What Are the Five Higher-Level Groups Within the Plant Kingdom?

• Plant life began in the water, as algae, before moving onto land. Normally/Photodisc/Getty Images

  Plants originated in the water, first moving onto land 400 million to 450 million years ago. Life on land started plants on an evolutionary course that involved progressive adaptations that better suited them for survival in environments without abundant and steady supplies of water, according to Stephen T. Abedon, a biology professor at Ohio State University. Today, botanists recognize four higher-level classifications within the plant kingdom based on significant evolutionary adaptations, and some botanists also include a fifth grouping, the algae.

Bryophytes

• Bryophytes -- the most common example of which is the mosses -- retain a high dependence on water in their environments. These plants do not have root systems or vascular tissue for transporting water to distant leaves, requiring them to remain small and colonize regions with abundant moisture. Bryophytes absorb water and nutrients directly through their leaves.

Ferns

• Ferns represent one of the earliest forms of vascular plants. Ferns not only have roots for extracting water and nutrients from the soil but vascular tissue to carry it throughout the plant, eliminating somewhat the fern's dependence on a constant supply of water. Sexual reproduction, however, continues to rely on water, as swimming sperm need abundant moisture to carry them to the female structures on the plant, according to retired Harvard biology professor John W. Kimball.

Gymnosperms

• Gymnosperms liberated the plant kingdom even further from water dependence with the significant developments of both heterospory and the seed. Heterospory refers to the production of pollen, which allows gymnosperms to rely on the wind rather than water to carry sperm cells to female structures. Seeds provide extra protection as well as nourishment for the developing plant embryo, removing the significant dependence on chance inherent in the spore-based reproduction used by bryophytes and ferns.

Flowering Plants

• Today, flowering plants, or angiosperms, form the most abundant and diverse grouping of plants, according to the University of Arizona's Tree of Life project. Flowering plants continued to produce seeds but protected them further inside an ovary, which often developed into a fruit, enticing animals to eat it and aid in dispersing the seeds inside. Many flowering plants also solicited the help of pollinators to carry pollen from plant to plant, removing reliance on wind to facilitate sexual reproduction.

Algae

• Contention continues about whether algae belong in the plant kingdom or the protist kingdom. Like other plants, algae produce food using photosynthesis, a process where energy from light powers the transformation of water and carbon dioxide into sugar. If you consider algae as plants, they gave rise to the first land plants hundreds of millions of years ago. Today, both green and red algae provide extant examples of this primitive plant grouping.