The internal parts of plants consist of specialized cells in the plant stem and leaf that make up the plant's structure and perform functions in the plant tissues. Cells that provide structure have thick cell walls that support the plant. Cells that provide function will move food, water and nutrients through the plant similar to how veins and blood vessels function in humans.
The parenchyma cells are cells in the plant that control most of the metabolic functions. Parenchyma cells are living when the cells reach mature size and are able to undergo cell division. The cells have a flexible cell wall and a large central vacuole. They are considered a non-specialized cell and can change into other cell types when placed under special conditions.
The collenchyma cells in a plant are those that support the structure in herbaceous plants. The primary cell walls are thick and often uneven in texture. Collenchyma cells are living when the cells reach mature size and function as plant support in new shoot growth and leaves. Collenchyma cells strengthen in plants that undergo stress from being shaken by wind.
The schlerenchyma cells are those with thick secondary cell walls that are not able to increase in length. The cells are found in areas of the plant that are no longer growing in length and become dead cells when plants reach mature size. Schlerenchyma cells support the plant tissue and are found as fibers and schlerids. The fiber type are long, thin cells that have a regular shaped cell wall. The schlerid types are short cells that have an irregular shape.
The xylem cells are those with a thick secondary wall that are able to stretch as they grow in an uneven coil. The cells play a role in the conduction of water and ions through the plant. Xylem cells are dead once they reach their functional maturity and are found as two types in the plant. The tracheid xylem cells are found in all vascular plants and grow as long, slim cells connected together by pits. The vessel xylem cells are only found in Angiosperms and grow as short, wide cells with perforated cell wall ends.
The phloem cells are those that transport sucrose, organic compounds and ions. The cells are living once they reach a mature size, however the cell protoplast may not have organelles or a nucleus. The end walls of the phloem cells connect to each other by sieve plates. Phloem cells are found as two types in a plant: sieve tube members and companion cells. The sieve tube members are those that serve as a tube for transporting sucrose. The companion cells are those that have a nucleus and control a portion of the sucrose loading.
Pith is the center area of the stem in vascular plants that consists of soft parenchyma cells. The pith is the storage area for plant impurities that come from the xylem during the plant growth. Most plants have a soft pith area; however, some trees and woody plants have solid or chambered pith.