1. Plant Structure and Function
Chapter 23
Plant Structure and Function

2. Section 23-1
Specialized Tissues in Plants

3. Structure of a Seed Plant
3 organs -> roots, stems, leaves
Linked by tissues that provide support, protection, nutrient production and transport

4. Structure of a Seed Plant
Roots
Anchor plants, prevent erosion, absorb nutrients/water and transport them, store food, hold plants upright
Stems
Produce leaves/reproductive structures, contain transport systems
Leaves
Photosynthesis, have adjustable pores to reduce water loss and help gas exchange

5. Plant Tissue Systems
3 main tissue systems -> dermal, vascular, and ground
Dermal Tissue - protective, outer covering
Single cell layer in young plants called epidermis, the outer surface often covered with a waxy cuticle
In older plants usually many layers, sometimes covered with bark
Some epidermal cells have trichomes, which protect leaves and give them a fuzzy appearance
In roots, incudes root hairs to absorb water

6. Plant Tissue Systems
Vascular Tissues – support plant bodies, transport water and nutrients
Xylem – transports water
Phloem – transports products of photosynthesis

7. Xylem Cells called tracheids
As they mature, they die and leave their cell walls which contain lignin (gives wood strength)
Cells have connecting openings for water to pass
Pits allow water to diffuse into ground tissue

8. Xylem
Angiosperms have second xylem tissue called vessel elements – wider than tracheids, arranged end to end
Mature and die, cell walls develop slits at each end for water to move freely

9. Phloem Alive at maturity
Main cells called sieve tube elements, arranged end to end forming sieve tubes
Small holes at ends so nutrients can move from cell to cell
Lose nuclei and most organelles as they mature

10. Phloem
Companion cells surround sieve tube elements - keep nuclei/organelles

11. Plant Tissue Systems
Ground tissue – produces/stores sugars, contributes to physical support
Parenchyma cells – thin walls, large central vacuole surrounded by thin layer of cytoplasm – chloroplasts in leaves
Collenchyma cells – thicker walls, flexible, provide support
Sclerenchyma cells - thickest walls, rigid, makes up seed coat

12. Parenchyma Collenchyma Sclerenchyma

13. Plant Growth and Meristems
Meristems – regions of unspecialized cells in which mitosis produces new cells ready for differentiation
Apical meristems found in places of rapid division – tips of stems and roots

14. Plant Growth and Meristems
At first, cells produced in apical meristems are all thin, unspecialized
Gradually mature and differentiate to form each tissue system
Meristems also create highly specialized cells of cones and flowers
Patterns of gene expression changes the stem’s apical meristem

15. Section 23-2
Roots

16. Root Structure and Growth
As soon as a seed sprouts, its first root brings in water/nutrients from soil
Cells divide rapidly, pushing root tips into soil, providing raw materials for developing stems and leaves

17. Root Structure and Growth
Taproot Systems
Primary root grows long and thick (taproot) giving rise to smaller branches
Can store sugars and starches
Fibrous Root Systems
Begin with one primary root, which is replaced by many equally sized branches that grow separately from the base of the stem
Help prevent soil erosion

18. Dandelion (taproot) Grass (fibrous root)

19. Anatomy of a Root
Epidermis made of dermal tissue – protection and absorption
Surface covered in root hairs – penetrate between soil particles and increase surface area
Cortex composed of ground tissue
Water/minerals move from epidermis
Stores products of photosynthesis and starches

20. Anatomy of a Root
Endodermis – layer of ground tissue enclosing vascular cylinder – moves water and minerals to center of root
Vascular cylinder in the center composed of xylem and phloem
Dicot roots have central column of xylem

21. Anatomy of a Root
Apical meristems near root tip allow roots to increase in length
Root cap protects meristem, secretes slippery substance to ease progress through soil
Cells at tip scraped away and replaced continually

23. Root Functions Uptake of plant nutrients
Soil contains sand, silt, clay, air, bits of decaying animal/plant tissue in varying amounts
Plants need inorganic nutrients like nitrogen, phosphorus, potassium, magnesium, calcium
Trace elements also important, but excessive amounts can be toxic

25. Root Functions Active transport of dissolved nutrients
Active transport proteins in root hairs, other epidermal cells
Bring in mineral ions from soil
Water movement by osmosis
Mineral ions accumulate in root, water “follows”

27. Root Functions Movement into vascular cylinder Move through cortex
Cylinder enclosed by endodermis – cells meet and cell walls from waterproof zone called Casparian strip
Casparian strip forces water/minerals to move through cell membrane rather than between cells – filter and control water
Ensures one-way flow

28. Root Functions Root pressure
Minerals pumped into vascular cylinder, water follows by osmosis creating pressure
Water has to go up - root pressure forces water through vascular cylinder into the xylem
Up and up!

29. Section 23-3
Stems

30. Stem Function Produce leaves, branches and flowers
Hold leaves up to sun
Transport substances
Xylem and phloem form continuous tubes from roots to stems to leaves
In many plants they function in storage and aid in photosynthesis

31. Anatomy of a Stem
Surrounded by layer of epidermal cells with thick cell walls and a waxy protective coating

32. Anatomy of a Stem Growing stems have nodes where leaves are attached
Buds contain apical meristems to produce new stems and leaves
Larger plants have woody stems to support leaves and flowers

33. Monocot Stems
Vascular bundles (clusters of xylem and phloem) scattered throughout ground tissue composed mainly of parenchyma cells

34. Dicot Stems Vascular bundles arranged in a ring pattern
Parenchyma cells inside ring called pith, outside form cortex
Complexity increases as stem increases in diameter

35. Primary Growth
Occurs in all seed plants – apical meristems increase plant length

36. Secondary Growth
Larger plants require older parts of stem to increase in thickness
Common in dicots and gymnosperms

37. Secondary Growth
Takes place in meristems called vascular cambium (produced vascular tissues, increase thickness of stem) and cork cambium (outer covering)

38. Growth from Vascular Cambium
Thin layer of cells between xylem and phloem
Xylem pushed in, phloem pushed out
Increases diameter of stem each year

39. Wood Formation Layers of secondary xylem produces by vascular cambium
Older xylem near center no longer carries water – heartwood (dark)
Surrounded by sapwood – active in fluid transport (light)

41. Tree Rings
In spring, vascular cambium produces light colored rings of xylem (early wood)
Cells grow less as season continues, have thicker cells walls, darker in color (late wood)
A ring = a year of growth
Thick rings mean favorable weather

42. Formation of Bark
Everything outside the vascular cambium in a mature stem (phloem, cork cambium, cork)
Expansion leads to oldest tissue splitting
Cork cambium surrounds cortex producing a thick layer of cork to prevent water loss
Outer layers may flake off as stem thickens

43. Section 23-4
Leaves

44. Anatomy of a Leaf
Blade – thin, flat part of leaf – maximum light absorption
Blade attached to stem by petiole
Outer covering of dermal tissue
Top and bottom covered by epidermis, tough irregular cells with thick outer walls
Covered by waxy cuticle – waterproof, prevents water loss

45. Anatomy of a Leaf
Vascular tissues bundles into veins that run from stem through leaf
Palisade mesophyll beneath upper epidermis – closely packed cells that absorb sunlight
Spongy mesophyll contains air spaces connected to stomata – small opening in epidermis allowing for gas exchange

47. Transpiration Mesophyll cell walls moist for easy diffusion
Water can evaporate from these surfaces by transpiration
May be replaced by water from xylem
Cools leaves on hot days, but can threaten survival

48. Gas Exchange
Exchange gases between air spaces in spongy mesophyll and exterior by opening stomata

49. Homeostasis
If stomata were always open, too much water would be lost to transpiration
Open just enough to allow photosynthesis
Guard cells control opening and closing of stomata, regulating movement of gases

50. Homeostasis
When water is abundant, increaser in water pressure in guard cells opens stoma by curving
When water is scarce, water pressure in guard cells drops and stoma closes
Stomata usually open during day, closed at night
Can be closed in bright sunlight or hot/dry conditions
Respond to environment

51. Transpiration and Wilting
Osmotic pressure keeps leaves/stems rigid
Water loss due to transpiration can lead to a loss of pressure in the cells

52. Adaptations of Leaves
Pitcher plant – attract/digest insects to obtain nitrogen
Living stone (rock plant) – 2 leaves for hot/dry conditions are round to minimize exposure to air, have very few stomata
Spruce – waxy epidermis, stomata sunken below leaf surface
Cactus – photosynthesis occurs in stems, leaves are thorns

53. Section 23-5
Transport in Plants

54. Water Transport
Combination of transpiration and capillary action moves water through xylem
Water evaporates through stomata, leaf dries out, water is pulled up through xylem

55. How Cell Walls Pull Water Upward
Water molecules attracted to each other by cohesion – H bonds form between molecules
Water molecules bond to other substances by adhesion

56. How Cell Walls Pull Water Upward
Capillary action is the tendency of water to rise in a thin tube because of cohesion and adhesion
Thinner tube, higher water will rise

57. Putting it All Together
Xylem tissue hollow, connected tubes (tracheids and vessel elements
Tubes are lined with cellulose cell walls (adhesion)
Transpiration removes water from the exposed walls, adhesion pulls water from interior of leaf
Pull is powerful - extends down through tips of roots to the water in the soil

58. Nutrient Transport Pressure-flow hypothesis
Membranes of sieve tube cells use active transport to move sugars from cytoplasm into sieve tube itself
Water follows by osmosis, creating pressure at the source of the sugars
If a plant region has a need for sugars, they are actively pumped out of the tube and into the tissue - water leaves the tube via osmosis, reducing the pressure

59. Nutrient Transport
Flow of nutrient-rich fluid from the sources of sugars (source cells) to the places where sugars are used or stored (sink cells)
Flexibility in changing seasons