Mycorrhizae are fungal roots found in many plants. These fungal associations are important for both the plant and for the fungal and are therefore considered to be mutualistic. Essentially, the fungus will have a greater capacity for absorbing phosphorus than root hairs alone. The fungus will also grow and increase the absorption of water and other nutrients. In return, the plant provides sugars and amino acids vital to the survival of the fungus. Plants with mycorrhizae generally have fewer root hairs than those without. Nearly all woody trees and shrubs found in forests have fungal associations in their root systems. However, it has been demonstrated that mycorrhizae are particularly susceptible to acid rain. This may have a direct impact on forest health and maintenance.
Some plants, such as dodders, broomrapes, and pinedrops do not have chlorophyll. They will parasitize other plants and utilize their chlorophyll and food making abilities. The parasitic mechanism involves rootlike projections called haustoria (singular haustorium). These projections develop along stems that are in contact with the host. They will penetrate the outer tissues of the host plant and will tap into the water and food conducting tissues (xylem and phloem). Other plants with chlorophyll, such as mistletoes, will also form haustoria in order to obtain water and dissolved minerals from host plants. They are capable of producing their own food and thus are considered to be partially parasitic.
Tropical trees may have large buttress roots at the base of the trunk. These roots add stability to the tree and give an angular look to the lower visible portion of the trunk.
Contractile roots are roots that pull the plant deeper into the soil. Lily bulbs are a good example, as each bulb is pulled a little further into the soil as additional contractile roots are developed each year. When a region of stable temperature is reached, the contractile roots quit pulling. Dandelions also have contractile roots, and their presence is noticeable because the lower leaves may look like they are coming right out of the ground. In reality, the roots are pulling the stem downward. The actual mechanism of contraction involves the thickening and constriction of parenchyma cells. This causes the components of xylem to spiral into a corkscrew shape. The portion of the root that contracts may lose up to two-thirds of its length within weeks.
Pneumatophores are spongy roots that develop in most plants that grow in water. Swamps, marshes, and coastal areas are good places to find plants with pneumatophores. These specialized roots account for the fact that water, even after having air bubbled through it, has less than one-thirtieth of the amount of free oxygen that is found in the air. Plants growing in water may require additional methods of obtaining oxygen for respiration. Pneumatophores fill that need by rising above the water surface and facilitating gas exchange.
There are many different kinds of aerial roots produced by a wide variety of plants. Orchids produce velamen roots, corn plants have prop roots, ivies have adventitious roots, and vanilla orchids even have photosynthetic roots that can manufacture food. Banyan trees have aerial roots that grow down from the tree branches until they touch find the soil. In a nutshell, aerial roots are roots that are not covered by soil hence out in the air. They can facilitate climbing and various types of support as demonstrated by ivies and creeper plants.
To propagate means to produce more of oneself. Propagative root structures are one way for a plant to produce more of itself. Adventitious buds are buds that appear in unusual places. Many plants will produce these buds along the roots that grow near the surface of the ground. Suckers, or aerial stems with rootlets, will develop from these adventitious buds. The ‘new’ plant can be separated from the original plant and can grow independently. Some plants will produce propagative roots up to 30 feet or more away from the parent plant. This can be a nuisance for some people, while others may enjoy the propagative qualities of their cherry tree, strawberries or horseradish plants.
Plants that grow in particularly arid regions are known for growing structures used to retain water. Some plants in the Pumpkin Family produce huge water-storing roots. The plant will then use the stored water in times or seasons of low precipitation. Some cultures will harvest the water storing root and use them for drinking water. Plants storing up to 159 pounds (72 kilograms) of water in a single major root have been found and documented.
Most plants produce a fibrous root system, a taproot system, or most commonly a combination of both. However, some plants have roots with modifications that allow specific functions in addition to the absorption of water and minerals in solution.
In certain plants, the roots, or part of the root system, is enlarged in order to store large quantities of starch and other carbohydrates. Sweet potatoes and yams, for example, have extra cambial cells that develop in the xylem portion of branch roots. The cambial cells produce numerous parenchyma cells that cause the organs to swell. Starches are then stored in the swollen areas of the root. Carrots, beets, and turnips have storage organs that are actually a combination of root and stem. Approximately, the top two centimeters of a carrot are actually derived from the stem. Although, you likely will not be able to see the origin of the cells just by looking at a carrot.