Botany Two - Roots

Root Anatomy

The anatomy of a root is divided in to four zones all of which exist in the first 1cm of the root…

• The Zone of Maturation – This area begins at the first root hair and is where cells mature and differentiate in to specialized functions.
• The Zone of Elongation – In this area the newly added cells increase in length and extend the length of the root.
• The Zone of Cell division – (or alternatively the zone of meristemic activity). The apical meristem is a region of cell division that sits at the apex of the root just behind the root cap. This is where new cells are created and added to both root cap and the root itself.
• The Root Cap – the cells of the cap are continually slough off as the root grows and extends through the soil. This creates a slippy surface which lubricates the root and prevents damage.

A cross-section of the root in the zone of maturation reveals its internal structure.

Working inwards…

• Root Hairs – these are extensions of epidermal cells and massively increase the effective surface area of the root. This is the main site for the absorption of soluble nutrients by osmosis. In Legumes t is the root hairs which encapsulate bacteria to form the nitrogen fixing nodules.
• Epidermis – this is outermost cell layer and “skin” of the root. It is formed of specialised Parenchyma cells from the Cortex. The epidermis provides protection to the roots inner structure and assists the root-hairs with nutrient absorption.
• Cortex – Root cortexes are comparatively thicker by proportion than stem cortexes. The cortex is made from thin-walled Parenchyma cells and has two functions; it transports water and soluble nutrients by diffusion from the root-hairs to the Vascular Cylinder, and it acts as a storage system for the plant by storing (usually) starches within its cells.

• Stele – (or Vascular Cylinder) is the collective term for all features including, and enclosed by the Pericycle.
• Pericycle – The outermost layer of the Stele (or Vascular Cylinder). The Pericycle is the origination zone of lateral or branching roots. It also has a strengthening function for the root and gives protection to the Vascular Bundles.
• Procambium – The Procambium is a meristem, an area of cell division and growth lying inside the Stele, as such it is the main site of secondary growth (thickening) in roots. On its outside it gives rise to the Cork Cambium secondary meristem, and on the inside the Vascular Cambium secondary meristem.
  • Cork Cambium – The outside of the Cork Cambium develops into the Pericycle (also into the Cork or Bark when in Stems but not in roots). The inside of the Cork Cambium develops into the Primary Phloem.
  • Vascular Cambium – The outside of the Vascular Cambium develops into secondary Phloem. The inside of the Vascular Cambium develops into the Xylem.
• Vascular Tissue – A plants vascular tissue is its circulation system and comprises long slim cells gathered in Vascular Bundles. The two types of vascular tissue, Phloem, and Xylem are often present in the same bundle with Phloem lying towards the outside of the root and Xylem towards the inside.

• Phloem – Phloem tissue transports the sap (water + products of photosynthesis, typically sugars and other carbohydrates) form where they are made to where they are needed or will be stored. Special cells called Sieve Elements are lined up end to end; when mature they are alive but almost empty, have their metabolic needs supported by other cells and are connected by enlarged pores that allow fluid to move between them. Sap can move in any direction as required and can often move in opposite directions in adjacent sieve tubes.

• Pith – Pith is the soft spongy tissue in the very center of roots, stems and flowering stems. When young it is white or pale and it gradually darkens with age, in woody plants it may eventually get replaced by xylem tissue.

Root System Architecture

Tap Root Systems

Tap root systems are usually, but not exclusively associated with Dicotyledonous plants, Dicots for short. Dicots are plants that exhibit two (di-) seed leaves (- cotyledons) when they first germinate.

Tap root systems develop a primary dominant root directly from the radicle, they are typically straight, thick, tapered and grow directly downwards. From this branch secondary roots, from them tertiary roots and from them rootlets. The primary taproot may persist for the plants entire life, or it may later die back, and a more fibrous system develop from the secondary (etc) roots. One of the best examples of a persistent tap root system is the Dandelion, most UK trees establish with tap roots but later move to fibrous systems.

These root systems provide good anchorage against wind and water, branched tap roots even more so. They allow access to deep nutrients and water giving tolerance to drought. They can be used as food stores as an aid to longevity. This deep ground penetration also aids percolation and storage of groundwater as well as sequestering carbon deeper underground and for longer.

Tap Root Adaptations

Tap root systems exhibit a number of adaptations…

• Food Storage Roots – the primary tap root is often modified for this in the following ways…
  • Conical, widest at the top, e.g. Carrot.
  • Fusiform, widest in the middle, e.g. Radish.
  • Napiform, very wide top and sudden taper, e.g. Turnip.
  • Tuberous, storage roots of no definitive shape, e.g. Cassava.
• Nodulated Roots – these are essentially a specialised feeding root. The root develops irregular nodules that contain millions of Rhizobium genera bacteria. These bacteria are able to fix atmospheric Nitrogen into organic forms the plant can use, while the bacteria receive shelter and protection from the nodules the plant forms. This type of relationship is known as Mutualism and most commonly occurs in leguminous plants, e.g. Peas and Beans so you will see it more often in the veg garden than in the border.
• Respitory Roots – also called Pneumataphores are an adaptation of plants growing in swamps, Mangroves are a well known example. Tap roots turn upwards and the protruding part is equipped with small pores (lenticels) to allow air to reach the roots.

Fibrous Root Systems

Fibrous root systems (sometimes referred to as Adventitious Root systems) are usually, but not exclusively associated with Monocotyledonous plants, Monocots for short. Monocots are plants that exhibit one (mono-) seed leaves (- cotyledons) when they first germinate.

In Fibrous root systems the radicle dies back as fibrous roots develop directly from the base of the plant stem. These roots are comparatively thin, moderately branched while maintaining diameter and tend to grow horizontally in all directions for a length about equal to the plants’ height. One of the best examples of a fibrous root system is that of most grasses.

Fibrous root systems only provide moderate anchorage against wind and rain. They have a much greater surface area than tap roots but at a shallower depth, this gives huge potential to harvest rain and nutrients, but only in surface layers leading to less drought tolerance. This densely matted surface rooting makes fibrously rooted plants and the surrounding soil much less prone to erosion.

Fibrous Root Adaptations

Fibrous root systems also exhibit a number of adaptations…

• Fleshy Fibrous Roots are the fibrous root systems version of food storage roots and occur in several types.
  • Tuberous Roots or Single Root Tubers – enlarged lateral roots of no particular shape that occur singly. Sometimes giving the appearance of a tap root the cells of these roots are modified to store food. E.G. Sweet Potato.
  • Palmate Tuberous Roots – occur in some Orchids where a new tuber grows and an old one dies each year resulting in a hand-like appearance.
  • Fasciculated Roots – these are formed the same way as tuberous roots but always occur in clusters. E.G. Dahlia or Asparagus.
  • Annulated Roots – Swollen roots with ring-like structures appearing like stacked discs. E.G. Ipeac (Cephaelis ipecacuanha)
  • Nodulous Roots – roots with swollen food-storing tips. E.G. Arrowroot.
  • Moniliform or Beaded Roots – with regularly spaced food-storing swellings.
• Epiphytic or Hygroscopic Roots – formed by some orchids and other epiphytes thy hang freely in the air and absorb moisture through specialised sponge-like tissues.
• Photosynthetic or Assimilatory Roots – very similar to epiphytic roots, ut are green and capable of photosynthesis.
• Saprophytic Roots – always associated with fungal hyphae for the absorption of water and minerals. E.G. Monotropa and Sarcodes.
• Stilt roots – these are a bit like tent guy ropes. They arise from the basal node of the stem, descend to the soil at an angle and once rooted form a fibrous system. They stabilise ad anchor the plant and allow water and mineral uptake. E.G. Maize.
• Prop Roots – These grow from aerial branches of plants such as the Banyan tree. They are initially hygroscopic but when reaching the soil develop fibrous roots and a column like structure that supports the spreading branches. Over extended periods they can take over from the main trunk.
• Buttress and Ballast roots – plank like broad but thin roots that add strength and stability to large, usually tropical trees. They are termed buttress roots when near the base of the stem and ballast roots when on the soil surface.
• Climbing Roots – Aerial adventitious roots that grow from the nodes or thin stems of climbing plants to support their climb into the light E.G. Ivy.
• Floating Roots – these develop from the stem nodes of floating plants, often store air, and support the plant towards the surface and the light.
• Contractile or Pull Roots – these are used by plants with underground stems, or Corms, to ensure the stem and aerial parts of the plant are kept at the right depth in the soil.
• Root Thorns – common on many Aroids and Palms these are short, hard and pointed to assist in defence.
• Reproductive roots – These are fleshy fibrous roots that produce buds and can form entire new plants. E.G. Dahlia.

Further Resources

Most of the main search engines will give you a result if you are looking for the meaning of a particular botanical word. If you wish to explore the subject in more detail here are a few website links to give you a starting point.

• Wikipedia – Plant Development.
• Wikipedia – Glossary of botanical terms.
• Angiosperm Morphology – Online self learning.
• Openstax – Online Biology self learning.
• Encyclopedia.com  – Online Encyclopedia – Biology section.