The plant kingdom is made up of some 350,000 species of shrubs, trees, land plants, ferns, mosses and algae. Each species has a specific function and plays an important role in the ecological food chain. Most plants grow on land, but several plants are adapted to live in water. Although plant characteristics vary from species to species, most plants have the same basic structure.
The cellular structure of all plants is basically the same. The cell wall, a rigid structure outside of the cell membrane, gives the plant cell a defined shape and helps support individual parts of the plant. Plant cells also contain chloroplasts which enable plants to convert energy from the sun into glucose and other carbohydrates. This process is called photosynthesis and is the way that plants create and store their own food.
Roots, Stems and Leaves
Vascular plants include flowering plants, ferns, clubmosses, conifers and other gymnosperms. These plants make up the majority of plants. They have vascular, or liquefied, tissues throughout their roots, stems and leaves. These tissues transport water, minerals and other nutrients for the nourishment of the plant. The roots, stems and leaves each perform specific roles in the growth and development of the plant.
Although vascular plants share basic plant structure, some species have specialized features. One such species is the flowering plant. Flowering plants make up about 90 percent of the plant kingdom. They can grow in a variety of conditions and have adapted to practically every region in the world. An important feature of the flowering plant is the fruit-producing flower which contains the seed. Seeds contain the embryo of a seeding plant and the food for that seedling to germinate. The dispersal of the seed is the way that new plant growth and development takes place.
Some plants do not have basic plant structure. Mosses and liverworts still perform photosynthesis but do not fall into the category of vascular plants. Mosses are small plants that require water to reproduce. Mosses and algae have no leaves, roots or stems and mosses need each other to stand upright. Worts are considered the simplest plants and generally grow flat along the ground in large leaf-like structures. Another deviant plant is the cactus, belonging to the Cactaceae family. Although the cactus is a flowering plant, it is unique in that its stems are succulent (water-retaining) and photosynthetic. Most cactus plants have no leaves.
Plants do not have a pre-programmed body plan. Although leaf shape and stem patterns can be constant, cells continue to divide and multiply throughout the life of the plant. This is different from the pre-programmed body plan of most animals that causes growth to cease after a certain time period. Thus, the plant growth pattern is termed "indeterminate."
Plants are classified into three major groups according to their growth pattern. Annual plants live and die within the same year. Biennials take two years to complete their life cycle and perennials live for many years, dying back every winter and returning in the spring.