Roots Objectives 1 List three benefits of roots

Roots

Objectives: 1. List three benefits of roots to plants. 2. Observe the internal features of a cross-section of a monocotyledonous and dicotyledonous root, specifically the xylem, phloem, cortex and pith. 3. Observe the internal features of a longitudinal-section of a dicotyledonous root, specifically the xylem, phloem, companion cells, and sieve tubes. 4. Compare the external appearance of the dicotyledonous root with modified roots.

Observe the diagram

Observe the diagram

Types of Root Systems

Tap Root System – It is the root system that develops from the radicle and continues as the primary root (tap root) which gives off lateral roots. – They provide very strong anchorage as they are able to reach very deep into the soil. – It is the main root system of dicots e. g. gram, china rose, neem

Tap Root System

Fibrous Root System – In this root system, the primary root is short lived. – A cluster of slender, fiber-like roots arises from the base of the radicle and plumule which constitute the fibrous root system. – They do not branch profusely, are shallow and spread horizontally, hence cannot provide strong anchorage. – Fibrous root system is the main root system of monocots, e. g. maize, grasses, wheat

Fibrous Root System

Types of Roots

Tap Root – It is the primary and the main root that develops from the radicle, bears many branches and remains underground. – It is usually found in dicots e. g. sunflower, mustard, carrot, mango

Adventitious root – These are roots that develop from any part of the plant except the radicle. – They may be aerial or underground. – They may grow from node (money plant, bamboo), stem cutting (rose), tree branch (banyan)or stem base (fibrous roots in monocots).

Regions of Root – Apical region of roots of any root system shows the same zones or regions

Regions of Root cap region: – It is a thimble- like structure produced by meristematic (rapidly dividing) zone and protects the tender apex (apical meristem) from harsh soil particles. – As the root grows further down in soil, root cap wears out but it is constantly renewed. – In aquatic plants (Pistia and water hyacinth) root cap is like a loose thimble called root pocket.

Regions of Root Region of meristematic cells: – Is a small region of actively dividing cells called the apical meristem. It consists of : (i) Dermatogen (outermost layer whose cells mature into epiblema and root cap); (ii) Periblem (inner to dermatogen whose cells mature into cortex) and (iii) Plerome (central region whose cells mature into stele). In monocots, cap is formed by independent group of cells known as Calyptrogen.

Regions of Root Region of elongation: – Lies next to the meristematic region, the cells elongate and enlarge to make the root grow in length.

Regions of Root Region of maturation – Lies next to the region of elongation. – The cells mature and differentiate into various tissues constituting (i) Root hair or piliferous region having unicellular hairs which absorb water and mineral salts from soil and (ii) Permanent region which lies behind the root hair zone and is without hairs. It produces lateral roots, anchors the plant in soil and conducts water and minerals upwards.

Function of Roots Anchorage – Roots anchor the plant firmly to the soil (mechanical function).

Function of Roots Absorption – Roots absorb water and mineral salts and conduct them upwards (physiological function). – To take up water and minerals in the root hair cells for transportation into the xylem vessel. – Once water is in the xylem vessel, it transports water and minerals to all parts of the plant. – The water moves into the root hair cells by osmosis. – The minerals move from the soil to the cell by means of active transport.

Function of Roots Special functions – By undergoing modifications in their structure, roots perform special physiological functions like food storage, assimilation, absorption of atmospheric moisture, sucking food from host, better gaseous exchange and mechanical functions like floating (buoyancy), stronger anchorage and climbing.

Root – The vascular bundle is in the centre of the root, unlike the stem where the bundles form a cylinder in the cortex. – The xylem carries water and salts from the root to the stem. – The phloem bring food from the stem to the root, to provide the root cells with substances for their energy and growth.

Root – The vascular bundle is found in the centre – There is a large central core of xylem- often in an xshape – This arrangement provides strength to withstand the pulling forces to which roots are exposed

Root – Around the vascular bundle are cells called the endodermis which help to get water into the xylem vessels – Just inside the endodermis is the pericycle which contains meristem cells that can divide (for growth)

Structure of Roots

Dicot Root

Dicot Root – Epiblema: Single, outermost layer of thin-walled cells. Some cells are prolonged to form unicellular root hairs. It protects and absorbs water. – Cortex: Large zone, many layered, cells thin-walled parenchymatous with intercellular spaces, stores food and water.

Dicot Root – Endodermis : Innermost layer of cortex, cells barrel-shaped, closely packed, show band like thickenings on their radial walls called casparian strips. Some cells (opposite the protoxylem) which lack these strips are called passage cells. They help in the movement of water and dissolved salts from cortex directly into xylem. – Stele : All tissues inner to endodermis comprise stele.

Dicot Root – Pericycle : Inner to endodermis lies a single layer of pericycle. It is the seat of origin of lateral roots and vascular cambium and cork cambium during secondary growth.

Dicot Root – Vascular Bundle : It consists of xylem and phloem patches lying on alternate radii i. e. , it is radial. Xylem is exarch where protoxylem (first formed , having narrow vessels and tracheids) lies towards the periphery and metaxylem (differentiates later, has wider vessels and tracheids) lies towards the center. Depending upon the number of xylem patches a root may be diarch (di-2 patches) to hexarch (hexa- 6 patches).

Dicot Root – Pith: Sometimes the metaxylem of all xylem patches meet in the centre, in that case pith is absent or is small and parenchymatous. – Conjunctive parenchyma: Parenchyma which separates xylem and phloem. .

Monocot Root

Buttress roots – Buttress roots are large roots on all sides of a shallowly rooted tree. Typically, they are found in nutrient-poor rainforest soils and do not penetrate to deeper layers. They prevent the tree from falling over (hence the name buttress) while also gathering more nutrients.

Tuber – Storage Root

Breathing roots – An aerial root may be defined as a root which, for part of the day at least, is exposed to the air. The mangrove mud is rather anaerobic (oxygen poor) and unstable and different plants have root adaptations to cope with these conditions

Prop roots – Adventitious roots, when modified for aerial support, are called prop roots, as in corn or some figs. In many tropical rain forest trees, large woody prop roots develop from adventitious roots on horizontal branches and provide additional anchorage and support.

Stilt roots or Prop Roots – Mangrove rainforest trees require a different kind of support system. Mangroves grow in wet, muddy soil at the water's edge which can be subject to tides and flooding. – Act as a stilt

Sucking Root – These are small root which penetrates the host tissue. – Partial parasites- penetrate and make contacts with host xylem tissue since they are in need of water and nutrients. – Total parasite makes contact with the phloem tissue, since they are in need of photosynthetic materials.

Modified Roots 1. Buttress roots- are large roots on all sides of a shallowly rooted tree. Typically, they are found in nutrient-poor rainforest soils and do not penetrate to deeper layers. They prevent the tree from falling over (hence the name buttress) while also gathering more nutrients. 2. Prop roots- provide additional anchorage and support. 3. Sucking roots- These are small root which penetrates the host tissue. They can be Partial parasites (penetrate and make contacts with host xylem tissue for water and nutrients. Total makes contact with the phloem tissue, since they are in need of photosynthetic materials. 4. Breathing Roots- The mangrove mud is rather anaerobic (oxygen poor) and unstable and different plants have root adaptations to cope with these conditions 5. Storage Roots- holds food