Roots

Kinds of Roots

1. Taproot system – a strongly developed main root which grows downwards bearing lateral roots much smaller than itself.

1. In most dicots, the radicle enlarges to form a prominent taproot that persists throughout the life of the plant.
2. Many progressively smaller branch roots grow from the taproot.
3. This system is called a taproot system; common in dicots and conifers.
4. In plants such as carrots and sugar beets, fleshy taproots store large reserves of food, usu. as carbohydrates.
5. Taproots are modified for reaching deep water in the ground: e.g. the long taproots of poison ivy (Rhus toxicodendron), dandelion (Taraxacum sp.) and mesquite (Prosopis sp.).

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.

2. Anchorage – to locate water and minerals, roots permeate the soil. In doing so, they anchor the plant in one place for its entire life.

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

1. Absorption – most water and nutrients are absorbed by roots hairs (in zone of maturation). Root hairs extend the absorptive surface of roots that is in contact with moist soil several thousand-fold. Root hairs are short-lived, single-celled extensions of the epidermal cells near the growing root tip. Root hairs form only in the maturing, non-elongating region of the root. They are fragile extensions of epidermal cells and are easily broken off.

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.

2. Anchorage – relatively little absorption occurs past a few centimeters beyond the root tip, because these parts of the root lack root hairs and have a heavily suberized endodermis (innermost layer of the cortex). (Suberin – waxy substance that occurs in both cork cells and in cells of underground plant parts. Consists of hydroxylated fatty acids. Impervious to water). These non-absorptive regions of roots anchor plants and may later produce branch roots.
3. Conduction and storage – water and dissolved minerals absorbed by roots move to the shoot in xylary elements.
4. Movement – Each root tip has a root cap, a protective thimble-like layer of many sells that covers the delicate root apical meristem. The root cap also appears to be involved in orienting the root so that it grows downward. It can sense light, pressure and, perhaps, gravity. It produces and secretes mucigel, which protects and lubricates roots.

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.

2. Pneumatophores – “breathing roots”
1. In swampy or tidal environments where the soil is flooded or water-logged, roots often grow upwards until they are above the high-tide level. Even though roots live in the soil, they still require oxygen for aerobic respiration. A flooded soil is depleted of oxygen, so these aerial, “breathing roots” may assist in getting O2 to the submerged roots.
2. Plants such as black mangrove (Avicennia germinans) avoid suffocation by producing these roots that impor oxygen from the atmosphere. The pneumatophores contain as much as 80% aerenchyma, grow up in the air, and function much like snorkels, thru which oxygen diffuses to submerged roots.
3. Epiphytes – plants that grow attached to other plants.
1. Epiphytes and climbing plants have aerial roots that anchor the plant to the bark, branch, or other surface on which it grows.
2. Aerial roots of some epiphytes are specialized not only for anchorage, but some have photosynthetic roots (some epiphytic orchids), some absorb water.
3. Some parasitic epiphytes, such as mistleltoe (Phorodendron sp, etc.), have roots that penetrate the host plant tissues and absorb nutrients.
4. Suckers – aboveground stems that develop from adventitious buds on the roots.
1. Each sucker develops additional roots and becomes an independent plant when the parent dies.
2. Plants that form suckers include cottonwood, Lombardy poplar, pear, apple, cherry, blackberry, etc.

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.