Kinds of Roots
2.Fibrous root system – has several to many roots of the same size that develop from the end of the stem, with smaller lateral roots branching off of them.
a. Most monocots (including grasses and onions) have a fibrous root system.
b. In these plants, the radicle is short-lived and is replaced by a mass of adventitious roots (from the Latin, adventicius, meaning “not belonging to”), which are roots that form on organs other than roots. Because these roots arise not from preexisting roots, but from the stem, they are said to be adventitious.
c. The adventitious roots of monocots are very extensive and cling tenaciously to soil particles. These plants are excellent for preventing erosion.
d. The fibrous root of a few plants are edible – sweet potatoes (Ipomoea batatas) are the fleshy part of a fibrous root system.
Adventitious roots – there are several types of adventitious roots besides those of monocots.
a. Adventitious roots are common along rhizomes (underground stems) of ferns, club mosses (Lycopodium), and horsetail (Equisetum).
b. In some plants, adventitious roots are a primary means of vegetative reproduction: forests of quaking aspen (Populus tremuloides) are often a single clone spread by adventitious roots. Prop roots of mangrove (Rhizophora sp.) are like stilt roots, an adventitious root that grows out from the lower part of a stem into the soil to support the stem, or grows down from a lower branch in to the soil to support that branch (strangler figs).
c. You might see adventitious roots of ivy (Hedera helix) growing along the stem as it climbs along a fence or into a tree.
Take home point – Most dicots have a taproot system consisting of a large taproot and smaller branch roots. Taproot systems maximize support and storage. Monocots have a fibrous root system consisting of similarly sized roots that maximize absorption. Adventitious roots are roots that form on organs other than roots.
Functions and structure of roots
1. Absorption – roots absorb large amounts of water and dissolved minerals (nitrates, phosphates, and sulfates) from the soil.
3. Storage – roots store large amounts of energy reserves, initially produced in the leaves of plants via photosynthesis, and transported in the phloem, as sugar, to the roots for storage, usu as sugar or starch, until they are needed.
Structure and function
a. Mycorrhizae – the roots of most plant species form a mutually beneficial relationship with certain soil fungi.
b. Mycorrhizae enable plants to absorb adequate amounts of certain essential minerals (such as phosphorus) form the soil.
c. Minerals absorbed from the soil by the fungus, travels to the roots, and carbohydrates produced by photosynthesis in the plant travel to the fungus.
d. Mycorrhizae often enhance plant growth, and when mycorrhizae are not present, neither the fungus nor the plant grows as well.
e. Roots of some plants, like legumes (peas, beans, mesquites) form an association with nitrogen-fixing bacteria (Rhizobium, Frankia). Swellings, called nodules, develop on roots and house millions of the bacteria.
f. Like mycorrhizae, the association between nitrogen-fixing bacteria and roots is mutually beneficial.
g. The bacteria receive products of photosynthesis from the plants while helping the plant to meet its nitrogen requirement.
h. Cortical cells are those infected with the Rhizobium, which infects the roots through the root hairs, and then forms infection threads that permeate the root.
Roots specialized for unusual functions
1. Prop roots - are adventitious roots that develop from branches or from vertical stem and grow downward into the soil to help support the plant in an upright position.
Roots generally grow away from the light, and light inhibits root growth in corn, wheat, peas, and rice. Light is sensed by the root cap and inhibits growth by slowing the rates of cellular division and elongation.