Plants belong to an ancient taxonomic kingdom that represents vast diversity. As time and evolution unfolded, plants developed from strings of algae in the primitive sea to the complex flowering plants that, today, are suited even to the most extreme environment. Representing opposite extremes in evolution, contrasting bryophytes and flowering plants unveils the tremendous evolutionary progress observable in plants today.
Bryophytes, part of the taxonomic division Bryophyta, include mosses and liverworts, small and simple plants usually found in moist locations. Bryophytes rely on spores for reproduction, lack any type of vascular system and exhibit a dominate gametophyte generation, meaning that the visible plant contains only half the expected number of chromosomes. Flowering plants, on the other hand, reproduce using seeds inside of an ovary, contain a complex vascular system and display a visible plant with a full arrangement of chromosomes.
Bryophytes and flowering plants sit at opposite ends of the timeline in plant evolution. Because the first plants formed in water, systems to take in and distribute water never developed. The first land plants--bryophytes--also lacked root systems or vascular tissue for conducting water to distant parts of the plant, requiring bryophytes to remain small and near a water source. As time passed, the first vascular tissue appeared in ferns, and the first seeds developed in gymnosperms. Flowering plants improved on both, developing both vascular and reproductive improvements, the most obvious of which is the flower.
Bryophytes completely lack vascular tissue, including root systems to draw water from the soil. Instead, they attach to surfaces with filaments called rhizoids that serve only to anchor them into place. Because each cell must remain close to water and nutrient sources, bryophytes remain small. Flowering plants, on the other hand, develop specialized vascular tissue for conducting both water and nutrients produced in the leaves. Alone of the plants, flowering plants form tubes of nonliving cells inside their stems that act like long drinking straws, pulling water from the roots to the tip of the furthest leaf.
Alternation of Generations
Characteristic of all plants is the alternation of generations, and plants exhibit both a sporophyte generation containing a full array of chromosomes and a gametophyte generation, in which chromosome numbers are halved. In bryophytes, the gametophyte dominates, meaning that the small green mosses have half the chromosomes possible in the plant. The sporophyte generation forms only briefly as part of reproduction. Flowering plants are exactly the opposite. When you see a flowering plant, it contains the full set of chromosomes--the sporophyte generation--and the gametophyte generation develops briefly to produce eggs and sperm.
When the egg and sperm fuse in a bryophyte, the result is a spore, a tiny structure that, carried away by wind or water, will develop into a new plant. Mosses require rainwater to carry the sperm to the egg on the plant. Flowering plants, on the other hand, reproduce with seeds, produced in the ovary of a flower when the egg and sperm fuse. Flowers have evolved bright colors and sweet scents that attract pollinators to carry male cells from plant to plant.