Seed plants, formally known as Spermatophytes, are the most successful and widespread group of plants on Earth, dominating nearly all terrestrial ecosystems. This diverse group includes everything from towering conifers to common grasses and flowering herbs. Their evolutionary success, starting approximately 350 million years ago, allowed them to colonize drier environments more effectively than earlier plant forms like mosses and ferns.
Their prevalence is a direct result of reproductive innovations that provided superior protection and nourishment for the next generation. They are responsible for the majority of the world’s biomass and play a major role in global ecology.
Key Evolutionary Traits
The defining structure of this group is the seed itself, a significant evolutionary innovation that allows for the dispersal of a protected, dormant embryo. A seed consists of a developing diploid embryo, a food supply, and a tough protective seed coat. This structure allows the embryo to remain viable for extended periods until conditions are favorable for germination.
Seed plants also possess highly developed vascular tissue, which enables efficient transport of water and nutrients throughout the plant body. This system includes xylem, which transports water and minerals upward, and phloem, which carries sugars produced during photosynthesis. The efficiency of this transport system allowed seed plants to grow to much larger sizes than non-vascular plants.
Another defining trait is heterospory, meaning the plant produces two distinct types of spores. Microspores develop into the male gametophyte (pollen grain), while megaspores develop into the female gametophyte, which is retained within the ovule. This led to the development of pollen, eliminating the need for external water to transport sperm to the egg.
The Reproductive Cycle
The life cycle of a seed plant is characterized by the dominance of the sporophyte generation, which is the large, familiar plant we see. The gametophyte generation is microscopic and retained entirely within the protective tissues of the sporophyte. This retention ensures the vulnerable gametophyte is nourished and shielded from desiccation.
Reproduction begins with pollination, the process where the male gametophyte (pollen) is transferred to the female reproductive structure, the ovule. This transfer is often accomplished by wind or animals, eliminating the need for water. Once the pollen reaches the ovule, it germinates and grows a tube that delivers the sperm cell to the egg cell.
Fertilization occurs when the sperm fuses with the egg, forming a diploid zygote. This zygote develops into the embryo, packaged with surrounding tissue and food supply into the seed. The ovule wall hardens to form the seed coat, and the structure is dispersed, ready to grow into a new sporophyte when conditions permit. This shift to airborne pollen and a protected seed was a major factor in the colonization of dry land.
Major Divisions of Seed Plants
The seed plants are categorized into two major divisions based on how their seeds are protected: Gymnosperms and Angiosperms.
Gymnosperms
Gymnosperms, whose name means “naked seeds,” are the more ancient group, with seeds that are not enclosed within an ovary. Instead, their seeds are typically exposed on the surface of specialized reproductive structures, like the scales of a cone.
This division includes familiar examples such as conifers (pines, firs, spruces), cycads, ginkgo, and gnetophytes. Gymnosperms lack true flowers and rely almost exclusively on wind for pollination, which is why their pollen production is often prodigious. They were the dominant plant group during the Mesozoic Era, and they remain ecologically important in colder climates.
Angiosperms
Angiosperms, or flowering plants, are the largest and most diverse group of seed plants, representing approximately 80% of all known green plant species. The name Angiosperm means “enclosed seed,” referencing their defining trait: the seed is contained within a protective structure called the ovary, which matures into a fruit.
The presence of flowers is another distinguishing feature, as these structures facilitate reproduction. Flowers allow for targeted pollination, often utilizing insects, birds, and other animals, which is a more efficient method than relying on the wind. Angiosperms also exhibit a unique reproductive event called double fertilization, which results in the formation of both the diploid embryo and a triploid nutritive tissue called endosperm. Common examples include virtually all food crops, hardwood trees, and garden flowers.