The Plant Kingdom
The 260,000 species of the plant kingdom are divided into several phyla. Bryophytes are three phyla of nonvascular plants, comprised of around 16,000 species including the mosses, liverworts, and hornworts. Called nonvascular, bryophytes lack a developed vascular system for the internal conduction of water and nutrients. It typically takes two generations to complete a plants life cycle, this is called alternation of generations, the regular alternation of a sexual with an asexual generation. The leafy plant of bryophytes is the sexual (gamete-producing) generation of the life cycle of the plant. Because they lack a vascular system and since the gametes require water for dispersal, bryophytes are generally small plants found in moist conditions, although, some have adapted to desert life.

The other phyla, tracheophytes, are called vascular plants. Vascular plants carry water and nutrients up and down the plant. The two types of vascular tissue are xylem, which conducts water and minerals from the ground to stems and leaves, and the phloem, which conducts food produced by photosynthesis in the leaves to the stems, roots, and for storage and reproductive organs. The presence of vascular tissue is not the only differentiating feature the tracheophytes possess. As opposed to bryophytes, the tracheophyte leafy plants are the asexual (spore-producing) generation of their life cycle.

As the tracheophytes evolved, the leafy asexual, spore-producing generation became much larger and more complex, and the gamete-producing generation became reduced in size and is contained in the sporophyte tissue. This evolution, combined with the vascular nature of these plants has allowed the tracheophytes to dominate all terrestrial habitats of the Earth, save higher Arctic regions, and they also provide food and shelter for animal life on Earth.

The Study of Plants
All of the more than 260,000 species of plants differ from one another in one or more, sometimes minute, ways. Just as they differ, plants also have many common features. This is how we are able to classify plants into their distinct groups. This study of plants is called botany, and the scientists who study plants are known as botanists.

The main types of plants in the plant kingdom are divided into five basic groups which are the seed plants, ferns, lycopsids, horsetails, and bryophytes.

Seed plants are a diverse group of plants, and as the name indicates, bear seeds at one point in their life cycle in order to reproduce. These plants can be divided into two main groups, angiosperms and gymnosperms. Flower and fruit producing plants are all angiosperms and include brightly colored garden plants, many kinds of wildflowers, and most trees, shrubs, and herbs. Most of the plants which produce the fruits, grains, and vegetables that people eat also are angiosperms.

Angiosperms make up almost 90 percent of the plant species we know and the sizes of angiosperms vary greatly, from the largest angiosperms, the eucalyptus trees, which can grow more than 300 feet (91 meters) tall, to the smallest angiosperms, the duckweed, about 1/50 inch (0.5 millimeter) long found floating on the surface of fresh water ponds. Some botanists will divide the angiosperms into two more smaller groups, one called the monocotyledons or monocots, which grow from seeds that contain one seed leaf called a cotyledon, the other group called the dicotyledons or dicots, differing from the monocots because they have two cotyledons in their seeds.