Seed Plants: An In-Depth Look at Spermatophytes
Spermatophytes: The Seed-Bearing Lineage
Spermatophytes are a monophyletic group encompassing all vascular plants that produce seeds. The term originates from the Greek words “sperma” (seed) and “phyton” (plant), translating to “seed plants.” This group aligns precisely with the traditional classification of flowering plants, as the pollen grain in seed plants generates a pollen tube to reach the ovum, facilitating fertilization.
Gymnosperms: Vascular Plants with Naked Seeds
Gymnosperms are vascular plants that produce seeds not enclosed within an ovary, hence the term “naked seeds.” These seeds typically reside on cone scales. Gymnosperms represent a primitive lineage, appearing approximately 350 million years ago. Today, around 700 species exist, primarily within the conifer class. Gymnosperms often appear as shrubs and trees, forming extensive forests. They are typically evergreen, with conducting tissues composed of elongated cells called tracheids rather than true vessels. These plants are often monoecious, meaning they bear distinct male and female flowers on the same individual.
Female Flowers and Cones
Female flowers consist of an bract, a scale, and two ovules. Grouped around a floral axis, they form an inflorescence known as a female cone or pineapple. Within the ovule, an embryo sac develops, containing two archegonia, each housing two female gametes. This structure eventually becomes the seed of conifers.
Male Flowers and Pollen
Male flowers comprise a scale and two pollen sacs or microsporangia, also clustered around a floral axis. Within the pollen sacs, stem cells produce pollen grains, each containing two anterozoids and two air sacs that aid in their transport to the female flower. Upon fertilization, one anterozoid fuses with an oosphere, forming a diploid zygote that develops into the embryo. The embryo, encased in albumen and a woody integument (derived from the ovule), takes approximately two years to fully develop.
Gymnosperm Divisions
Cycads
Cycads likely originated in the Carboniferous period, with the oldest fossils dating back over 230 million years to the Permian. They comprise around 360 species across 11 genera and 3 families, including the Cycadaceae family and the Cycas genus. Cycads feature pinnate or bipinnate leaves, often bright green, with a palm-like stalk. They can range from partially underground tubers to tall, tree-like structures. All species possess roots with negative geotropism, forming coral-like structures associated with nitrogen-fixing cyanobacteria. Cycads are dioecious, with male strobili ranging in size and number. Male gametes are often flagellated.
Coniferophyta
Coniferophyta includes families such as Pinaceae (pines, spruces, firs), Araucariaceae (Araucaria), Podocarpaceae, Sciadopityaceae (umbrella pine), Cupressaceae (cypresses, junipers, redwoods), Cephalotaxaceae, and Taxaceae (yew). This division comprises about 50 genera and 550 species distributed globally. Most are large, branched trees with resin-producing structures. Leaves are simple, small, and often evergreen. Most species are monoecious, with sporophylls arranged in strobili. Pollination occurs via wind, and seeds often have wing-like extensions for dispersal.
Ginkgophyta
Ginkgophyta includes the Ginkgo biloba species. This deciduous tree can reach 35 meters in height, with a narrow, pyramidal crown. The bark is grayish-brown, and the leaves are fan-shaped with dichotomous venation. Ginkgo is dioecious, with male trees bearing yellow catkin-like inflorescences and female trees bearing groups of 2-3 flowers. The seed is soft and yellowish-brown. Ginkgo is a living fossil, with related fossils dating back 270 million years to the Permian.
Gnetophyta
Gnetophyta consists of three genera: Ephedra, Gnetum, and Welwitschia (with the species Welwitschia mirabilis). This division comprises 71 species characterized by vessels and tracheids in the xylem. Ephedra species are shrubs found in dry climates, known for their medicinal properties. Gnetum species are tropical trees, shrubs, and vines with large leaves and tracheids in the xylem. Welwitschia mirabilis, found in African deserts, has a short trunk and two continuously growing leaves.
Angiosperms: The Flowering Plants
Angiosperms, derived from Latin and Greek meaning “enclosed seed,” are the flowering plants. With approximately 230,000 species, they dominate most natural landscapes. Angiosperms have ovules enclosed within an ovary, which develops into the fruit after fertilization. Fruits protect and aid in seed dispersal. Angiosperms often have xylem vessels for efficient water transport and unique phloem structures. Most are monoecious, with stamens and pistils present in the same flower.
Vascular System
Xylem
Xylem conducts water and mineral nutrients from roots to the rest of the plant. Its cells are elongated, lignified, and dead at maturity.
Phloem
Phloem transports photosynthetic products throughout the plant. Its cells are living and have sieve areas for cytoplasmic communication.
Life Cycle
Angiosperms have a double fertilization process. One male gamete fertilizes the egg cell to form the zygote, while the other fuses with polar nuclei to form the endosperm. The zygote develops into the embryo, consisting of a radicle, plumule, and cotyledons. The seed comprises the embryo, endosperm, and a protective covering (testa and tegmen). The ovary wall develops into the pericarp, which, along with the seed, forms the fruit.
Angiosperm Groups
Monocotyledons
Monocots have a single cotyledon, a fasciculated root system, and often unbranched stems. Leaves typically have parallel veins. Flowers usually have parts in multiples of three. Examples include lilies, palms, orchids, and grasses.
Dicotyledons
Dicots have two cotyledons, often exhibit tree-like growth, and have vascular bundles arranged concentrically. Leaves typically have reticulate venation. Flowers have parts in multiples of four or five. Examples include roses, oaks, beans, and sunflowers.