No, not all plants produce seeds. The Kingdom Plantae, which encompasses all plants, includes a diverse range of organisms that have evolved various methods of reproduction. While seed-bearing plants have become the most dominant life forms across many terrestrial environments, older and less complex plant groups rely on a different reproductive strategy.
Plants That Rely on Seeds
The majority of the world’s visible flora, from towering trees to garden flowers, belong to the category of seed plants. A seed is a complex, multicellular reproductive unit that consists of an embryonic plant, a supply of stored food to nourish the embryo, and a protective outer coat called the testa. This structure provides a significant evolutionary advantage because it shields the embryo from harsh environmental conditions like desiccation, fire, or freezing.
The two major divisions of seed plants are the Angiosperms and the Gymnosperms. Angiosperms, commonly known as flowering plants, are the most successful and widespread group, characterized by producing their seeds inside a protective ovary, which often develops into a fruit. This mechanism aids in seed dispersal, frequently by enlisting the help of animals.
Gymnosperms, whose name literally translates to “naked seeds,” include conifers, cycads, and ginkgos. Their seeds are not enclosed within a fruit but are typically exposed on the scales of cones. Both groups developed the use of pollen, a microscopic structure that carries the male gamete, which eliminated the requirement for water to achieve fertilization, allowing them to colonize drier habitats.
Plant Groups That Do Not Produce Seeds
Plant groups that do not produce seeds represent older lineages, and they include both non-vascular and vascular species. These seedless plants instead rely on spores for reproduction. The two most prominent categories of non-seed plants are Bryophytes and Pteridophytes.
Bryophytes are non-vascular plants, meaning they lack the specialized internal tissues that transport water and nutrients efficiently throughout the plant body. This group includes mosses, liverworts, and hornworts, which typically remain small and are often found in damp, shaded environments. Their lack of vascular tissue and their reproductive strategy link them closely to the earliest plants to colonize land.
Pteridophytes, which encompass ferns and horsetails, are classified as seedless vascular plants. They possess true vascular tissue for water transport, allowing them to grow larger than bryophytes, but they still reproduce without producing seeds. Their presence on Earth pre-dates the evolution of seed plants.
How Non-Seed Plants Reproduce
Instead of seeds, the Bryophytes and Pteridophytes reproduce using spores, which are microscopic, single-celled reproductive units. These spores are produced in specialized organs called sporangia, found on the underside of fern leaves or at the top of moss stalks. A spore lacks the multicellular embryo and the stored food supply found in a seed.
Upon release, spores are dispersed into the environment, mainly by wind. If a spore lands in a suitable, moist location, it germinates and grows into a tiny plant structure called a gametophyte. This small plant then produces the male and female reproductive cells, or gametes.
A defining feature of this reproductive cycle is its dependence on water for successful fertilization. The male gamete, or sperm, must swim across a film of water to reach the female gamete, which fuses with it to form a new embryo. This requirement for moisture is the primary reason why mosses and ferns are most commonly found in damp, humid habitats, making their reproductive process constrained by the environment than that of seed plants.