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  Vegetative Structure of Seed Plants-Root, Biology tutorial

Introduction:

In vascular plants, the root is the main organ of a plant which generally lies beneath the surface of the soil. Though, a root can as well be aerial (that is, growing over the ground) or aerating (that is, growing up above the ground or particularly above water).

The first root which comes from a plant is termed as the radicle. The two main functions of roots are (a) Absorption of inorganic nutrients and water (b) Anchoring of the plant body to the ground. In response to the concentration of nutrients, roots as well synthesize cytokinin that acts as a signal as to how fast the shoots can grow up. Roots a lot function in storage of nutrients and food. The roots of most of the vascular plant species enter to the symbiosis having certain fungi to form Mycorhiza and a big range of other organisms comprising bacteria as well closely relate with the roots.

Root Growth:

Early root growth is one of the main functions of the apical meristem situated near the tip of the root. The meristem cells less or more continuously divide, generating more meristem, root cap cells (that is, these are sacrificed to protect the meristem) and undifferentiated root cells. The later become the main tissues of the root, first undergoing elongation, a procedure which pushes the root tip forward in the growing medium. Gradually such cells differentiate and mature into the specialized cells of the root tissues. Roots will usually grow in any direction where the right environment of mineral nutrients, air and water exists to meet up the needs of plant. Roots will not grow in dry soil. Over time, given the right conditions, roots can crack the foundations, snap water lines and lift side-walks. At the time of germination, roots grow downward due to gravitropism, the growth method of plants which as well causes the shoot to grow upward.

Growth from the apical meristems is termed as primary growth that encompasses all elongation. Secondary growth comprises all growth in diameter, a main component of woody plant tissues and numerous non-woody plants. For illustration: storage roots of sweet potato encompass secondary growth however are not woody. Secondary growth takes place at the lateral meristems, namely the vascular cambium and cork cambium.

In plants all along with secondary growth, the vascular cambium, originating among the xylem and the phloem, makes a cylinder of tissue all along the root and stem. The cambium layer forms new cells on both the internal and external of the cambium cylinder, with those on inside forming secondary xylem cells, and those on the outside forming secondary phloem cells. As secondary xylem builds up, the girth (that is, lateral dimensions) of the stem and root rises. As an outcome, tissues beyond the secondary phloem (comprising the epidermis and cortex, in numerous cases) tend to be pushed outward and are ultimately sloughed off (shed).

Types of roots:

There are two major kinds of root according to origin of growth and branching pattern in the angiosperms: taproot system and fibrous system.

Taproot system:

A taproot system is one in which the prime root becomes the main root of the plant having minimal branching comprising of secondary, smaller lateral roots. The taproot system takes place in dicot plants and is one of the bases of differentiating these plants from the monocots that encompass fibrous roots.

In plants containing a taproot system, the trunk-like primary root builds up directly from the embryonic root termed as radicle and grows downward into the soil. From this taproot, lateral roots build up that might initially grow horizontally then turn downward. Such roots repeatedly make finer roots that terminate in a root tip having a minute, dome-shaped, protective root cap at the tip-most portion. As the root grows up, it pushes its root cap forward, probing the soil, absorbing nutrients and water and mainly via fine root hairs. The hairs of root are extensions of the epidermis that build up in the region of differentiation. Such plant organs are short-lived and continually replaced.

Fibrous root system:

In grasses and other monocots comprising lilies and palm plants, the root system is a fibrous root system comprising of a dense mass of slender, adventitious roots which occur from the stem. A fibrous root system consists of no single large taproot as the embryonic root dies back if the plant is still young. The roots grow downward and outward from the stem, branching continually to form a mass of fine roots.

Specialized Variations of Roots:

1) Storage roots:

These comprise of a thickened roots due to the accumulation of high-energy storage compounds, generally starch. These are further sub-categorized into fleshy and tuberous roots. Illustrations of crops with fleshy roots are the carrot, ginseng and sugar beet.

2) Aerial roots:

These are adventitious roots which are general in most of the epiphytes like in the monocot plants belonging to the arum or gabi family and orchid family (Orchidaceae). Generally, these fibrous roots remain aerial, that is, they don't enter the soil.

3) Contractile roots:

As in most of the plants which form bulbs (example: lily) or corms (example: Gladiolus), these fibrous roots contract vertically to pull the plant downward in the soil.

4) Haustoria:

These are specialized roots in the parasitic plants which penetrate the tissues of a host plant, as in watch-weed (Striga) and broomrape (that is, Orobanche).

5) Prop roots:

These are the aerial roots which occur from a stem and then sink into the soil to give additional support to the plant like in corn and Ficus.

6) Buttress roots:

These are enlarged, frequently thickened roots which spread horizontally from the base of trees to give extra support. In tropical trees such as fig (Ficus) which are shallow rooted, big buttress roots are made at the base of the trunks.

Root architecture:

The prototype of growth of a root system is known as root architecture and is significant in giving a plant having a secure supply of nutrients and water and also anchorage and support. The architecture of a root system can be considered in the same manner to above-ground architecture of a plant that is, in terms of size, branching and distribution of the component parts. In roots, the architecture of fine roots and coarse roots can both be explained by variation in topology and distribution of biomass in and between the roots.

Having a balanced architecture let's fine roots to make use of soil efficiently around a plant; however the 'plastic' nature of root growth lets the plant to then concentrate its resources where nutrients and water are more simply available. Balanced coarse root architecture having roots distributed relatively evenly around the stem base, is essential to give support to bigger plants and trees.

Tree roots generally grow outward to around three times the branch spread. Only half of a tree's root system takes place among the trunk and the circumference of its canopy. Roots on one side of a tree generally supply the foliage on the similar side of the tree. Therefore when roots on one side of a tree are injured the branches and leaves on that similar side of the tree might die back and/or wilt. For some trees though, such as the maple family, the consequence of a root injury might show itself anywhere in the tree canopy.

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