Plants, like other living organisms, are made of cells. All plant cells are eukaryotic cells, cells that have nuclei. But plant cells have specific structures, depending on their function and tissue type. The basic cells of a plant include sclerenchyma, collenchyma, parenchyma, phloem, and xylem cells.
- A sclerenchyma cell is a hard, woody cell that, along with other cells, provides structural support. It contains an organic polymer called lignin that helps make the tissues stiff and woody. Sclerenchyma may be fibers (elongated cells with long, tapered ends that interlock) or sclereids (thickened cell walls that form small bundles of tissue layers), both of which can protect other structures because they have thick, strong cell walls. These cells form uniform walls and die once the cell wall is completed, yet they continue to function in stems and leaf veins. Sclerenchyma cells assist in fluid transport by supporting stems and branches and protecting vascular structures. Fibrous sclerenchyma cells make cloth, such as linen or hemp, and give a gritty texture to pears.
- A collenchyma cell is an elongated cell that supports new growth, particularly in areas of a plant needing extra strength. The narrow stalk connecting a leaf to a branch is constructed of collenchyma cells. The strings of celery stalks are composed of collenchyma cells.
- A parenchyma cell is a thin-walled, large cell with a very large central vacuole, and numerous plastids, which performs metabolic processes, including photosynthesis. Parenchyma cells store water and transport water over short distances. In plant parts that do not get any light exposure, parenchyma contain colorless plastids and function primarily as carbohydrate storage units. Carbohydrates provide structural components for plants, such as cellulose, as well as deliver energy plants require for growth. In areas exposed to the sun's light energy, such as leaves, the primary plastids are chloroplasts—the location in which photosynthesis takes place. Parenchyma cells are living cells. They can also store plant starches, repair damaged areas, and take part in photosynthesis.
- Xylem is a vein-like tissue that carries water from roots and stems to leaves. Phloem is the series of tissues that transport sugars and other materials from the leaves of plants to their roots. Together xylem and phloem cells are the transport cells that move fluids through plant structures. Water is the primary fluid flowing through both xylem and phloem since both nutrient-rich water and organic compounds from photosynthesis must spread throughout a plant's structures to maintain health. Translocation is the transport of materials through the phloem of a vascular plant (a plant that has tissues for transporting water or sap).