A seed is a protective package containing an embryonic plant and a reserve food supply. This structure is the primary mechanism by which flowering plants ensure the survival and dispersal of their offspring. The entire process, from the initial transfer of genetic material to the final hardened, dormant package, is a sophisticated biological sequence. Understanding how a seed is made involves tracing the precise steps of this transformation, starting with specialized reproductive structures.
Reproductive Structures Required
Seed production begins with specialized organs within the flower of an angiosperm. The female reproductive structure, known as the carpel or pistil, houses the ovules within a lower, swollen section called the ovary. Each ovule contains a female gametophyte, or embryo sac, which holds the egg cell ready for fertilization.
The ovary is the precursor to the fruit, designed to protect the developing seeds and aid in their eventual dispersal. On the male side, the anthers produce pollen grains, which are microscopic capsules containing the male gametes. Each pollen grain is protected by a tough outer layer called the exine, ensuring the genetic material survives the journey to the female part of the flower.
The Pollination Process
The journey toward seed formation is triggered by pollination, the physical transfer of the pollen grain from the anther to the receptive surface of the stigma. This transfer is mediated by various agents, including wind, water, and animals such as insects, birds, and bats. The specific shape and composition of the flower often dictates which agent is responsible for carrying the male gametes.
Once a compatible pollen grain lands on the sticky stigma, it begins to germinate, extending a slender cellular protrusion called the pollen tube. This tube grows down through the style, navigating the maternal tissue toward the ovule within the ovary. The growth of the pollen tube is a highly directed process, often guided by chemical signals secreted by the embryo sac.
The pollen grain’s tube cell nucleus directs this growth, while the generative cell travels inside the tube. As the tube nears the ovule, the generative cell divides to produce two distinct sperm nuclei. The pollen tube finally penetrates the ovule, typically entering through a small opening called the micropyle, and releases these two sperm nuclei into the embryo sac.
Fertilization and Double Fusion
The moment the two sperm nuclei are released marks the beginning of a process unique to flowering plants called double fertilization. This event involves two separate fusion actions occurring almost simultaneously within the embryo sac. The first fusion occurs when one of the haploid sperm nuclei unites with the haploid egg cell.
This union creates a single diploid cell known as the zygote, which is the first cell of the future plant embryo. The second sperm nucleus travels further into the embryo sac to fuse with the two polar nuclei located in the large central cell. This second fusion involves three haploid nuclei, producing a triploid cell (3n) called the primary endosperm cell.
This primary endosperm cell quickly begins to divide, forming the endosperm, a nutrient-rich tissue that serves as the food reserve for the developing embryo. Both the resulting diploid zygote and the triploid endosperm are necessary for the development of a viable seed. The success of this double fusion event signals the surrounding flower parts to begin their transformation.
Maturation of the Embryo and Seed
Following double fertilization, the zygote undergoes a series of highly regulated cell divisions and differentiations to develop into a mature embryo. The initial division of the zygote is often asymmetrical, creating a small terminal cell that will form the proembryo and a larger basal cell that develops into a stalk-like structure called the suspensor. The suspensor anchors the embryo and facilitates the transfer of nutrients from the parent plant tissue.
The proembryo progresses through distinct morphological stages, eventually forming the rudimentary structures of the new plant, including the embryonic root (radicle) and one or two seed leaves (cotyledons). Simultaneously, the triploid endosperm tissue proliferates around the embryo, accumulating starches, oils, and proteins that constitute the seed’s food supply. In some species, the endosperm is entirely absorbed by the growing cotyledons, which then store the nutrients.
The final stage of seed development involves structural hardening and dehydration. The ovule’s outer protective layers, known as the integuments, harden and dry out, transforming into the tough seed coat, or testa. This protective layer ensures the embryo is shielded from mechanical damage and desiccation. As the seed loses water, often dropping to five to fifteen percent moisture content, it enters a state of metabolic arrest known as dormancy, ready to be dispersed and await favorable conditions for germination.