Plant Root Guide

Roots are irregularly branched, underground structures that serve two main functions; to firmly anchor the plant to a fixed spot (with good access to water, nutrients, and light), and as a transport systems, allowing the plant to suck up the water and nutrients.

Two Types of Root Systems

In general, plant root systems are either a taproot system (with primary root) or a fibrous root system (adventitious roots) with attached branch roots and finer rootlets having root hairs close to the tip.

Taproot System:

taproot

Distinguished by having one large, central, and dominant root (taproot), growing directly downward, from which smaller roots sprout laterally.  When the seed germinates, the first root to emerge is the radicle, or primary root.  In conifers and most dicotyledons, seed that has two embryonic leaves, this primary root develops into the taproot.

Taproots are sometimes modified for use in storage (mainly carbohydrates) like those found in sugar beet or carrot.  Taproots are also an important adaptation for searching for water.

There are many different shapes a taproot can take on, the main ones being:

  • Conical root: this type of root tuber is cone-shaped: carrot.
  • Fusiform root: this root is widest in the middle and tapers towards the top and the bottom: radish.
  • Napiform root: the root has a top-like appearance. It is very broad at the top and tapers suddenly like a tail at the bottom: e.g. turnip.

Fibrous Root System:

fibrous

Distinguished by having many similarly sized roots.  In this case the radicle from a germinating seed is not the main root.

Adventitious roots are also classified under fibrous root.  They are roots that form on plant organs other than roots.  Most monocotyledons, or seed typically containing only one embryonic leaf, have fibrous root systems.  Some fibrous roots are also used for storage; sweet potatoes form on fibrous roots.  Plants with fibrous roots systems are great for erosion control, because the mass of roots cling to soil particles.

There are many types of fibrous roots, the main ones being:

  • Aerial roots: Adventitious roots that are common in the arum or gabi family (Araceae) and orchid family (Orchidaceae). In general, these fibrous roots remain aerial and do not enter the soil.
  • Haustoria: These are specialized roots in parasitic plants that penetrate the tissues of a host plant, as in watchweed (Striga) and broomrape (Orobanche).
  • Prop roots: Aerial roots that arise from a stem and sink into the soil to provide additional support to the plant. Similar to the pillars of a bridge making it possible for branches to extend horizontally to great distances.
  • Pneumatophores: Specialized roots in some plants that grow in watery places and which function mainly for oxygen absorption. The raised pneumatophores of many mangrove species serve as entry of atmospheric oxygen which diffuses to roots growing in anaerobic soils. These “knees” are described as cone-shaped extension of the root that protrude from the ground.
  • Buttress roots: These are enlarged, often thickened roots that spread horizontally from the base of trees to provide additional support. In tropical trees that are shallow rooted, large buttress roots are formed at the base of their trunks.

 

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The quantity and dispersal of plant roots has a major impact on the absorption of water and nutrients. The depth and spread of the roots depends on the inherited characteristics of the plant and the texture and structure of the growing media. Roots will penetrate much deeper in a loose, well-drained media than in a heavy, poorly-drained media.

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Root Anatomy

Roots have specialized parts that develop from the three major types of plant tissue: ground, dermal, and vascular.

Plant Cross

 Dermal Tissue System:

Dermal tissue is found in the epidermis and periderm of a plant. This dermal tissue is used for protection and for the prevention of water loss.

Ground Tissues System:

There are three types of ground tissue. They are parenchyma, collenchyma and sclerenchyma. They includes all the tissues of the plant body except epidermal and vascular tissues. This ground tissue is used for photosynthesis, food storage, regeneration, support, and protection.

Vascular Tissue System:

Vascular tissue includes the xylem and the phloem (along with some collenchyma and parenchyma cells). The xylem is the main tissue for carrying water and minerals throughout the plant body. The phloem transports sugars, food, and other solutions to the different parts of the plant.

Parts of a Root

 

roottip

Internally, there are three major parts of a root. The meristem is at the tip and produces new cells. It is an area of cell division and growth. Behind it is the region of elongation, in which cells grow in size through food and water intake. These cells by expanding in size, push the root through the soil. The third major root part is the maturation zone, in which cells undergo changes in order to become specific tissues such as epidermis, cortex, or vascular tissue.

Plant Root Functions:

  • Anchorage and support: A plants root system anchors the plant body to the soil and provides physical support. In general, however, taproot system provides more effective anchorage.
  • Absorption and conduction: The plant root system absorbs water, oxygen and nutrients from the media in mineral solution, mainly through the root hairs. They are able to absorb inorganic nutrients in solution even against concentration gradient. These nutrients are moved upward from the roots. Plants with a fibrous root system are more efficient in absorption from shallow sources.
  • Storage: The root serves as a storage organ for water and carbohydrates. Fibrous roots generally store less starch than taproots.
  • Photosynthesis: Some roots are capable of performing photosynthesis, as in the epiphytic orchids and aerial roots of mangrove trees.
  • Aeration: Plants that grow in stagnant water or other watery places have modified roots called pneumatophores to which oxygen from the air diffuses.
  • Movement: In many bulb- and corm-forming plants, contractile roots pull the plant downward into the soil where the environment is more stable.
  • Reproduction: The plant root system also serves as a natural means of perpetuating a species. Plants with a fibrous root system are easier to transplant than those with tap roots.

Root pressure

Root pressure is how water travels from the soil through the roots of a plant and up its stem. The xylem conducts the water, while the pressure is produced by a combination of osmosis, which forces water from the soil into the cells of the root, and an active pumping mechanism that creates a concentration gradient of salt ions in the xylem. Excessive root pressure may cause ‘bleeding’ of sap from a wound in the plant or the collection of drops of water from undamaged leaf margins.

 

 

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