EXTERNAL STRUCTURE OF ROOTS

1. EXTERNAL STRUCTURE OF ROOTS • Roots • cylindrical structures • no nodes and internodes • do not generally bear buds, leaves, flowers and fruits • but the enlarged roots of sweet potato give rise to adventitious buds

2. EXTERNAL STRUCTURE OF ROOTS • Classification of Roots based on Origin: • Primary root – first root formed from the embryonic root (radicle, growing tip of the hypocotyl) • Secondary root – roots arising from primary roots • Tertiary roots – arising from secondary roots • rootlets – branches of tertiary roots • Adventitious roots – arising from structures other than the hypocotyl, such as stems and leaves

3. EXTERNAL STRUCTURE OF ROOTS • Kinds of Root Systems • Taproot System • has prominent primary root • Fibrous (diffuse) System • Primary root is lost and replaced by numerous adventitious roots arising from the lower portion of the stem • Slender in form and are more or less equally prominent

4. Types of root systems Tap root system Fibrous root system Tap root is common in dicots; the first or primary root grows straight down and remains dominant root of a plant; often fleshy and adapted to store food (e.g., carrots, beets) The fibrous root system of monocots is a mass of slender roots and lateral branches that hold the plant secure in the soil.

5. EXTERNAL STRUCTURE: LONGITUDINAL REGIONS ROOTS • Root cap region • thimble-shaped mass of cells at the tip of the root • for protective covering for the growing point as it pushes its way through the soil • Embryonic (Meristematic) Region • actively dividing region (apical mersitem) • Region of Elongation or Cell Enlargement • Cells increase in size, particularly the length

6. EXTERNAL STRUCTURE: LONGITUDINAL REGIONS ROOTS • Region of Maturation or Cell Differentiation • region where cells are differentiated (cells attain their final structural characteristic and perform their respective functions) • In Dicots: divided into 3 zones: • Root Hair Zone – numerous hairs; young root • Zone of Primary Permanent Tissue (young) • derived from apical meristems; fully differentiated (primary tissue) • Zone of Secondary Tissues (older) • Derived form the lateral meristems or cambia • In Monocots: only 2 zones: • Root Hair Zone (young) • Zone of Primary Permanent Tissue (young but old at the base)

7. protoderm ground meristem procambium Elongation region Meristematic region Actively dividing cells Root cap Produces mucigel for protection, lubrication, water and nutrient absorption

8. ROOT HAIRS Increase the surface area for the absorption of water

9. Roots-External • Root cap -indicates growth of new cells • Root hairs -absorb moisture (water) and minerals

10. Root development - The main difference is in the vascular cylinder or stele. Dicot Monocot • there is pith in the very center composed of parenchyma. Instead there is xylem tightly packed with branching arms looking somewhat like a star. The phloem fills in the spaces in between the arms • the phloem and xylem are in loose rings. With the phloem towards the outside and the xylem towards the inside. There is no pith in the center.

11. Root development Dicot Monocot 1. usually 3-5 groups of phloem and xylem arranged alternately 2. xylem endarch (relating to a xylem whose early development is toward the center) 3. cambium present 1. there r many vascular bundles alternately arranged 2. xylem exarch (relating to a xylem whose early development is away from the center and toward the periphery) 3. no cambium

12. INTERNAL STRUCTURE: CROSS SECTION OF A YOUNG DICOT ROOTS • Three Regions: • Epidermal or Dermal Region (outer region) • Single layer of living cells; may or may not have root hairs • Cortex (middle region) • Consists of 2 zones: • Outer zone consisting of several layers of parenchyma • Inner zone consisting of a single layer of thick-walled living cells (endodermis) • Passage cells – cells of the endodermis opposite the xylem rays

13. INTERNAL STRUCTURE: CROSS SECTION OF A YOUNG DICOT ROOTS • Three Regions: • Stele or Vascular Cylinder (inner region) • Pericycle (outermost part) • Single layer of thin-walled living cells (parenchyma) • Primary Xylem • tracheary elements organized in the form of a star • direction of development from outer (tip of the xylem ray) towards the inside – pattern called exarch • Primary Phloem • Patches of tissues located between the rays or radial arms of the primary xylem • Vascular Cambium • Layer of meristematic cells between primary xylem and phloem

14. INTERNAL STRUCTURE: CROSS SECTION OF A OLD DICOT ROOTS • Two groups of tissue which enter into the tissue composition of old dicot root: • Secondary vascular tissues • consists of secondary xylem and phloem • Periderm • Phellogen (cork cambium) • Phellem (cork) • Phelloderm (secondary parenchyma)

15. INTERNAL STRUCTURE: CROSS SECTION OF AN OLD DICOT ROOTS • What happens? --- once differentiated: • Cambium gives rise to the secondary phloem outwardly and secondary xylem inwardly • Secondary xylem would eventually occupy the places formerly occupied by primary phloem • Secondary xylem → vascular cambium → secondary phloem • Pericycle becomes meristematic and gives rise to cork cambium (phellogen) • Cork cambium produces phelloderm inwardly and phellem or cork outwardly (serves as the outermost tissue)

16. INTERNAL STRUCTURE: CROSS SECTION OF A MONOCOT ROOTS • Three Regions: • Epidermal or Dermal Region (outer region) • Cortex (middle region) • Stele (inner region) – consists of • Pericycle • Xylem – star-shaped like of a young dicot, except for: • star-shaped xylem has more radial arms (polyarch) • center of star-shaped xylem may be occupied by sclerenchyma fibers and parenchyma; tracheary elements are found only within the radial arms • Phloem – occupies the spaces between the radial arms of xylem • No vascular cambium

17. FUNCTIONS OF ROOTS • Support • presence of brace roots, prop roots, clinging roots, and buttresses (extensions from the lower part of stem) • Food storage • enlarged, fleshy or succulent roots • Photosynthesis • green aerial roots • Reproduction • Development of adventitious bulbs • Protection • Presence of spines • Aeration • Development of pneumatophores (roots with spongy tissues protruding above the ground

18. Functions of Roots Root images from a rice plant • Absorption-take water and nutrients from the soil and conduct them to the stem • Anchor the plant and hold it upright • Store food for plant use • Asexual reproduction in some plants

19. List at least 5 practical human uses for roots • Food storage • sugar, beets, turnip, rutabagas, parsnip, radish, carrot • Spices • Sassafras, sarsaparilla, licorice, • Dyes • reds, browns, coffee bean • Drugs • gentian, reserpine (tranquilizer) • Insecticide • rotenone

20. Fig. 35-7, p. 753

21. Young dicot root epidermis cortex vascular cylinder endarch relating to a xylem whose early development is toward the center) there is pith in the very center composed of parenchyma. Instead there is xylem tightly packed with branching arms looking somewhat like a star. The phloem fills in the spaces in between the arms

22. Monocot root metaxylem vessel protoxylem vessel actinostele polyarch exarch exarch exarch - xylem whose early development is away from the center and toward the periphery

23. Monocot root -there are many vascular bundles alternately arranged - the phloem and xylem are in loose rings. With the phloem towards the outside and the xylem towards the inside. There is no pith in the center.

24. Dicot root usually 3-5 groups of phloem and xylem arranged alternately