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Plant parts such as leaves, stems, roots, flowers, fruits and seeds are known as plant organs. Each organ is an organized group of tissues that work together to perform a specific function. These structures can be divided into two groups: sexual reproductive and vegetative. Sexual reproductive parts produce seed; they include flower buds, flowers, fruit and seeds. Vegetative parts include roots, stems, shoots, nodes, buds and leaves; they are not directly involved in sexual reproduction. Vegetative parts often are used in asexual forms of reproduction such as cuttings, budding or grafting. Roots Often roots are overlooked, probably because they are less visible than the rest of the plant. However, it is important to understand plant root systems because they have a pronounced effect on a plant’s size and vigor, method of propagation, adaptation to soil types and response to cultural practices and irrigation. Roots typically originate from the lower portion of a plant or cutting. They have a root cap, but lack nodes and never bear leaves or flowers directly. Their principal functions are to absorb nutrients and moisture, anchor the plant in the soil, support the stem and store food. In some plants, roots can be used for propagation. Root Structure Internally, there are three major parts of a root: The meristematic zone is at the tip and manufactures new cells; it is an area of cell division and growth. Behind the meristem is the zone of elongation. In this area, cells increase in size through food and water absorption. As they grow, they push the root through the soil. The zone of maturation is directly beneath the stem. Here, cells become specific tissues such as epidermis, cortex or Epidermis vascular tissue. The epidermis of this young region Cortex forms root hairs that replace old root hairs destroyed as the root penetrates the soil. A root’s epidermis is its outermost layer of Endodermis cells. These cells are responsible for absorbing water and minerals dissolved in water. Cortex cells Xylem are involved in moving water from the epidermis to the vascular tissue (xylem and phloem) and in Phloem storing food. Vascular tissue, located in the center of the root, conducts food and water. Externally, there are two areas of importance: the root cap and the root hairs. The root cap is the root’s outermost tip. It consists of cells that are sloughed off as the root grows through the soil. Its function is to protect the root meristem. Root hairs are delicate, elongated epidermal cells that occur in a small zone just behind the root’s growing tip. They generally resemble fine down to the naked eye. Their function is to increase the root’s surface area and absorptive capacity. Most water absorption occurs in the area of the root hairs. Root hairs usually live 1 or 2 days. When a plant is transplanted, they are easily torn off or may dry out in the sun. Many roots have naturally occurring symbiotic (mutually beneficial) relationships with certain fungi. Fungi grow inside plant roots and in the surrounding soil helping to facilitate mineral uptake. The fungi hyphae (threadlike cells of the fungus) growing through the soil increase the area available for the absorption of water and nutrients. Plants provide carbohydrates (the products from photosynthesis) or food for the fungi. This beneficial association is called mycorrhizae (fungus + root). Legumes and a few plants such as alder produce root nodules, which contain nitrogen-fixing bacteria. These bacteria can fix atmospheric nitrogen into a form that can be used by plants. Garden seeds such as peas, beans and clover are inoculated with nitrogen fixing bacteria when planted. Because inoculums are living organisms they must be fresh and handled properly. There are two types of root systems. After the primary root emerges from the seed, it may continue to grow straight down and become a tap root, or it may branch, forming secondary roots, and become a fibrous root system with many side, or lateral, roots. Some plants, such as grasses, naturally produce a fibrous root system. In other cases, the taproot can be encouraged to branch by removing or breaking the apical root meristem, or growing point of the root. This often happens when seedlings, trees, and other plants are transplanted. This process favors a fibrous root system. The depth that tree roots penetrate depends on the types of tree and soil, and water status. A dense compacted layer in the soil or a high water table will restrict or terminate root growth. Adventitious roots are those that form at any place on the plant other than the primary root system. They may arise from cells adjacent to vascular bundles, cambium, or young phloem cells. They also form on other plant parts besides stems, such as leaf petioles or leaf blades or even on older root pieces. The ability of many plants to produce adventitious roots allows the propagation of plants vegetatively. However, roots have adapted to fulfil a variety of other functions including storage, support and aeration. Aerial roots develop on stems above ground. Epiphytic plants grow on other trees using them for support. Their aerial roots rarely reach the soil so absorb water from rain or mist, but if they reach soil they will become ground roots. The aerial roots of some orchids contain chlorophyll for photosynthesis and absorb moisture and nutrients from the air. Certain swamp-dwelling trees form pneumatophores, or breathing roots, since there is little oxygen available to the roots in waterlogged conditions. They grow out from the water surface and facilitate the aeration necessary for root respiration in hydrophytic trees such as many mangrove species. Adventitious attachment roots grow from aerial stems, on the side in contact with a surface. Prop roots grow down from the stem into the soil to brace the plant. Absorption Aerial roots Epiphytic orchid (Oncidium) Aeration Attachment Support Pneumatophores Mangrove (Sonneratia) Attachment roots Ivy (Hedera helix) Prop roots Maize (Zea mays) Storage A tuberous root or storage root, is a modified lateral root, enlarged to function as a storage organ. The enlarged area of the root-tuber, or storage root, can be produced at the end or middle of a root or involve the entire root. It is thus different in origin but similar in function and appearance to a stem tuber. Root tubers store nutrients over periods when the plant can not actively grow, thus permitting survival from one year to the next. Examples of plants with notable tuberous roots include the sweet potato, cassava, yam and dahlia. Tuberous roots Sweet potato (Ipomoea batatas)