A seed is a reproductive unit produced by plants after fertilization. It contains an embryo, a miniature plant that can develop into a new plant when conditions are suitable. Seeds are a crucial stage in the plant life cycle, enabling propagation and establishment.
Anatomy of a Seed
A typical seed comprises three main components: the embryo, food storage tissue (either endosperm or cotyledons), and a protective seed coat. The embryo is the undeveloped plant within the seed, containing structures that will form the future plant. This includes the radicle, the embryonic root, and the plumule, which develops into the shoot system.
The cotyledons, often called seed leaves, are part of the embryo. They can store food reserves or absorb nutrients from a separate storage tissue. This stored food, whether in the endosperm or cotyledons, provides energy for the embryo’s initial growth. The endosperm is a nutritive tissue rich in starches, oils, and proteins, supplying nourishment to the developing embryo and seedling during germination.
Encasing these inner parts is the seed coat. This tough outer layer shields the embryo from physical damage, desiccation, and pathogens. The seed coat also influences germination by regulating water and gas entry. Its varying thickness and permeability ensure the seed’s survival until conditions are favorable for growth.
Function and Life Cycle
Seeds enable plant reproduction, protect the young plant, and facilitate dispersal. Seed formation begins after an ovule is fertilized by pollen, forming a zygote. This zygote develops into an embryo within the ovule, which matures into the seed. For flowering plants, the ovary surrounding the ovule ripens into a fruit, often aiding in seed dispersal.
Once dispersed, a seed remains dormant until optimal environmental conditions for germination. These conditions include adequate water, suitable temperature, and sufficient oxygen. Some seeds also require specific light conditions.
Germination begins with imbibition, where the dry seed absorbs water, causing it to swell and soften the seed coat. This reactivates the seed’s metabolic processes, suspended during dormancy. The radicle, the embryonic root, is usually the first to emerge, anchoring the seedling and absorbing water and nutrients. The plumule, the embryonic shoot, then emerges and grows upwards, developing into stem and leaves. Stored food reserves provide energy for this initial growth, sustaining the seedling until it can begin photosynthesis.
Significance of Seeds
Seeds are highly significant, both ecologically and for human societies. Ecologically, seeds are crucial for plant propagation and maintaining biodiversity. They enable plants to spread, colonize new areas, and contribute to genetic diversity. Seeds also serve as a primary food source for wildlife, forming a foundational element of many food webs.
For humans, seeds are vital, forming the basis of global agriculture and food security. Staple crops like rice, wheat, and maize are cultivated from seeds, providing most of the world’s dietary calories and nutrients. Beyond food, seeds yield products like vegetable oils, fibers, and raw materials for industrial applications. Many medicines originate from compounds extracted from seeds. Preserving diverse seed varieties in seed banks safeguards genetic resources against environmental and agricultural challenges.