Flowering plants come into this world as a single-celled zygote. Scientists define the zygote as a fertilized, diploid, eukaryotic cell that carries an organism’s blueprint for continuing the species. Even mighty oaks grow from tiny acorns that start out as microscopic zygotes. The zygote forms when a male gamete fertilizes a female gamete; each haploid gamete contributes equally to the genome of the diploid zygote.
Types of Plant Reproduction
The plant kingdom includes simple nonvascular plants and complex vascular plants that have specialized vessels for transporting food, water and gasses. Non-vascular seedless plants (Bryophytes) were the first plants to live on land. Examples include mosses, hornworts and liverworts. Reproduction can be asexual or sexual through fragmentation or spores.
Seed-bearing vascular plants include the broad categories of angiosperms and the gymnosperms. Angiosperms are flowering plants that are more common today than more gymnosperms. Both types of vascular plants produce male and female gametes that can fuse to form a zygote under the right conditions, such as the availability of water for sperm motility.
How is a Zygote Formed in Angiosperms?
Angiosperms are flowering plants with male and female reproductive structures that produce haploid sperm and egg cells via meiosis. Pollen in the anther of the stamen contain sperm, which is dispersed by pollinators like birds, bees, bats and wind. Some species can self-pollinate but most plants exchange genetic material to increase variance and diversity within the population.
When pollen grains reach the stigma of a flower’s female structures, a pollen grain containing two sperm makes its way into the ovary. One sperm fertilizes one egg, which becomes a diploid zygote. The other sperm fuses with polar nuclei to form endosperm that will nourish the zygote as it matures into a seed.
Zygote Example in Angiosperms
In garden plants such as bell peppers, string beans and zucchini, fertilization takes place in the flowers on the plant. Male and female gametes fuse, forming a zygote in an ovary of the flower. The ovary thickens and ripens into a “fruit” that nourishes growing embryonic seeds. Fruits entice birds and animals to eat the seed and disperse it away from the parent plant, which encourages plant reproduction.
How is a Zygote Formed in Gymnosperms?
Gymnosperms are vascular, nonflowering plant species dating back to the days of the dinosaur. Ancient species like pine, spruce and fir trees have male and female cones instead of flowers. Inside the cone, diploid microsporocytes in the male cones and megaspores in female cones undergo meiosis to form haploid cells.
Fertilization occurs when the haploid male gametophytes (pollen) lands on a female cone and slowly grows toward the female gametophyte. Fertilization results in a zygote that develops into embryonic seed while protected inside the female pinecone. The seed itself is not coated and is called “naked” seed in biology.
Zygote Example in Gymnosperms
Conifers such as pine trees are sporophytes with male and female cones. Wind carries pollen to female cones. The larger female cones are closer to the top of the tree and the smaller male cones are at the bottom, which is nature’s way of discouraging self-pollination.
After fertilization in conifers, the zygote develops into an embryo. Slowly, the embryo grows into a seed on a scale inside the female cone. When the seed is mature, the pinecone opens and the seed is dispersed by the wind or by being eaten by animals such as birds and squirrels.
Zygote in Plant Life Cycle
The plant life cycle includes an alternation of generations where the plant alternates between the diploid sporophyte stage (two sets of chromosomes) and the haploid gametophyte stage (one set of chromosomes). Diploid germ cells divide by meiosis to create haploid gametes containing half as much genetic material as the parent plant. Male and female gametophytes release sperm and eggs.
When an egg is fertilized, it becomes a diploid zygote that matures into a sporophyte on the gametophyte plant. Sporophytes make haploid spores that mature into an entirely new plant. For instance, in the fern life cycle, the tall leafy fronds seen growing in the woods are sporophytes and small heart-shaped plant structures are the fern gametophytes.
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