Seeds

Seeds are where the miracle of plant life is stored.

All plants, aside from mosses, liverworts* and some ferns, can be grown from seed. Seeds come in all shapes and sizes. They can be as tiny as an orchid seed, which weighs in at 1/1,00,000th of a gram, to the oceanic giant, coco de mer, at 40 pounds!

After a flower is pollinated, a seed develops. Most seeds aren’t very impressive looking from the outside. Usually, a hard shell, or seed coat, covers what looks like starch. But locked in that seed coat is an embryo, waiting to be born.

Seeds are categorized as either angiosperms or gymnosperms. Angiosperms are flowering plants. Their seeds are surrounded by fruit, such as the apples, tomatoes, and cucumbers in your garden. In contrast, gymnosperm seeds are naked. Conifers and ginkgoes are examples of gymnosperms.

A seed is called viable when it can sprout on its own. Most garden seeds only remain viable for 1 - 5 years, but the oldest recorded viable seed was 1,300 years old when it sprouted! In 2007, scientists extracted embryos from Siberian Arctic flower seeds. The seeds were germinated in vitro. These test tube babies grew, flowered, and created their own viable seeds. They were nearly 32,000 years old! Outside of the lab, a 2,000 year old Judean date palm seed that was excavated from Herod's garden was germinated in 2005. How’s that for tenacity? What makes this even possible? Let’s find out.  

Seed development

Seeds have six stages of development:

1. They start out as fertilized eggs called zygotes. Zygotes are the combined DNA from both parents.
2. The zygotes develop into a series of cells called the pro embryo.
3. The next phase is called the Globular Stage. This is where cell specialization really gets going and you can begin distinguishing between root and shoot development.
4. As the cotyledons develop, they have a heart shape at first. This is called the Heart Stage.
5. The next stage is called the Torpedo Stage because, well, the cotyledons have elongated into more of a torpedo shape.
6. The final stage of seed development is when a mature embryo is formed. 

Stages of seed development (TheLAW14) CC BY-SA 3.0

Once a seed has developed fully, it can float on a breeze, sit in the ground, or, in some cases, pass through someone’s digestive system, unscathed and still viable. ​


Seed structure

Seeds are fertilized eggs called ovules. There are three parts to a seed: the embryo, endosperm, and seed coat. The seed coat surrounds its innards, providing protection from the elements, insects and disease. The seed coat also helps keep water out. This prevents germination from occurring at the wrong time of year. It takes a specific combination of moisture, heat and sunlight to break the seed coat.

Held within the seed coat is a large food supply called endosperm. Endosperm is made of carbohydrates, proteins, or fats, depending on the type of plant.

The embryo is a miniature version of the mature plant, held in a state of limbo. The embryo has several parts: 

• cotyledon - first leaves (monocots have one, dicots have two)
• plumule - shoot
• hypocotyl - stem (only found in dicots)
• radicle - root

Seeds also have scars. One scar, called the hilum, is formed where the seed was attached to the ovary wall. Another scar, called the raphe, occurs where the seed was attached to the placenta, making it the seed's bellybutton.

It is the structure of a seed that makes it so durable. Until the correct conditions exist, that little bundle of potential will stay the way it is for a good long while. So, what triggers it to change?

​Germination

Germination only occurs when all the conditions of light, temperature, moisture, air, and seed viability are right. When these conditions occur, certain enzymes are activated. From the Greek words that mean “leavened within”, enzymes are protein molecules that act as catalysts for biochemical changes.

These newly activated enzymes start converting the endosperm (carbs) into sugars that the plant can use for energy. At this point, the seed coat ruptures, the stems (plumules) go up and roots (radicles) go down and a new plant is born!

Today's garden challenge

Take half a dozen paper cups, fill them soil, and place a bean or sunflower seed in the medium, about 1/2" down. Gently saturate the soil with water and place the cups on a sunny window sill. While you're at it, pop a couple of seeds open and see if you can identify its parts! Use the Comments section to let us know which seeds you planted and how long they take to germinate!
 
*Liverworts (and hornworts) are the simplest form of plant life. They have no roots or vascular bundles. Liverworts are believed to have been the first group of plants to evolve from algae.