Lesson Overview
This lesson is a more detailed look at the propagation of plants from seeds. It includes information on the germination requirements of seeds and the special treatments that some seeds require.
Goals for this lesson
- understand what the different methods of propagating plants are
- understand the requirements for successfully germinating seeds including methods of breaking seed dormancy
- learn proper techniques for planting
seeds
Tasks
- botany logbook entry
- read and study the information below
- complete and submit activity
Vocabulary
propagation, pollination, sexual reproduction, asexual reproduction, germination, dormancy, stratification, scarification
Assessment
- quality and completeness of worksheet
Reading
Walk into the nearest large supermarket and look around at the huge selection of food products available. With all the luxury of this vast selection we have become more and more disconnected from where our food comes from. "Where does our food come from?" is a very important question we should be asking more often. Since the majority of the food products available in the store come from plants or from animals that eat plants this lesson will focus on how plants are grown from seeds.
There are many ways of propagating or reproducing plants. The most common method of propagating many plants is through the use of seeds. Seeds come from the plant flower. Pollination fertilizes the egg within the flower, and the results of this sexual reproduction are seeds. Seeds are truly amazing self-contained units. They are each unique individuals (although they are made up of genetic material from both parents) containing an embryonic plant and a reserve of food to get the plant started.
Other plant propagation methods have been developed that produce exact duplicates of the original plant. These methods are all asexual and have been practiced by humans since before recorded history. Asexual reproduction involves regenerating a new plant by dividing the roots of an existing plant; by taking cuttings of leaves, stems, or roots; or by budding or grafting. These processes are possible because each single cell of a plant contains all the characteristics of the entire plant and can regrow any missing part. Thus, a section of stem cut from the roots of the plant develops new roots from cells along the base and edge of the stem. A section of root can develop new stem tissue in the same manner.
Some of the commonly used methods of propagating plants are shown below:
Propagation begins with new, high quality seeds (older seeds produce lower quality, slower growing plants). Generally 60-85% of high quality seeds can be expected to germinate and 60-75% of those will produce vigorous, sturdy seedlings. For successful germination, the proper environmental conditions for that seed species must be met. These conditions include temperature, moisture, light, and oxygen.
temperature: Although seeds and plants vary in their soil temperature requirements, most seeds have trouble germinating when the soil temperature is below 50 degrees F. A temperature of 65-75 is optimal for germinating most seeds. Extreme high and low temperatures can damage the seed and prevent germination. You can control the soil temperature using heating pads placed underneath the flats containing the seeds.
moisture: An adequate continuous supply of water is important to insure germination. Water aids in germination by making the seed coat soft so that the embryonic plant within the seed can begin growing. Water itself is an important nutrient and also acts to dissolve other nutrients present in the soil, making them available to the growing plants. Once a seed begins germinating it is important to keep the soil moist but not too wet. If the soil dries out even once, it might kill germinating seedlings. During germination water from above with a very fine mist or water from below by setting the containers in shallow pans of water and removing them when the soil is moist.
light: Light can stimulate or inhibit germination of some seeds. The light reactions involved in the seed are complex. It is known that seeds of certain plants (lettuce and begonias for example) require direct exposure to light before they will germinate while other seeds (phlox and verbena) require a period of darkness before germinating. For other plants, light doesn't matter at all. During germination, light is provided naturally by the sun or artificially with fluorescent lights hung close to the soil surface.
oxygen: In all viable seeds, respiration, which requires oxygen, takes place. When the seed germinates, the amount of oxygen it needs increases. The medium (soil) in which the seed is planted should be loose and well aerated (filled with oxygen). Make sure the containers you seed in have drainage holes in the bottom and the soil isn't constantly saturated.
Other factors including the depth that seeds are planted, the need to keep pots and soil disease-free, and the need to provide appropriate nutrients at the right time are important for proper seed germination.
Before the seeds of many plants will germinate, they go through a dormancy stage. One of the functions of dormancy is to prevent a seed from germinating until it is in a favorable environment. Plants have adapted to many different environments by protecting their seeds in this way. Imagine if all the seeds of a plant living in a really cold climate were to germinate right away in the fall. Most, if not all the germinating seedlings would be killed off by subsequent cold weather and snow.
Various methods have been developed by people to break seed dormancy so these plants can be raised at times other than what their natural life cycle allows. The more common methods are described below.
Seed stratification: This is a procedure that provides an artificial chilling period for seeds that require a cold, dormant period before germinating. To mimic winter for these seeds fill a container with sand or vermiculite place the seeds on the surface and cover with a thin layer of sand or vermiculite, dampen the medium, seal in a plastic bag and place in the refrigerator for a month or two.
Seed scarification: Many plants have seeds with hard seed coats that must be soaked or scratched before the seeds are able to germinate. Seed scarification involves breaking, scratching, or softening the seed coat so that water can enter and begin the germination process.This process occurs naturally over time or with the aid of an animal's digestive system. It can be simulated with sandpaper, hot water, or an acid solution.
Next time you visit the grocery store reflect on what conditions needed to be met in order for the food you see in the store to be produced from animals that eat plants or from plants and seeds themselves.