Structure of a Stem
Stems are composed of sclerenchyma, collenchyma, and parenchyma cells. In an herbaceous plant, such as a dandelion, the stem that develops in the shoot system does not grow much thicker. Herbaceous plants do not have a woody stem above ground. In woody plants, such as elms or cypress, the stem or trunk grows taller and adds girth each year, with every year added shown in an age ring. Cells combine to form basic tissues the plant stems need to function. Within the stem, ground tissue provides support, vascular tissue transports water and sap, and dermal tissue provides protection and prevents water loss.
Initially, a plant begins with a slender shoot containing a vascular column. Within the column lies pith at the center, pipe-like structures for the transport of water and sap (xylem and phloem), and sclerenchyma cells for strength. Pith forms the spongy central tissue of a stem and is utilized for storage. Multiple xylem and phloem structures are arranged around the outside of the pith. These structures are adjacent to each other to allow the flow of water between xylem and phloem.The outer coating of a leaf and stem which has a protective cuticle or waxy layer is the epidermis, which is typically a single cell layer. The epidermis is much like the skin that covers and protects human beings. It has a protective cuticle, which is the clear, waterproof layer on leaves and stems that is secreted from epidermal plant cells and prevents water loss. It also prevents invasive infection from attacking the plant. The epidermis is composed of similar, specialized cells. These epidermal cells are fairly uniform in structure. Just under the epidermis is the cortex. This layer is composed mostly of parenchyma cells and functions to transport materials to the roots and also stores carbohydrates. A stoma (plural, stomata) is a small opening mostly found on the undersides of leaves that allows for gas exchange between the plant and the external environment. Stomata are part of the epidermis, as are the layers of bark called cork cells, found on woody plants. A trichome is a hair-like outgrowth on a plant shoot that functions to deter herbivores. Trichomes protrude from the epidermis and offer an added layer of protection. They reduce evaporation from plant surfaces, reflect the sun's light energy, and may contain toxins that protect plants from herbivores. The stinging trichomes of the nettle plant are an example of this toxic protection.
Xylem and Phloem
Xylem is comprised of several structures, some of which are limited to specific plant groups. Vessel members (tracheae), found in advanced angiosperms, such as magnolia trees and tulips, are tubular in shape and connected to each other by means of thin plates perforated with pores. Tracheids, found mostly in gymnosperms, such as pines, and pteridophytes, such as ferns, perform water and mineral transport, through narrow tubes. Both tracheids and vessel members perform the same functions in plants.
Since the primary function of xylem tissue is to transport water up from roots to the rest of the plant's structures, xylem needs pathways for this type of transport. Tracheids are more primitive cells for fluid transport. Water moves from one tracheid to the next by means of bordered pits—thinner areas of cell walls. Vessel elements are several times thicker than tracheids and thus deliver water and minerals more easily than tracheids. Vessel elements may be up to three meters long, and their porous end plates makes transport much easier.
Phloem is plant tissue that carries sugars, organic compounds and minerals throughout a plant, including roots, stems, branches, and leaves. The fluid carried is the result of the photosynthesis process. It is the food produced in chloroplasts. When the bark of a tree is cut and sap oozes out, that is the liquid normally traveling through phloem. The sap travels by diffusion, the random movement of molecules along a concentration gradient from an area of higher concentration to an area of lower concentration. The diffusion in phloem is translocation, which is the movement of liquid food from one plant part to another. The translocation occurs true whether the plant is large, like a lodgepole pine, or small, like a blade of grass.The xylem and phloem systems are adjacent in plant stems, allowing for the flow of water to move to all plant structures. Water also flows between xylem and phloem, since not all water is used in photosynthesis and water is needed to carry carbohydrates to other plant parts. Like xylem, phloem is part of the vascular column found in all vascular plants. The flow of liquid through phloem is not always in one direction; liquid sometimes flows up and sometimes flows down. Much of this depends on the time of year and a plant's age. In deciduous trees, phloem does not flow during the winter when the tree enters a dormant period. However, it does flow during the times of increased growth occurring in spring and summer.