Specialized Roots

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As mentioned earlier, most plants produce either a fibrous root system, a taproot system, or, more commonly, combinations of the two types. Some plants, however, have roots with modifications that adapt them for performing specific functions as well as the absorption of water and minerals in solution.

Food-Storage Roots

Most roots and stems store some food, but in certain plants, the roots are enlarged and store large quantities of starch and other carbohydrates (Fig. 5.8), which may later be used for extensive growth. In sweet potatoes and yams, for example, extra cambial cells develop in parts of the xylem of branch roots and produce large numbers of parenchyma cells. As a result, the organs swell and provide storage areas for large amounts of starch and other carbohydrates. Similar food-storage roots are found in the deadly poisonous water hemlocks, in dandelions, and in salsify. In carrots, beets, turnips, and radishes, the food-storage tissues are actually a combination of root and stem. Although the external differences are not obvious, approximately 2 centimeters (0.8 inch) at the top of an average carrot is derived from stem tissue that merges with the root tissue below.

Potato Storage Roots
Figure 5.8 A sweet potato (Ipomoea) plant. Note the food-storage roots.
Marah Root
Figure 5.Q A manroot (Marah) water-storage root that weighed over 25.3 kilograms (60 pounds). (Courtesy Robert A. Schlising)

Water-Storage Roots

Some members of the Pumpkin Family (Cucurbitaceae) produce huge water-storage roots. This is particularly characteristic of those that grow in arid regions or in those areas where there may be no precipitation for several months of the year. In certain manroots (Marah), for example, roots weighing 30 kilograms (66 pounds) or more are frequently produced (Fig. 5.9), and a major root of one calabazilla plant (Cucurbita perennis) was found to weigh 72.12 kilograms (159 pounds). The water in the roots is apparently used by the plants when the supply in the soil is inadequate.

Propagative Roots

Many plants produce adventitious buds (buds appearing in places other than stems) along the roots that grow near the surface of the ground. The buds develop into aerial stems called suckers, which have additional rootlets at their bases. The rooted suckers can be separated from the original root and grown individually. Cherries, apples, pears, and other fruit trees often produce suckers. The adventitious roots of rice-paper plants (Tetrapanax papyrifera) and tree-of-heaven (Ailanthus altissima) can become a nuisance in gardens, often producing propagative roots 10 meters (33 feet) or more from the parent plant. Horseradish (Rorippa armoracia), Canada thistle (Cirsium arvense), and some other weeds have a remarkable facility to reproduce in this fashion as well as by means of seeds. In the past, this capacity has made it difficult to control them, but some biological controls being investigated (see Appendix 2) may be an answer to the problem in the future.

Pneumatophores

Water, even after air has been bubbled through it, contains less than one-thirtieth the amount of free oxygen found in the air. Accordingly, plants growing with their roots in water may not have enough oxygen available for normal respiration in their root cells. Some swamp plants, such as the black mangrove (Avicennia nitida) and the yellow water weed

Ludwigia Repens Pneumatophores

Figure 5.10 Pneumatophores (foreground) of tropical mangroves rising above the sand at low tide. The pneumatophores are spongy outgrowths from the roots beneath the surface. Pneumatophores facilitate the exchange of oxygen and carbon dioxide for the roots, which grow in areas where little oxygen is otherwise available to them. (Courtesy Lani Stemmerman)

Figure 5.10 Pneumatophores (foreground) of tropical mangroves rising above the sand at low tide. The pneumatophores are spongy outgrowths from the roots beneath the surface. Pneumatophores facilitate the exchange of oxygen and carbon dioxide for the roots, which grow in areas where little oxygen is otherwise available to them. (Courtesy Lani Stemmerman)

(Ludwigia repens), develop special spongy roots, called pneumatophores, which extend above the water's surface and enhance gas exchange between the atmosphere and the subsurface roots to which they are connected (Fig. 5.10). The woody "knees" of the bald cypress (Taxodium dis-tichum), which occurs in southern swamps (see Fig. 22.18), were in the past believed to be pneumatophores, but there is no conclusive evidence for this theory.

Aerial Roots

Velamen roots of orchids, prop roots of corn and banyan trees (Fig. 5.11), adventitious roots of ivies, and photosyn-thetic roots of certain orchids are among various kinds of aerial roots produced by plants. It was formerly assumed that the epidermis of velamen roots, which is several cells thick, aided in the absorption of rain water. It appears, however, it may function more in preventing loss of moisture from the root. Corn prop roots, produced toward the base of

Velamen Orchid
Figure 5.11 The aerial (velamen) roots of orchids have a thick epidermis that reduces water loss from internal tissues.

the stems, support the plants in a high wind. Some tropical plants, including the screw pines and various mangroves, produce sizable prop roots extending for several feet above the surface of the ground or water. Debris collects between them and helps to create additional soil.

Many of the tropical figs or banyan trees produce roots that grow down from the branches until they contact the soil. Once they are established, they continue secondary growth and look just like additional trunks (Fig. 5.12). Banyan trees may live for hundreds of years and can become very large. In India and southeast Asia, there are several banyan trees that have almost 1,000 root-trunks and have circumferences approaching 450 meters (1,476 feet). The oldest is estimated to be about 2,000 years old.

The vanilla orchid, from which we obtain vanilla flavoring, produces chlorophyll in its aerial roots and, through photosynthesis, can manufacture food with them. The adventitious roots of English ivy, Boston ivy, and Virginia creeper appear along the stem and aid the plants in climbing.

Contractile Roots

Some herbaceous dicots and monocots have contractile roots that pull the plant deeper into the soil. Many lily bulbs are pulled a little deeper into the soil each year as new sets of contractile roots are developed (Fig. 5.13). The bulbs continue to be pulled down until an area of relatively stable

Chapter 5

Pneumatophores
Figure 5.12 A banyan (Ficus) tree with many large prop roots that have developed from the branches.
Modified Adventitious Roots Prop Roots

Figure 5.13 How a lily bulb over three seasons is pulled deeper into the soil by the action of contractile roots. A. A seed germinates. B. Contractile roots pull a newly formed bulb down several millimeters during the first season. C. The bulb is pulled down farther the second season. D. The bulb is pulled down even farther the third season. The bulb will continue to be pulled down in succeeding seasons until it reaches an area of relatively stable soil temperatures.

Figure 5.13 How a lily bulb over three seasons is pulled deeper into the soil by the action of contractile roots. A. A seed germinates. B. Contractile roots pull a newly formed bulb down several millimeters during the first season. C. The bulb is pulled down farther the second season. D. The bulb is pulled down even farther the third season. The bulb will continue to be pulled down in succeeding seasons until it reaches an area of relatively stable soil temperatures.

Roots and S oil 75

Specialized Roots Buttress Roots
Figure 5.14 Buttress roots of a tropical fig tree.

temperatures is reached. Plants such as dandelions always seem to have the leaves coming out of the ground as the top of the stem is pulled down a small amount each year when the root contracts. The contractile part of the root may lose as much as two-thirds of its length within a few weeks as stored food is used and the cortex collapses.

parasitic plants are capable of manufacturing at least some of their own food through photosynthesis. Other plants lacking chlorophyll (e.g., Indian pipes) are not parasitic at all. Instead, these plants are saprophytic, obtaining all the nutrients they require from organic materials in the soil.

Buttress Roots

Some tropical trees growing in shallow soils produce huge buttresslike roots toward the base of the trunk, giving them great stability (Fig. 5.14). Except for their angular appearance, these roots look like a part of the trunk.

Parasitic Roots

Some plants, including dodders, broomrapes, and pine-drops, have no chlorophyll (necessary for photosynthesis) and have become dependent on chlorophyll-bearing plants for their nutrition. They parasitize their host plants via peglike projections called haustoria (singular: haustorium), which develop along the stem in contact with the host. The haustoria penetrate the outer tissues and establish connections with the water-conducting and food-conducting tissues (Fig. 5.15). Some green plants, including Indian warrior and the mistletoes, also form haustoria. These haustoria, however, apparently aid primarily in obtaining water and dissolved minerals from the host plants, since the partially

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Responses

  • AZZEZA
    How suckers develop from the root of plant?
    8 years ago
  • Randy
    What is the function of pneumatophores?
    8 years ago
  • lisa
    What are the specialized root?
    8 years ago
  • LINDA
    What type of specialized roots do dodders, broomrapes have?
    8 years ago
  • sebhat
    What type of specialized root does a manroot have?
    8 years ago
  • consolata
    How are flowering specialized?
    8 years ago
  • jeanett
    Where can storage roots be found?
    8 years ago
  • SEBHAT
    What plants are not a food storage root?
    8 years ago
  • maik
    What trees have buttress and prop roots?
    8 years ago
  • Bisrat
    What propagative roots?
    3 years ago
  • maria
    Are suckers a specialized root?
    2 years ago
  • antje
    What flower has a thick root?
    2 years ago

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