The Anatomy of Seeds

Seeds are some of the most important and fascinating feats that the botanical world has to offer. After all, they’re the end-product of all seed-bearing plants; their lifelong investment which is intricately designed to ensure the survival of the next generation.

While they come in all the shapes, sizes, colours, and forms you can imagine, all seeds share a common anatomy that dates back hundreds of millions of years.

Despite every plant having unique strategies when it comes to the development of their seeds, they all contain the same principal structures that make up the seed.  These are:

1. The Seed Coat: The hard tissue that surrounds the seed.
2. The Embryo: The tissues that develop into the future stems, leaves, and roots of the plant. This also includes the cotyledons.
3. The Endosperm: This is a storage organ found within many different types of plants, particularly monocots.

1. The Seed Coat

The seed coat is the external layer that protects the seed and mediates many important functions related to dormancy and germination. It is the part of the seed that we see when we look at a seed; it can be thick, smooth, textured, and come in a variety of colors.

During dormancy, it protects the internal structures from pests and potential predators, While they are usually semi-permeable, they are designed to prevent bacteria, fungi, or viruses from entering inside and compromising the future health of the plant. Thick seed coats can prevent predators from gaining access to the nutrient-rich insides, be it by preventing them from breaking them open for food or by keeping the nutrients unavailable as they pass through the digestive system of an animal. Likewise, by being protected in the digestive system of an animal, it also facilitates the dispersal of the seeds as they become deposited in the nutrient-rich feces of the animal.

Seed coats are also important for mediating the proper timing of seed germination. For example, fire-dependent seeds have thick seed coats that only burst open after being exposed to hot temperatures. This ensures that they germinate with low competition in the aftermath of a wildfire. Other seeds that germinate in the presence or absence of sunlight have thin seed coats that are capable of being penetrated by certain light waves.

Parts Of The Seed Coat

• Testa: This is the outer layer of the seed and makes up most of the seed coat. It is the thick protective layer of the seed coat.
• Tegmen: This is a thinner layer that exists below the testa and above the internal parts of the seed.
• Micropyle: This is a small pore that facilitates the entry of water. It is often visible on some seed coats.
• Hilum: This is a small “scar” on the seed coat which marks where the seed was attached to the rest of the fruit.
  
  

2. The Seed Embryo

Below the seed coat is the embryo; it is arguably the most important part of the seed. All other parts of the seed are intended to protect and ensure the survival of the embryo. That’s because it contains the primitive tissues, which are destined to become all of the future parts of the plant. All of the leaves and roots that you see in a developed plant, originated from these embryonic cells.

The Embryo consists of four primary structures;

• Cotyledon: For many seeds, the largest portion by volume and mass consists of the cotyledons. Dicots such as Beans and Tomatoes contain two cotyledons, while monocots such as grasses contain one. The cotyledons act as nutrient/energy reserves and are important for nourishing the developing seed during germination. In many plant species, the cotyledons are lifted above ground and can conduct photosynthesis to further promote plant development. In other plants, cotyledons stay below ground and nourish the developing plants from there.
• Epicotyl and Hypocotyl: The epicotyl and hypocotyl are stem tissues located on either side of the cotyledons within a developing seedling. The hypocotyl is located below the cotyledons and attaches to the embryonic root below. The epicotyl is located just above the cotyledons and supports the plumule.
• Plumule: The plumule is located at the tip of the epicotyl and often looks like a small feather-like structure (hence the name plumule from the Latin “pluma” which means feather). Within the tissues are the meristems, which will develop into all the branches, and leaves of the plant. Generally speaking, all above-ground tissues we see in a plant originate from the epicotyl.
• Radicle: Also known as the embryonic root, the radicle is the first root that emerges from a developing seed. It often swells to be relatively large, and its first job is to anchor the seed into the soil. Once in the soil, the radicle begins developing root hairs, which can absorb water and nutrients. In some plants, the radicle becomes a large tap root that deeply anchors the plant and reaches for below-ground nutrients.

3. The Endosperm

The endosperm is present in the seeds of many flowering plants and acts as a storage organ for the developing embryo. It mostly contains starches but also fats, minerals, and all other nutrients needed for growth. Despite their being present in the basic anatomy of all seeds, their function and prevalence greatly vary between dicots and monocots.

• Endosperms in Dicots: While dicots (those with two cotyledons like beans, tomatoes, and mint) do contain endosperms, in most species this part of the seed is largely undeveloped. During the development of these seeds, the nutrients within the endosperm are quickly mobilized to the cotyledons, which replaces the role of the endosperm. Certain dicots are considered “endospermic dicots”, meaning that they do contain well-developed endosperms.
  
  
• Endosperms in Monocots: Endosperms are particularly important in the anatomy of monocots. These are plants that only contain one cotyledon, such as grasses and grass-like flowering plants. This includes corn, wheat, rye, and all the true grains consumed by humanity. In fact, it is the nutrients contained within the endosperm that make these grains so valuable to our society. 

Other Seed Structures

Since their dawn about 400 million years ago, plants have developed many creative strategies to improve the success of their seeds’ survival. Many additional structures have been developed by certain plant lineages that help in the dispersal or protection of the seed.

• Burs: Burs are prickly structures that surround the seed and help in seed dispersal. It is these burs that often get stuck in the socks, shoes, and pant legs of hikers. They are also notorious for getting stuck in the fur of dogs.
  
  
• Wings: These structures improve the aerodynamic properties of the seed. This facilitates the dispersal by wind and allows the seeds to travel greater distances. Seeds that contain wings are known as “samaras”.
  
  
• Fleshy Fruits: This is something we are all familiar with. Fleshy fruits are modified portions of the former ovaries of the mother plants. They are rich in nutrients and help attract animal dispersers, which will help get the seeds to new locations.
  
  
• Capsules: These are specialized fruit structures that are dry and tough, helping protect the seeds before they are ready to be released.
  
  
• Elaiosome: This is a specialized structure attached to the outside of seeds that is rich in lipids and proteins. These share a similar purpose as fleshy fruits but are particularly important in dispersal by ants and other insects.
  
  
• Pappus: Pappus is the cotton fluff found in dandelion seeds, which helps in wind dispersal. It is found in many members of the sunflower family.
  
  
• Mucilage: Certain seeds have a sticky mucilaginous membrane surrounding the seed coat. When wet, this can stick to unsuspecting animals and help in dispersal.
  
  
• Trichomes: These are small, crystal-like hairs that can help in dispersal or deter herbivory.