Vegetative Structure And Function 656v4

Vegetative Structure and Function

• Origin of the Root – Hypocotyl – portion of the embryo axis located below the cotyledonary node that give rise at its tip to the radicle – Radicle - lowermost porion of the embryonic axis of a seed. Responsible for the production of the primary root – Primary Root – from the main trunk or from the hypocotyl – Secondary roots – arise from the primary root – Adventitious roots – from stems, leaves and other parts of plants • Developed from structures other than the hypocotyl or primary root.

ROOTS

Types of Root System • Taproot System – Characteristic of dicots – Primary roots continue to grow and send out lateral smaller roots

• Fibrous or Diffuse Root System – Characteristic of monocots – Primary root stops growing and numerous adventitious roots grow and develop from the base of the stem and take the place of the primary root.

Functions of the Roots • • • •

Anchorage-locate Absorption- extracts Storage- store and hoard Conduction-transport

Structure of Roots • Root tip – Root cap • Composed of elongated columella cells that later develop into peripheral cells • Protect growing root tip and meristem • Senses light and pressure exerted by soil particles • Secretes slimy substance mucigel, that aid in protection, lubrication, water and nutrient absorption

Quiscent center • Located just behind the root cap • Composed of 500-1000 inactive cells arrested in G1 phase • Divides only once in 15-20 days • Unaffected by radiation and other extreme environmental conditions • Functions as reservoir to replace damaged cells of meristem • Reorganizes patterns of primary growth in roots

Subapical region • Zone of cellular division • Zone of cellular elongation • Zone of cellular maturation

Zone of cellular division – Dome-shaped apical meristem surrounding the quiscent center – Located 0.5-1.5 mm behind the root itp – Composed of densely undifferentiated cytoplasmic cells – Divides every 12-36 hours – Produces almost 20,000 new cells per day

Zone of cellular elongation • 4-15 mm behind the root tip • Long vacuolated cells • Process of differentiation starts

Zone of cellular maturation • Process of differentiation complted • 10-50 mm behind root tip • Non-elongated, matured cells • Presence of many ephemeral root hairs

Mature region • Epidermis • Cortex • Stele

Epidermis • Covers root except root cap • Usually one cell thick; lacks stomata • Lacks a cuticle or have a thin layer cuticle

Cortex • Interior to epidermis • Occupies largest crosssectional area of root • Consists of 3 concentric layers – Hypodermis protect roots – Parenchyma tissue stores energy reserves – Endodermis lined with Casparian strips, which diverts water and dissolved minerals into cytoplasm of endodermal cells

Stele • Includes all tissues inside the cortex • Pericycle produces branch root • Vascular tissue xylem and phloem form in laternating strands interior to the pericycle • Most dicots have solid core of xylem • Most monocots have parenchymatous pith

STEMS

• Origin of Stems – Epicotyl • Portion of the embryo axis in the seed • Short cylindrical structure bearing a small mass meristematic tissue and frequently a pair or more of tiny leaves at its tip • Immature shoot that later becomes the stem

Kinds of Stem • As to location – Aerial or epiterranean - above surface – Underground or subterraneanbeneath soil – Rhizome – horizontal direction bears most of the feature of typical stem e.g. ginger – Corn – arise from base of an aerial shoot covered with dry leaves e.g. taro (gabi) – Bulb – stem of bulb is in the form or reduced, flattened disc e.g onion

Functions of Stem • Store materials – parenchymal cells • leaves- turgor pressure • Transport water and solutes betwwen roots and leaves – vascular system • Produce carbohydrates chlorophyll

External Structure of Stem • Node – leaves and buds arise • Internode- between 2 successive nodes • Lenticel- tiny raised pores on surface of mature dicot stem for gas exchange • Scar- remnant mark from leaf, bundle, bud, fruit, flower, twig • Bud – immature shoot, largely meristematic

Internal Structure of Stem • Epidermal tissue • Transparent cells surrounding stem • One cell thick that often bears trichomes • in dicot trees, modified into bark as plant grows older

Vascular tissue • Embedded in the ground tissue • Composed of xylem and phloem occurred in vascular bundles – Phloem for the transport of food from the leaves down to the roots and other parts of the plant, and xylem for the transport of water from the roots up to the leaves.

• Vascular bundles are arranged differently in different groups of plants • Monocots have vascular bundles embedded throughout the ground tissue, where phloem oriented outward and xylem inward • Most dicots have a single ring of vascular bundles embedded in the ground tissue • Many non-flowering plants and few dicots have concentric cylinders of xylem and phloem

Ground tissue • In dicots, the parenchymatous ground tissue is composed of cortex and pith • Because monocots have vascular bundles throughout their ground tissue, their stems do not have cortex or pith

LEAVES • Origin of Leaves – Leaf primordia • Outgrowths of the apical meristem in terminal and lateral buds • give rise to mature leaves • its position in the bud determines the relative position of leaves on the stem • the first pair of leaves are produced during the development of the seedling

• Petiole – stalk of the leaf that connects the leaf blade to the node of the stem • Blade – broad, flat, photosynthetic portion of a leaf divided into: – Veins – vascular tissues within a leaf located on both sides of the midrib – Midrib – narrow, thickened structure which is a continuation of the petiole and extends through the center of the blade to the opposite end; the major vein in a leaf that divides the blade into two halves

Leaf Morphology

• Stipule – small paired leaf-like structures at the base of the leaf stalk found on cetrtain plants • Leaf sheath – the base of the leaf blade that completely encircles the portion of the internodal segment of the stem, present among grasses

Leaf Anatomy

• Upper and lower Epidermis – – – –

Transparent, not pigmented Coated with waxy material, cuticle Cells are modified in the form of hairs and glandular cells Presence of chloroplast-containing guard cells (small opening is called stoma)

Mesophyll

• Located between 2 epidermal layers • Made up of parenchymatous, photosynthetic tissues • Consists of 2 distinct types: – Palisade layer- vertically elongated comunar cells arranged below the upper epidermis – Spongy layer –irregular parenchymatous cells with many intercellular spaces (called stomata chambers) arranged above the lower epidermis

Vascular bundles

• Located midway between the upper and lower epidermis • Consists of primary xylem and primary phloem surrounded by a bundle sheath • Also conatin sclerenchymatous fibers or collenchyma cells • No vascular cambium • Xylem – composed of vessel elements located toward the upper epidermis • Phloem- composed of sieve tubes, and companion cells located toward the lower epidermis