Plant Root

What is the root of a plant?

Roots are plant organs that are typically part of the belowground network of the plant. Although, as we will learn later, plant roots can also be aboveground, for extra support.

Roots help provide support, or anchorage, to plants since they cannot move and must be able to maintain themselves in variable conditions. Most importantly, however, roots help absorb water and nutrients the plant needs to produce food and maintain its health.

Plant root structure

The structure of plant roots and their growth vary dependent on the plant species, as is true for the other plant organs. However, a basic structure that characterizes plant roots can be discussed to understand more about the tissues and cells that make up the root.

Root growth

Starting at the tip of the root, there is a root cap, which covers and protects the actively growing root in the soil. Above the root cap, the plant root is split into different zones, from the tip upwards: zone of cell division, zone of elongation, and zone of differentiation.

1. Zone of cell division: the zone of cell division is right above the root cap (which is protecting it) and is made up of meristem. Meristems are areas of cell division, and they give rise to new plant organs. Each root has its own root apical meristem or area of cell division which helps it extend into the soil.

2. Zone of elongation: this is an area of rapid cell growth above the meristem.

3. Zone of differentiation: this is an area above the zone of elongation where the cell division and growth have stopped, and cells become different types of cells that will constitute the different root tissues.

The root apical meristem is the zone of cell division that supplies the roots with new cells as they continue to grow. The roots will continue to go throughout the plant's life to head in the direction where water is readily available. These zones only make up the very small fragment at the beginning of the root, beyond these zones, the root has the three types of plant tissue that also appear in leaves and stems: ground tissue, vascular tissue, and epidermal tissue.

Root anatomy

Epidermal tissues in roots make up the outer layers of cells. Inside the root past, the epidermis is the cortex. The cortex is made up of plant ground tissue and includes a few different layers: the mesocortex or “middle cortex” and the endodermis. The middle cortex is usually a site for storage, while the endodermis is the separation between the cortex and the middle of the root known as the stele.

The endodermis has a Casparian strip which forces water and other molecules entering the root to first go through the selectively permeable membrane of plant cells. Thus, this helps protect roots from harmful pathogens or chemicals. In the stele are the vascular bundles and other ground tissue. The very middle of the cell is composed of the pith, which is also a storage site for sugars. Some plants (eudicots) do not have a pith.

Primary versus secondary growth

In the root primary growth refers to the growth of roots, in particular getting longer, and the addition of new roots.

Primary growth is the main growth of herbaceous plants. Woody plants may undergo another round of growth known as secondary growth.

Secondary growth is lateral growth that increases the width of a plant root or stem. This occurs by the growth of the vascular tissue and produces layers that cause the cortex to shed and be replaced with a layer known as the periderm. K

Knowing all the names of root layers is not necessary for the AP test, but it's good to know that different plants have different root tissues!

The function of roots in plants

Roots help to anchor plants in the ground. Plants inhabit many different biomes, from the desert to the tropical rainforest, meaning they face all kinds of conditions. Having a root system has ensured plants the protection they need in different climates.

Typically, root systems are more extensive than the shoot system (stem and leaves). In addition to providing stability, roots, being a novel characteristic of vascular plants, absorb water, minerals, and nutrients from the soil which travel through the xylem into other parts of the plant.

Roots are also important because they:

• form symbiotic relationships with bacteria and fungi to help the plant get nutrients or minerals,

• act as storage of nutrients/food and,

• sequester carbon in plants and in the soil.

Types of roots in plants (with examples)

Taproot system

In angiosperm (flowering) plants, the radicle is the first root to arise after the seed germinates. If this radicle develops into the main central root, known as a tap root, and all the other roots develop off of the taproot, the system is called a taproot system.

Taproot systems can grow to be quite extensive, making them efficient absorption systems. Taproots themselves can grow deep into the soil, anchoring the plant, and also sometimes acting as a nutrient and food storage system for the plant.

Fibrous root system

In the fibrous root systems, there is no main taproot, and the primary root does not last. Instead, the roots are formed from low nodes on the stem, and are often, thin, roughly the same size, and form mat-like structures.

Prop roots

Prop roots are a type of adventitious root that helps provide extra support for the stem, typically aerial support.

Root adaptations to environmental pressures

Different root systems may be found in the same environment, as plants have evolved different ways of obtaining resources. Largely depending on the climate, the quality of the soil, and the needs of the plant, roots can vary between species.

Root nodules

In a group of plants known as the legumes, special nodules, or chambers, exist to allow for the presence of nitrogen-fixing bacteria in the roots. This symbiosis is important to the plants since the bacteria make nitrogen readily available in a form accessible to the legume plants where it might not be otherwise. We eat lots of plants from this family including lentils, beans, and chickpeas.

Root hairs

Root hairs are outgrowths from a type of epidermal cells in the roots of almost all vascular plants, and they increase the surface area of the root system significantly. Being extensive and having adaptations to increase surface area is important since the more surface area, the more water will diffuse into plant cells via osmosis.

Lenticels

Lenticels are openings in the epidermis that allow for gas exchange with the tissues below. Unlike stomata, lenticels remain open and do not close. Some plant roots (woody dicots) may have them to increase the presence of gases, such as oxygen in the roots (remember oxygen is still needed by plants for cellular respiration!).

Pneumatophores are special roots that grow upwards out of the water in plants that are coastal or live in swampy places. These roots have lenticels on them to remedy the lack of gas exchange in submerged roots.