Germination of seeds

Germination of seeds is certainly a topic in most gardeners’ minds at this time of the year.  In fact, while I was cleaning out my old pots and collecting seeds to plant, my mind wandered to wonder…wondering how the tiny, hard seeds can indeed produce shoots to become wonderful and inspiring plants from something that almost seems dead.

group pf germinating seeds

group of germinating seeds

A seed surely looks lifeless as it displays no obvious signs of life rolling around in its package.  According to several articles in fact even biochemical tests for metabolic processes such as respiration, show very little to indicate whether the seed is alive. Of course for us the seed collectors, we hope the seeds have been stored correctly so they are not dead, but merely dormant.

 

Dormancy

 

While all living seeds are inactive, awaiting hydration, some require specific environmental conditions which generally mean a longer time before germinating.  In the case of home germination of seeds, gardeners need to know what those conditions are, as generally stated on seed packages or by checking out good sources of planting information.

Common vegetable garden seeds generally lack any kind of dormancy and are ready to sprout with some moisture and warm enough temperature for their biochemistry to begin. Many flower seeds and those from wild plants in particular may have deeper forms of dormancy that require further conditions before their growth is triggered.

While we may be familiar with scarifying and stratifying, both terms that describe external treatments   of a seeds outer membrane that some seeds require to leave their dormant state, temperature, and light also play a role before hydration can begin.  There are also physiological factors that signal the end of dormancy, such as plant hormones of Gibberellins which activate plant growth.  Other seeds may have a naturally found coating of Abscisic Acid which prevents germination until environmental conditions have worn this layer off and provided the conditions for germination of those seeds.

Germination

 

Germination can be explained simply with a few brief diagrams, or in great detail with many technical terms. When I bought seeds recently and began to collect pots and planting materials, the gardener and biochemist in me wondered how a few molecules of water and some sun could cause the tiny speck of a seed to ultimately form a large, beautiful plant.

Regardless of its size, each seed contains an embryo and in most cases, a store of food to help that seedling begin its formation.

Avocado seed diagram

Avocado seed diagram

 

A “typical” seed   has fundamental parts:

  • a seed coat
  • a storage area (in this case the endosperm which houses food and genetic material)
  • a dormant embryo

 

The embryo has three parts:

  • the cotyledon (or seed leaf)
  • the epicotyl (becomes the shoot)
  • the radicle (becomes the root)

 

456px-Salix_scouleriana_seed

 

 

 

Physiology of Germination

 

When water penetrates through the seed coat and begins to soften up the hard tissues inside, causing the seed to swell up and then split.  Then water enters even faster and begins to activate the biochemistry of the dormant embryo.   Once such substance activated there is Gibberelli Acid (GA), which is a plant hormone similar to steroids.

Then the GA is carried with the water through out the seed tissues and in to the endosperm until it activates genes inside the seed and a complex DNA/RNA reaction takes place. Basically this “blue print” allows the genetic characteristics of the parent plant to be passed on through the seed. Also as this is going on, the embryo feeds on the starch and sugar found in the endosperm as well.

Under proper conditions, the seed begins to germinate and the embryonic tissues begin to protrude from the seed’s casing in the form of what is called the Radicle. This primary root anchors the seedling in to the ground as well as absorbing more moisture.  From here other growth takes place, but the components vary.

In most plants another shoot emerges, many of which have three parts:

– the embryonic stem called the hypocotyls

– the cotyledons ( first leaves)

– the epicotyls

 

 The hypocotyls

The hypocotyls grows out and lifts the growing seedling up as it becomes the stem of the seedling and carries the cotyledons (embryonic leaves)   clear of the ground.  At this point growth components change depending on the classification .Flowering plants are classified in to two groups, depending on whether they have one or two cotyledons.

The cotyledons

Monocots have only one single shoot or cotyledon after the seed breaks out of the ground. In these plants the roots develop from the stem instead of from the base of the embryo. Included in this group are flowers such as orchids and lilies as well as wheat, onions and corn, asparagus and many other plants and grasses.

 

 

Monocot diagram

Monocot diagram

 

Dicots have two initial leaves or cotyledons that grow from the embryo.  Then the the epicotyls forms the initial stem which holds up the true leaves, while the radicle grows and forms the plant’s the root-system. Most flowering plants found in the garden belong to this classification. Beans are also a dicot.

 

 

Dicots

 

The epicotyls

The epicotyls, which are above the cotyledons and below the plants real leaves, are not found in every plant. Where they are present, they become the plants stem, which rises up and gives way to the plant’ s further  leaf and stem development. The growth of plants is another matter that requires far more complex physiological terminology than this author/gardener can explain.

 

Germination of seeds

 

In addition to Monocot and , Dicot germination there are other forms including but not restricted to Epigeous, Hypogenous and Precocious germination. There are many names, classifications, conditions the seed requires and other complexities that my brain can barely contain!

Despite that, germinating seeds each spring is part of my celebratory ritual and I am glad to have some basic understanding of what really is going on inside each seed.  Based on several definitions, seed germination is said to have occurred when growth of the radicle bursts the seed coat and protrudes as a young root… but to us gardeners the no names matter as the excitement builds when the green shoots poke their heads above the ground and inspire us to bury more seeds for germination!

 

-_Seedling_-001