Seeds and Germination

Seeds and Germination

Look closely at a sliced tomato. You can see the start of many new tomato plants inside. Every tomato seed contains a tiny tomato plant that is alive but dormant. Dormant means that it is not growing. It is just waiting. When the conditions around it are right, the seed will germinate. This is when the tiny plant sprouts from the seed and begins to grow.

Parts of a Tomato Seed

Flowering plants, like tomatoes, create more of themselves by making seeds. The tiny plant inside a seed is called an embryonic plant. Embryonic means something is at an early stage of development. The embryonic plant has an embryonic root called a radicle. It also has one or two embryonic leaves called cotyledons. There is a protective shell, called a seed coat or testa, around the whole embryonic plant.

Embryonic tomato plants need energy to germinate. This energy comes from the endosperm. The endosperm is usually made of starch. Starch is a complex carbohydrate. When broken down, starch provides energy for the growing embryo.

Some plants have seeds without an endosperm. These seeds are called non-endospermic seeds. Instead of storing energy in starch, they store energy in their cotyledons. Peas and green beans are two examples of plants with non-endospermic seeds.

Flowering plants are classified by how many cotyledons they have in their seeds. Plants with seeds that have one cotyledon are called monocotyledons, or monocots for short. Plants with seeds that have two cotyledons are called dicotyledons, or dicots for short. Tomato plants are dicotyledons.

The Life of a Seed Before Germination

The purpose of seeds is to bring the offspring of plants to new places when they can have space to grow. This means that seeds must be able to travel. Seeds can be moved by the wind, by water, or even by the fur of animals. Tomato seeds use animals for transportation in a different way. When animals eat tomatoes, the seeds pass through the digestive system. They are then “planted” in the waste that the animals leave behind. 

Try This!

In this lesson, students work collaboratively to design and build models of seeds that can travel as far as possible.

Germination of Tomato Seeds

Seeds are alive, but they cannot live forever. Some seeds, like onions, can only germinate for one year. Others, like cucumber and lettuce, can still germinate after five years or more. Tomato seeds can live up to four years.

Did you know?

The oldest seeds to germinate successfully were 32,000 years old! They were from a flowering plant called a narrow-leafed campion. Russian scientists discovered the seeds preserved in ice in Siberia.

Tomato seeds stay dormant until the environment around is right for germination. This is true of all seeds. They need water, oxygen and warmth to start growing.

When water is added, the testa softens to make it easier for the plant inside it to sprout. The cells of the plant take in water and grow bigger.

Oxygen also enters the seed. Plant cells need oxygen for cellular respiration, which is the process of getting energy from food. At this stage, the plant gets its energy from the endosperm.

Warmth is needed as well. Different seeds germinate best at different temperatures. In general, very cold temperatures prevent germination. Warmer temperatures cause faster germination. Tomato seeds will not germinate below 10°C and their preferred range is 16-30°C.

Let’s see how this looks for a tomato seed. 

Dry seeds are dormant seeds. The photo on the right shows a dry, dormant tomato seed. The seed coat is covered with fine hairs called trichomes.

Image - Text Version

Shown is a colour photograph of a tomato seed labelled Day 1. 

The seed is shaped like a teardrop and covered with something that looks like thick, pale golden fur. A white arrow points to the fur. This is labelled "Testa". A ruler runs along the right edge of the photograph. The space between two horizontal lines on the ruler is labelled 1 mm. This scale indicates that the seed is just over 3 mm from top to bottom. The same ruler appears on each photograph of the tomato seed.

 

After 2 days in a warm, moist environment the radicle has started to push through the weakened testa.

Image - Text Version

Shown is a colour photograph of a tomato seed labelled Day 2. 

The seed is shaped like a teardrop and is mostly deep, golden yellow in colour. The top pointed edge of the seed is pale gold. This part is labelled with a white arrow and the words "Emerging radicle". The rest of the seed is covered in what looks like thin, translucent, fur.  The ruler indicates the seed is now about 4 mm long.

 

After 4 days the radicle has grown several millimetres.

Image - Text Version

Shown is a colour photograph of a tomato seed labelled Day 3. 

A long, white structure now grows from the top of the seed. The ruler indicates this structure is over 7 mm long. This part is labelled with a white arrow and the words "Growing radicle".

The seed is still about 4 mm long and deep yellow, but it looks much thinner than on Day 2, as if it is deflated.

 

After 6 days, the green cotyledons have emerged from the testa. The radicle has also continued to grow. As long as water, oxygen and warmth are available, the plant will keep growing. Its roots extend downward and the shoot and cotyledons push upward.

Image - Text Version

Shown is a colour photograph of a tomato seed labelled Day 4. 

A long structure emerges from the top of the seed and curves up to the top of the frame and back down beside the seed. It is white at the bottom, and becomes light green at the top, where it splits into two pointed ends. This structure is labelled with a white arrow and the word "Cotyledon". The ruler indicates it is about 15 mm long. 

The seed is still deep yellow and about 4 mm long. The radicle still grows down from the seed and out of the picture.

Did you know?

Different seeds require different conditions to germinate. Some seeds must go through a cold period to be able to germinate. We call this process vernalization.

Many things are going on inside the seed as well. For example, embryonic tomato plants make growth hormones called gibberellins during germination. Gibberellins help weaken the endosperm and testa so that the embryonic plant can sprout.

As the tomato seedling grows, it stops using energy stored in the endosperm and starts using its cotyledons for photosynthesis. This is when the plant starts to need light and carbon dioxide as well as water, oxygen and warmth.

Did you know?

Tomato seeds do not need soil to germinate, but they do need the nutrients and minerals in soil to become healthy adult tomato plants.