Botany Chapter 4 Plant Structure Roots Stems and

Botany Chapter 4 Plant Structure Roots, Stems and Leaves

Objectives – Section 1, Roots 1. Describe the main functions of roots 2. Compare & contrast tap roots and fibrous roots 3. Describe the pattern of tissue development & growth in roots 4. Explain how root structure contributes to its function of absorbing water & minerals 5. Compare & contrast the organization of tissues in monocot roots with those of dicots and gymnosperms 6. Discuss some variations in root structure that correspond to specialized functions 7. Discuss the benefits to plants gained by mycorhizal associations and nitrogen fixing bacteria

The Functions of Roots � Anchor the plant in the soil � Absorb and transport water and minerals � Receive and store organic materials from the stems and leaves

Types of Roots Tap Root vs. Fibrous Roots � Tap root systems have a main root, usually vertical, with lateral roots and smaller branching roots � Fibrous roots have no main root. All roots are approximately equal in size

Tap Root Development � The tap root is derived from the embryonic radicle � Lateral roots grow from the tap root, and branch roots grow from the lateral roots � Tap root systems are characteristic of larger, longer living plants

Fibrous Root Development � In plants with fibrous root systems, the embryonic radicle dies, and roots are derived from the lower stem (“adventitious” roots) � Lateral roots branch off from the adventitious roots and extend horizontally

Which type of root system would have the advantage. . . �In deep soil? �In shallow soil? �For rapid growth? �For larger, taller plants? �Initially after a light rain? �In periods of drought? �In porous soil? �In storing food, water and minerals?

Root Growth and Development

Root Growth and Development � Growth initiates near the root tip (“apical” meristem) � Meristematic initials form a small “quiescenter” just inside the root cap � Derivative meristem differentiates into procambium, ground meristem, and protoderm

Primary Root Growth Zones � The quiescenter and fresh derivative cells form the Zone of Cell Division � Older derivative cells that are growing rapidly form the Zone of Elongation � Cells that are fully grown and differentiated form the Zone of Maturation � Root hairs are derived from the zone of maturation

Adaptions for Absorption �Water uptake occurs mostly in the root hairs, which are each only one cell thick. The fine, threadlike nature of the root hairs maximizes the surface area of contact with the soil and its moisture �Some of the water that is absorbed will enter the cytoplasm by osmosis �Some of the water will remain outside of the cell, associated with the cellulose of the cell wall

Transport to the Vascular Bundle � Water traveling between cells (“intercellular”), conducted by the cell walls is “Apoplastic” � Water traveling through the cytoplasm and across cell membranes is “Symplastic”

Endodermis and Casparian Strip �In roots, the vascular tissue is at the core of the root, in the Stele �The endodermis forms a boundary between the Cortex and the stele �The intercellular spaces in the endodermis are sealed off by the Casparian Strip. The casparian strip is impermeable to water flow, preventing Apoplastic flow into the stele

Why does it matter? �It matters because apoplastic transport is nonspecific. The cell walls and intercellular spaces are not selectively permeable. Only the cell membranes are. �By forcing only symplastic transport (through the cells, not around them) into the stele, the plant can control the materials that enter the vascular tissue

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Structural Variations in Roots � The arrangement of vascular tissue varies depending on the type of plant � In Monocots, the xylem and phloem alternate with each other in a ring surrounding a core of parenchyma � In Dicots, the xylem forms a lobed core with phloem between the lobes

Monocot vs. Dicot Roots (cs)

Specialized Functions (see p. 78) � Aerial Roots • Epiphytes – Absorb water from humid air • Climbing plants – Anchor the plant to a host • As prop roots – Adventitious roots to stabilize and anchor (usually in monocots) � Buttress roots – Lateral roots for support in thin soil (usually in dicots) � Pneumatophores – air roots to provide oxygen (if roots are submerged in water) � Storage roots – like carrots, radishes, beets

Symbiotic Associations �A symbiotic association is a close ecological relationship between two different species, where one depends upon the other. If both benefit, it is called Mutualism. �Plants live in close association with a variety of microorganisms (fungi and bacteria). Some are parasitic, but some benefit the plant in specific ways

Mycorhizal Associations �Myco = fungus, rhizal = root �Fungi are heterotrophs, usually decomposers, but mycorhizal fungi obtain food from the roots of a host plant �The host plant loses some food to the fungus, but benefits because the fungus provides water and minerals in greater amounts than the plant could obtain on its own – so both benefit

Nitrogen Fixing Bacteria � Plants are photoautotrophs. They produce sugars by photosynthesis � Producing amino acids and nucleotides is a more complex issue because it requires nitrogen, which must be obtained from the soil and in a particular form � Soil nitrogen originates from the activity of nitrogen fixing bacteria � Legumes (peas, beans, alfalfa) have nodules full of nitrogen fixing bacteria in their roots

Symbiotic Associations � Mycorhizal Associations � https: //www. youtube. c om/watch? v=JFi 6 c. Jecm 0 � https: //www. youtube. c om/watch? v=Drs. Nuw Ono. EM � Legumes, with nodules full of nitrogen fixing bacteria � https: //www. youtube. c om/watch? v=4 NKGS 4 bj 7 cc � https: //www. youtube. c om/watch? v=9 j 6 Jk 8 Y Lgf. Y

Section 1 Review Questions 1. 2. 3. 4. 5. 6. How do taproot and fibrous root systems differ? Describe cell development and maturation through the zones near a root tip What are the functions of the root cap, mucigel, and root hairs? What are the roles of the pericycle and endodermis? What are some examples of specialized adaptions in roots? What are mycorhizae, and how are they beneficial?

Objectives – Section 2, Stems 8. 9. Discuss models of shoot primary growth � Zone model � Cell layer model Discuss the arrangement of vascular bundles in stems 10. Compare and contrast the structure of stems in monocots, dicots, and ferns 11. Describe patterns of leaf arrangement on stems � Alternate � Opposite � Whorled 12. Discuss specialized functions of modified stems (see page 86 in textbook)

Shoot Primary Growth � Primary growth results from apical meristem activity � Primary growth in stems follows a similar pattern to root growth: • The apical meristem initials form a small ball of slowly dividing cells • The surrounding meristematic derivatives differentiate into the three primary tissues: protoderm, procambium, and ground meristem

Patterns of stem growth � There are two models of stem growth (the Zone model and the Cell Layer model) � There are variations between the two, but they share some fundamentals � For diagrams of the growth models see page 82 in the text

Stem Growth - The Zone Model � Apical meristem will give rise to 3 primary meristems: protoderm, ground meristem, and procambium � The apical meristem is divided into 3 zones: • The central mother zone gives rise to the other 2 zones (peripheral and pith zones) • The peripheral zone surrounds the central mother zone, divides rapidly, and can derive all 3 primary meristems laterally and distally to the central mother zone • The pith zone derives ground meristem only, on the proximal side of the central mother zone

Stem Growth: Cell Layer Model � The outer 2 layers of initials in the apical meristem form the Tunica. These cells divide perpendicular to the surface (“anticlinal), and give rise to the protoderm � The 3 rd layer of meristematic initials and underlying derivatives forms the Corpus. These cells divide in both planes (“anticlinal” and “periclinal”) and give rise to the procambium and ground meristem

Simplifying the Growth Models �The zone model emphasizes the regions of cell division, while the cell layer model emphasizes the cell divisions themselves �Both models share the same basic pattern • Derivative cells at the top and sides of the ball of initials grow laterally to form the Leaf Primordia and Vascular tissue • Derivatives at the base of the initials grow to form the pith parenchyma

Vascular Bundles � Vascular tissue in the shoot system forms a consistent pattern of bundles � Generally, xylem cells are located toward the center of the stem, while phloem is more toward the surface

Organization of Vascular bundles � As in the roots, vascular bundles form distinct organizational patterns that vary depending on the type of plant � The trend from root to stem is a periferal migration of vascular bundles (from middle towards surface)

Monocot vs. Dicot Stems

Vascular Variations in Stems �See diagrams in textbook, page 83 �Bryophytes • No vascular tissue �Seedless Vascular Plants • Vascular tissue in stems is the same as in roots �Monocots • Vascular bundles dispersed �Dicots & Gymnosperms • Vascular bundles in a ring (eustele) outside the pith

Leaf Arrangement on Stems

Modified stems

Modified Stems � Rhizomes • Underground, horizontal stems - Storage � Corms • Resembles a bulb – stores food in an enlarged stem surrounded by thin leaves � Bulbs • Stores food in fleshy leaves on a short stem � Tubers • Underground storage stem � Stolons • Above ground horizontal stems for asexual reproduction

Section Review p. 87 1. 2. 3. 4. 5. How does stem growth differ from root growth? Describe the basic types of steles in stems What is phyllotaxy? What do the differences between palm trees and wheat plants reveal about stem growth? What are some similarities between modified stems and modified roots?

Objectives – Section 3, Leaves 13. Describe the process of leaf formation 14. Identify characteristic parts of a leaf and describe the structure and function of each • External � Blade, Petiole, Stipules • Internal � Epidermis, Cuticle, Guard Cells, Stomata � Mesophyll: Palisade and Spongy � Vascular Bundle: Xylem, Phloem, Bundle Sheath 15. Explain variations in leaf structure • Simple vs. compound leaves • Shape variations • Venation pattern variations 16. Distinguish between deciduous and nondeciduous (evergreen) trees 17. Explain the role of abcission zones in seasonal leaf loss (“fall”) 18. Discuss specialized functions of modified leaves (see page in textbook)

Leaf Formation � Leaf Buttress • Bulges that are the first indications of leaf growth � Leaf Primordium • Elongate from the leaf buttress and form the petiole and blade � Petiole and Blade • Petiole �May range from stem-like to leaf-like �May be absent (“sessile” leaves – usually monocots) �May have 2 “stipules” at the node • Blade �Usually flattened �Growth rates of ridges near the edge of the leaf blade produce variations in leaf margins

Leaf Primordia � Leaf primordia form from the leaf buttress � Primordia are associated with the apical meristem � Note also the axillary buds at the next more mature node. The growth rate of these will depend upon the level of auxin hormone � Note “apical dominance”

Petiole, Blade and Stipules

Stipules

Comparison to Sessile Leaf

Blade Margins

Internal Structure of Leaves

Mesophyll and Vascular Bundles

Palisade vs. Spongy Mesophyll

Stomate and Guard Cells

Identify the Labeled Parts � Spongy mesophyll � Guard Cells � Palisade mesophyll � Upper epidermis � Stomate � Lower epidermis

Simple vs. Compound Leaves Compound leaves have more than one blade sharing the same petiole � A node bearing a simple leaf will have an axillary bud at the juncture of petiole to node � A blade on a compound leaf will not have axillary buds where the blades attach to the petiole �

Compound leaves

Pinnate vs. Palmate

Leaf Shape Variations

Venation Variations

Adaptation to Seasonal Change �Deciduous trees lose their leaves due to seasonal changes (autumn). Evergreens keep their leaves year round �Some evergreens are well adapted to seasonal change, but others simply live in tropical regions where seasonal change is minimal �Most deciduous trees are angiosperms while gymnosperms are often evergreen

Deciduous Forest in Autumn

Traditional Evergreens

Adaptation to Seasonal Drought

Pine Leaves � Pine leaves are shaped to minimize surface area in proportion to volume � A thick epidermis and heavy waxy cuticle prevent water loss

Tropical Evergreens

Ginkgo Trees are Deciduous Gymnosperms

Abscission of Deciduous Leaves � Deciduous trees drop their leaves as a result of changes at the point of attachment of the petiole to the stem � The abcission layer forms after the plant has reabsorbed water and other materials from the leaf � The abcission layer is basically a protective scar

Abcission

Modified Leaves �Tendrils • Coiling structures used for attachment by climbing plants �Spines • Note: Thorns are modified stems (arise from axillary buds), spines are modified leaves �Bracts • Modified leaves superficially similar to petals, found at the base of flowers

Tendrils

Cactus Spines

Bracts