5.7.1. Structure of a Dicotyledonous Seed

The outermost covering of a seed is the seed coat. The seed coat has two layers, the outer testa and the inner tegmen.

The seed coat serves as a protective layer for the seed. It has two parts: the outer layer called the testa and the inner layer called the tegmen. This structure is important because as a seed develops, it needs to be shielded from environmental hazards such as pests, pathogens, and extreme weather conditions that could harm the embryo inside.

Think of the seed coat like a tough wrapping around a fragile product you order online. The outer wrapping protects the item from damage during shipping, similar to how the seed coat protects the embryo before it germinates.

The hilum is a scar on the seed coat through which the developing seeds were attached to the fruit. Above the hilum is a small pore called the micropyle.

The hilum is where the seed was connected to the fruit, essentially marking the point of attachment. The micropyle, located above the hilum, is a tiny opening that plays a crucial role during germination. It serves as a channel for water and gases to enter the seed, initiating the germination process.

You can think of the hilum like a small hole you might find on a sealed bag after it’s been tied. It’s the point that attaches it to the larger container, and the micropyle is like a small air vent that allows air to flow in when you’re ready to open the bag.

Within the seed coat is the embryo, consisting of an embryonal axis and two cotyledons. The cotyledons are often fleshy and full of reserve food materials.

The embryo is the future plant that will grow once the seed germinates. It consists of the embryonal axis, which will develop into the stem and roots, and two cotyledons, which provide nutrition until the plant can produce its own food through photosynthesis. The fleshy cotyledons store essential nutrients that help the embryo start growing.

Imagine the embryo as a baby bird inside an egg. The cotyledons would be like the yolk, providing all the necessary nutrients for the baby bird until it’s strong enough to leave the egg and find food on its own.

At the two ends of the embryonal axis are present the radicle and the plumule.

The radicle is the part of the embryo that will develop into the root, while the plumule will develop into the shoot and eventually form the leaves and stems of the plant. This differentiation is crucial for the seed's successful growth into a mature plant, as roots and shoots have distinct roles in nutrient uptake and photosynthesis respectively.

Think of the radicle and plumule like a person getting ready to start a journey. The radicle represents the person’s feet (roots) that dig into the ground for support, while the plumule is like their head and arms (shoot) reaching out towards the sky, eager to explore and grow.

In some seeds such as castor, the endosperm formed as a result of double fertilisation is a food storing tissue and called endospermic seeds. In plants such as bean, gram, and pea, the endosperm is not present in mature seeds and such seeds are called non-endospermous.

Endosperm is a tissue that provides nutrition to the developing embryo within the seed. In endospermic seeds (like castor), this tissue remains and nourishes the embryo after germination. In contrast, non-endospermous seeds (like bean and pea) utilize all of their stored nutrients in the cotyledons, so there is no endosperm present at maturity.

Consider endospermic seeds as a meal kit with leftovers. The endosperm is like the leftover ingredients that help the baby plant grow, while non-endospermous seeds are like a meal that uses up all the ingredients at once, leaving no extra for later.

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