optimum: Tomato plants germinate best between 17°C and 20°C.
  • Maximum: Tomato plants will not germinate at temperatures which exceed 35°C.

Germination


 

Seeds are protected by a hard shell or seed coat. This shell can be extraordinarily tough. Some seeds require that they be eaten by animals to soften the seedcoat and permit germination. For others, simply soaking the seed in warm water will soften the seed coat enough to allow the seed to begin to germinate.

In nature suitable conditions are provided by climate and weather. In agriculture the correct conditions can be simulated in greenhouses and "hot beds".

 

Hidden inside every seed is a tiny embryonic plant complete with root, stem and leaves, ready to sprout when suitable conditions appear.

The seed's plant-parts are not "true" leaves, stem and roots, but are effective enough to launch the plant into its growth phase when true leaves, roots and stems appear.

Flowering plants whose seeds have one embryonic leaf (cotyledon) are called monocotyledons. Flowering plants whose seeds have two cotyledons are called dicotyledons.

Tomato plants are dicotyledons.

 

When warmth, water, and oxygen become available a seed begins to germinate. The embryonic plant begins to grow and the seed coat swells and breaks open under the pressure of the growing seedling within the shell.

Before photosynthesis begins the spouting plant uses food stored within the seed to grow.

As long as the environmental conditions remain favourable the plant will continue to grow, the stem and the cotyledon(s) pushing upwards and the root(s) extending downwards.


As the emerging seedling begins to grow its dependence on stored food diminishes and the transition to its own photosynthetic food production begins. It will not survive unless ample light is added to the water, warmth, and oxygen needed for germination.

At what point can we say
that germination has been successful?

In the Tomatosphere project we are dealing primarily with tomato seed germination. Germination is, of course, a process whose duration varies according to the type of seed and the local environmental conditions to which the seed is exposed.

Our interest is in both the rate (time), and the success, of tomato seeds in the germination process.

New tomato seedling approximately one week beyond the "successfully germinated " point.

In order to ensure, as much as possible, the self-consistency of all observations made by numerous independent observers, the following criteria will be used to define the condition "successfully germinated".

For purposes of this experiment, a seed can be considered to have successfully germinated when two (2) distinctly separate cotyledons (embryonic leaves) can be seen.