34.4: Introduction to Seed Plants
Most plants are seed plants—characterized by seeds, pollen, and reduced gametophytes. Seed plants include gymnosperms and angiosperms.
Gymnosperms—cycads, ginkgo biloba, gnetophytes, and conifers—typically form cones. The pollen cones contain male gametophytes. The ovulate cones contain female gametophytes and form exposed seeds when fertilized.
Angiosperms, the most diverse and ubiquitous group of land plants, form flowers, and fruit. Like the cones of gymnosperms, the flowers and fruit of angiosperms enable sexual reproduction.
Flowers facilitate pollen dispersal. The fertile flower structures—stamens and carpels—contain male and female gametophytes, respectively. Fruits facilitate seed dispersal, often forming after flowers have released pollen. As seeds develop from a flower’s fertilized ovules, the ovary wall thickens, forming a fruit containing seeds.
Angiosperms were historically categorized as monocots or dicots based on their number of cotyledons—or seed leaves. However, based on genetic evidence, most species classically considered dicots are now called eudicots. Legumes (e.g., beans) and most well-known flowering trees (e.g., oaks) are eudicots.
The other former dicots belong to one of four small lineages. Three of these—Amborella, water lilies, and star anise and its relatives—are considered basal angiosperms due to their early divergence from ancestral angiosperms. The fourth group—the magnoliids—contains thousands of species, including magnolias.
Examples of monocots include orchids, grasses, palms, corn, rice, and wheat. Aside from cotyledon number, other characteristics distinguish monocots from eudicots. Leaf veins are typically parallel in monocots and netlike in eudicots. In stems, the vascular tissue is often scattered in monocots and ring-like in eudicots. Unlike eudicots, monocots generally lack a primary root. Pollen grains typically have one opening in monocots and three openings in eudicots. Finally, flower organs are often found in multiples of three in monocots and multiples of four or five in eudicots.