The Protective Functions of a Seed Coat
The primary role of the seed coat is to shield the internal structures from external threats. This physical barrier provides protection against mechanical damage, such as being crushed or abraded in the soil. A hard testa prevents the delicate embryo and its food supply from being destroyed. This defense is a factor in the longevity and viability of many seeds.
The seed coat also offers protection from environmental stressors. It creates a barrier that prevents desiccation by limiting water loss and can insulate the embryo from extreme temperatures. In some species, the seed coat contains chemical compounds that guard against damage from fire.
The seed coat also defends against biological threats. Its tough exterior deters insects and herbivores. If eaten, a hard coat may allow a seed to pass through an animal’s digestive tract unharmed, aiding in dispersal. The coat’s cuticle is often impermeable to pathogens like fungi and bacteria, preventing infection.
Seed Coat Structure and Variations
The seed coat’s structure is derived from the integuments of the ovule and is often composed of two layers: the outer testa and the inner tegmen. The testa’s strength and water resistance are due to durable biological polymers like cellulose and lignin, as well as waxy substances such as cutin and suberin.
The diversity of seed coats reflects adaptations to various ecological niches. This variation is evident in the contrast between the thick, woody shell of a coconut and the thin covering of a peanut. Some seeds, like pomegranates, have a fleshy, edible outer layer known as a sarcotesta, which encourages animals to consume and disperse them.
Specialized structures on the seed coat often aid in dispersal. The wings on a maple seed and plumes on a dandelion seed are modifications that allow for wind transport. Other seeds have hooks or barbs on their surface that attach to the fur of passing animals, facilitating transport to new locations.
The Role of the Seed Coat in Germination
The seed coat regulates germination timing, often by enforcing dormancy. This period of arrested development prevents germination until environmental conditions are suitable for seedling survival. The seed coat can enforce dormancy by being impermeable to water and gases or by containing chemical inhibitors that suppress growth.
For germination to begin in seeds with permeable coats, imbibition must occur. This is the absorption of water, which causes the internal structures to swell and rupture the seed coat. This influx of water activates the metabolic processes within the embryo, initiating the growth of the new plant.
Seeds with hard, impermeable coats require scarification to break dormancy. This process wears down the seed coat. Mechanical scarification occurs from abrasion by soil or rocks. Chemical scarification happens when a seed passes through an animal’s digestive tract. Thermal scarification, from fire or freeze-thaw cycles, also creates cracks in the testa, allowing water to enter and germination to proceed.