It is a common belief that all plants reproduce by producing seeds, but this is not entirely true. While the majority of the world’s plant species rely on seeds, a large and ancient group of plants reproduces through a different, less complex method. The production of seeds marked a significant evolutionary step, allowing certain plant lineages to colonize drier environments and become the dominant flora across much of the globe.
Defining the Seed Producers
A seed is a sophisticated package designed to contain and protect the next generation of a plant. Each seed includes three primary components: a multicellular embryo, a store of food reserves, and a tough protective outer layer known as the seed coat. This structure allows the embryonic plant to remain dormant until conditions are favorable for growth.
The plant groups that utilize this method are collectively called Spermatophytes, or seed plants. Within this group, a crucial distinction exists between Angiosperms and Gymnosperms. Angiosperms, commonly known as flowering plants, enclose their seeds within a mature ovary, which we recognize as a fruit.
Gymnosperms, such as conifers and cycads, are characterized by “naked seeds” that are not enclosed in a fruit. Instead, their seeds are typically borne exposed on the scales of cones or similar structures.
The Mechanics of Seed Formation
The formation of a seed begins with the transfer of genetic material through pollination. In flowering plants, this involves the movement of pollen from the male part of the flower to the receptive female part, the stigma, leading to the growth of a pollen tube that delivers the male gamete to the ovule.
Following successful fertilization, the ovule begins its transformation into a mature seed. The fertilized egg cell divides to form the multicellular embryo, the miniature plant that lies dormant within the seed. Simultaneously, in flowering plants, a second fertilization event creates the endosperm, a nutrient-rich tissue that feeds the developing embryo.
The protective layers surrounding the ovule, known as integuments, harden and dry out to become the tough, outer seed coat. As the ovule matures, the ovary wall surrounding it often develops into the fruit.
Plants That Use Spores Instead
The plants that do not produce seeds, such as ferns, mosses, and liverworts, rely on microscopic reproductive units called spores. These plants, including Pteridophytes (ferns) and Bryophytes (mosses), represent older evolutionary lineages. Spores are fundamentally different from seeds because they are single-celled and haploid, containing only one set of chromosomes.
Unlike a seed, a spore contains no pre-formed embryo, no multicellular food supply, and lacks a robust protective coat. Spores are typically dispersed by wind and require a moist environment to germinate and grow. This dependence on water for reproduction limits these plant groups primarily to damp habitats.
The relative simplicity of the spore highlights why the seed was a significant evolutionary advancement. Spores are much smaller and lighter, making them effective for wide dispersal, but their lack of stored resources makes them less resilient to harsh or dry conditions.
How Seeds Ensure Survival and Dispersal
The internal structure of the seed is directly responsible for its enhanced survival capabilities compared to a spore. The seed coat acts as a physical barrier, protecting the delicate embryo from mechanical damage, desiccation, and predation. Furthermore, the stored food, which may be in the form of endosperm or cotyledons, allows the embryo to sustain itself during a period of dormancy.
Dormancy is a mechanism that allows the seed to delay germination until environmental conditions are optimal, such as adequate moisture and temperature. This ability to “wait out” unfavorable periods is a major factor in the widespread success of seed plants. The seed’s structure also facilitates effective dispersal, which is necessary to prevent competition between the new offspring and the parent plant.
Plants have evolved specialized structures to use various agents for dispersal, including wind, water, and animals. Wind-dispersed seeds, like those of dandelions, are lightweight and often possess wing-like or parachute structures to catch air currents. Seeds contained within fleshy fruits are dispersed when animals consume them and pass the seeds through their digestive system.