Learn the ins and outs of selecting, planting, and caring for seeds in your garden. Get expert tips on harvesting and saving seeds for future plantings.The seed . It is the fertilized and mature ovule, inside which the embryo is found in a latent state of life, protected or not by food reserves and surrounded by various protective covers (Episperma).

Seed morphology

The seed internally presents food reserves such as: Proteins (like lentils), Oils (like sunflowers), Starches ( Beans , Corn, Wheat , Barley and in general all Cereals ), Vitamins and Hormones .

Seed morphology is highly variable. This is due to three fundamental factors: the botanical family to which the species belongs, the type of outer covering, and the shape and size of the seed. In large families that are subject to greater adaptive pressures, variability is greater.

The seed coat is variable and may be protected by the testa, the pericarp of the fruit, the stony endocarp and a wide variety of floral remains. The shape of the seed is variable, even within the same plant. As for size, the smallest seeds are found in orchids and parasitic plants, many of which weigh less than 5 millionths of a gram. The largest seeds are those of the Coco .

Internal structure or anatomy of the seed

The seed is made up of:

• The outer covering, husk , integuments or episperm.

Seed coat or Episperm

These covers are made up of two layers of fabric:

• Outer tissue or Testa .
• Internal tissue or Tegmen , which come from the protective integuments of the seminal rudiment, primina and secundina respectively.

External morphology of the head

Externally, in the seed coat, structures such as:

Micropyle

Point-shaped orifice, sometimes microscopic, which corresponds to the micropyle of the seed rudiment. It is through this that the radicle emerges from the seed during germination, since the impermeability to water is lower in this area.

Thread

Scar of various shapes left by the funiculus when the seminal rudiment is detached.

Calaza

It is a protuberance that sometimes corresponds to the place where the ovule’s chalaza was located . Its position with respect to the micropyle and the hilum depends on the type of seminal rudiment; it is more apparent in seeds from campylotropic ovules .

Rafe

It is the mark left on the testa by the fibrovascular bundle, which in the seminal rudiment joined the funiculus with the calaza.

The anatomy of the episperm is very variable, which is why its study is important when analyzing germination problems. The functions of the integuments are:

• Protect the internal structures of the seed.
• Prevent desiccation of the endosperm .
• Ensure gas exchange.
• Prevent the attack of pathogens.

In order to meet these objectives, the teguments have the following characteristics:

• It is a lignified layer.
• It stores a large amount of cellulose and lignin .
• It is an extremely porous layer.
• It presents a large number of adaptations with the purpose of perpetuating the species, such as:

Aril

Protuberance that forms on the surface of a seed (mamoncillo). Caruncle. They are two fleshy projections. Eg: castor oil plant, which develops a small micropylar aril. Strophiolo . Rough seed with an appendix.

Winged

This type of seed adaptation is found in all conifers, which have wings so that they can be carried by the wind, Pine . Others have adaptations such as:

• Fibrous tegument, as in the avocado seed .
• Filamentous, hair-like covering as in Cotton .
• Presence of noticeable pores.

Smooth cover

The testa has a large amount of pigments and this is the reason for its varied colouring. The tegmen is attached to the testa and also to the endosperm. The tegmen is poorly lignified, it is fragile, it accumulates a large amount of cutin, which forms the cuticle, to avoid excessive transpiration and it is waxy and porous to allow gas exchange to give life to the embryo .

The embryo

Plant primordium or miniature plant , in which the root, stem and leaves are already outlined, joined to the reserve material in the cotyledons themselves. It is enclosed in the seed and can remain in a latent state for a long time. The embryo is structured as follows:

• Embryonic axis: It is made up of the plumule , hypocotyl and radicle.

Embryonic axis

• The plumule: It is made up of primary meristematic tissue, protected by two embryonic leaves. It gives rise to the stem of the plant.

Hypocotyl

It is the part of the embryonic axis that is inserted under the cotyledons and above the radicle. In seeds with epigeal germination, the hypocotyl pushes the cotyledons towards the surface of the earth because it has negative geotropism. In seeds with hypogeal germination, in which the cotyledons remain underground during the process, the hypocotyl does not differentiate or grow.

• Radicle: Portion that is opposite in position and development to the plumule, originates the root of the plant and is made up of primary meristematic tissue. It is the first to emerge from the seed when it germinates and has positive geotropism.

Cotyledons

The cotyledons are inserted into the embryonic node, below the plumule, they are the first leaves of the new plant, they can store reserve substances as occurs in the Bean , or be reduced, in these cases they serve as intermediaries in the transport of substances from the endosperm to the embryonic axis, although in some plants they serve as reserve material when the endosperm is depleted .

Endosperm

Tissue that stores reserve substances, appears surrounding the embryo, in the seeds in which it is present. At the beginning it is always liquid, but when the seed matures it solidifies. However, there are some mature seeds with liquid endosperm, as occurs with the coconut. When the seed has endosperm, the cotyledons are thin, like leaves .

Seed classification

By the cover or episperm

• Seeds with woody cover. E.g. Mamey .
• Seeds with hairy covering. E.g. Majagua .
• Winged seeds. E.g. Cedar , Mahogany , Pine .
• Seeds with caruncle. E.g. Higuereta .
• Seeds with aril E.g. Mamoncillo , Tamarind .

By the shape of the seed

• Spherical seeds. Ex. Pea .
• Lenticular seeds. E.g. carob tree .
• Reniform seeds. E.g. Majagua .

By the position of the embryo

The embryo, according to its position, can be: basal, axial and peripheral, adopting different forms within these positions, which are:

• Basal rudimentary.
• Wide basal.
• Basal capitate.
• Basal lateral.
• Linear axial.
• Axial spatulate.
• Curved axial.
• Axial folding.
• Invested axial.

This last aspect is used only for research work.