The integument in plants is a specialized layer of tissue that surrounds the ovule, which contains the female reproductive cells. This tissue serves as a protective covering for the developing reproductive unit before fertilization, after which the ovule matures into a seed. The integument is the outer layer of the megasporangium (the ovule’s precursor tissue), and its presence distinguishes seed plants from non-seed-bearing plants.
Integument Anatomy: Location in the Ovule
The integuments are physically located around the central mass of tissue within the ovule, known as the nucellus. The nucellus houses the female gametophyte, or embryo sac, which contains the egg cell. Structurally, the integuments are connected to the ovule body at the chalaza, which is the base region where the nucellus and the protective layers meet.
The number of integuments present varies between major plant groups. Angiosperms, or flowering plants, typically possess two integuments, a condition described as bitegmic. Gymnosperms, such as conifers, commonly exhibit a unitegmic ovule, meaning they have only one integument layer.
The layers of the integument do not completely fuse at the apex of the ovule, leaving a small, distinct opening called the micropyle. This tiny channel is important because it provides a pathway for the pollen tube to enter the ovule and deliver the male gametes for fertilization.
Transformation into the Seed Coat
The integuments undergo a transformation after fertilization, developing into the seed coat, or testa, as the ovule matures into a seed. This developmental process involves reorganization and histological changes within the tissue layers. The former outer covering transitions from a soft, living tissue to a hard, non-living, protective structure.
In species with two integuments, the outer integument differentiates to form the outer layer of the seed coat, known as the testa. The inner integument differentiates to form the inner seed coat layer, called the tegmen. This transformation often involves the accumulation of sclerenchyma, which are cells with thickened, rigid cell walls, providing mechanical strength to the developing seed.
As the seed matures, the integument tissue also undergoes significant dehydration, which contributes to the seed’s overall desiccation tolerance. Specialized cells within the outer integument may secrete mucilage, a polysaccharide substance that dehydrates and collapses onto the seed surface. This structural change, coupled with the thickening of cell walls, results in a hardened, sometimes impermeable barrier necessary for the survival of the embryo.
Essential Roles of the Seed Coat
The mature seed coat, derived from the integuments, assumes several roles. A primary function is providing mechanical protection for the delicate embryo and the stored nutrients (endosperm) housed within the seed. This dense outer layer shields the internal structures from physical damage and from attacks by pathogens and foraging animals.
The seed coat also plays a prominent role in regulating the seed’s water content and gas exchange, which is necessary for maintaining a state of dormancy. The incorporation of lipophilic polymers like suberin and cutin into the cell walls creates a water-impermeable barrier that prevents the seed from drying out or from imbibing water prematurely. This impermeability ensures that the seed remains viable and does not germinate until environmental conditions are favorable.
Furthermore, the seed coat often contains chemical inhibitors or physical features that impose seed dormancy, ensuring that germination only occurs after specific environmental cues, such as temperature changes or prolonged cold periods, have been received. The texture, color, and appendages of the seed coat, such as hooks or wings, also influence how the seed is dispersed by wind, water, or animals.