1. Essentials of the Living World Plant Form and Function
Second Edition
George B. Johnson
Jonathan B. Losos
Chapter 33
Plant Form and Function
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. 33.1 Organization of a Vascular Plant
A vascular plant is organized along a vertical axis
the part below ground is called the root
the root penetrates the soil and absorbs water and ions
it also anchors the plant
the part above ground is called the shoot
the shoot consists of the stem and leaves
the stem serves as a framework for positioning the leaves
the leaves are where most photosynthesis takes place

3. Figure 33.1 The body of a plant

4. 33.1 Organization of a Vascular Plant
Plants contain growth zones of unspecialized cells called meristems
meristems are not only areas of actively dividing cells that result in plant growth, but also continuously replenish themselves
in this way, meristem cells function much like stem cells in animals

5. 33.1 Organization of a Vascular Plant
Primary growth is initiated at the tips (of roots and shoots) by the apical meristems
the growth of these meristems results primarily in the extension of the plant body
Secondary growth involves the activity of the lateral meristems
the continued divisions of their cells results primarily in the thickening of the plant body

6. 33.1 Organization of a Vascular Plant
There are two kinds of lateral meristems
vascular cambium
gives rise to thick accumulations of secondary xylem and phloem
cork cambium
forms the outer layers of bark on both roots and shoots

7. 33.2 Plant Tissue Types Most plants have three tissue types
ground tissue
in which the vascular tissue is embedded
dermal tissue
the outer protective covering of the plant
vascular tissue
conducts water and dissolved minerals up the plant and conducts the products of photosynthesis throughout

8. 33.2 Plant Tissue Types
There are three kinds of cells in plant ground tissue
parenchyma cells
they are alive at maturity
they carry out the basic functions of living, including photosynthesis, cellular respiration, and food and water storage
collenchyma cells
they are also living at maturity
they provide much of the support for plant organs in which secondary growth has not occurred
sclerenchyma cells
they usually do not contain living cytoplasm when mature
they have tough cell walls called secondary cell walls

9. Ground Tissue Examples
Figure 33.2 Parenchyma cells
Figure 33.3 Collenchyma cells

10. 33.2 Plant Tissue Types There are two types of sclerenchyma
fibers which are long, slender cells that usually form strands
sclereids which are variable in shape but often branched
clusters of sclereids form the gritty texture one feels in the flesh of pears

11. Figure 33.4 Sclerenchyma cells in sclereids

12. 33.2 Plant Tissue Types
All parts of the outer layer of a primary plant body are covered by flattened epidermal cells, which are often covered by a waxy layer called the cuticle
they protect the plant and provide an effective barrier against water loss

13. 33.2 Plant Tissue Types
There are several specialized epidermal cells that make up dermal tissue
guard cells are paired cells that flank an opening called a stoma
the guard cells regulate the passage of oxygen, carbon dioxide, and water vapor across the epidermis
trichomes are outgrowths of the epidermis that occur on the shoot and give it a “fuzzy” appearance
they play an insulating role and affect heat and water balance
root hairs are extensions of the epidermis below ground and keep the root in intimate contact with soil particles
root hairs increase the surface area of the root

14. Figure 33.5 Guard cells and trichomes

15. 33.2 Plant Tissue Types There are two types of vascular tissues
xylem is the plant’s principal water-conducting tissue
it forms a continuous system that runs throughout the plant body
water (and dissolved minerals) pass from the roots to the shoots
when water reaches the leaves, most exits through the stomata
phloem is the principal food-conducting tissue

16. 33.2 Plant Tissue Types
There are two principal conducting cells in the xylem, both of which are dead at maturity
tracheids are elongated cells that overlap at their ends
water flows from tracheid to tracheid through openings called pits
vessel elements are elongated cells that line up end-to-end
the end walls of vessel elements are almost completely open or be perforated to allow for the flow of water
vessels conduct water much more efficiently than tracheids

17. Figure 33.6 Comparison of vessel elements and tracheids

18. 33.2 Plant Tissue Types
Food conduction in phloem is carried out through two kinds of elongated cells
sieve cells have smaller perforations between cells
sieve-tube members have some sieve areas with larger pores than do sieve cells
these areas are called sieve plates
sieve-tube members occur end to end, forming longitudinal series called sieve tubes
specialized parenchyma cells, known as companion cells, occur in association with the sieve tubes

19. Figure 33.7 Sieve tubes

20. 33.3 Roots Roots have a central column of xylem with radiating arms
alternating within the radiating arms of xylem are strands of primary phloem
surrounding the central column, and forming its boundary, is a cylinder of cells called the pericycle
branch, or lateral, roots are formed from cells of the pericycle

21. 33.3 Roots The outer layer of the root is the epidermis
the mass of parenchyma in which the root’s vascular tissue is located is called the cortex
its innermost layer lies just outside the pericycle and is called the endodermis
the cells making up the endodermis are encircled by a thickened, waxy band called the Casparian strip
this strip blocks the movement of water between the endodermal cells and instead forces the movement of water through the plasma membrane of the endodermal cells

22. Figure 33.9 A root cross section

23. 33.3 Roots
The apical meristem of a root is really three primary meristems
protoderm becomes the epidermis
procambium produces primary vascular tissues
ground meristem differentiates into ground tissues, which is comprised of parenchyma tissue
If apical meristem growth is outward, the cell division forms a thimblelike mass of unorganized cells called the root cap
the root cap protects the root’s apical meristem as it grows through the soil

24. 33.3 Roots
The root elongates rapidly just behind its tip in the area known as the zone of elongation
Abundant root hairs, extensions of single epidermal cells, form above the elongation zone
this area is called the zone of differentiation

25. 33.3 Roots
Roots branching is initiated as a result of cell divisions in the pericycle
The developing lateral roots grow out of the cortex toward the surface of the root
Figure Lateral roots

26. 33.4 Stems Stems often experience both primary and secondary growth
stems are the source of an economically important product—wood
In the primary growth of a shoot, leaves first appear as leaf primordia
these are rudimentary leaves that cluster around the apical meristem
they unfold and grow as the stem elongates

27. 33.4 Stems The places of the stem where leaves form are called nodes
the portions of the stem between these leaf attachment points are called internodes
As the leaves expand to maturity, a bud develops in the angle between the leaf and the stem from which it arises
this area is called the axil

28. 33.4 Stems Buds have their own immature leaves called stipules
buds may either elongate or remain dormant
their activity is controlled by a hormone that moves downward from the terminal bud of the shoot
the hormone suppresses expansion of buds in the upper portions of the stem
buds begin forming in the lower down portions of the stem where the amount of hormone is reduced

29. Figure A woody twig

30. 33.4 Stems
Within soft, young stems, the vascular tissue strands are arranged differently in dicots versus monocots
in dicots, vascular bundles (containing primary xylem and primary phloem) are arranged around the outside of the stem
in monocots, vascular bundles are scattered throughout the stem

31. Figure 33.12 A comparison of dicot and monocot stems

32. 33.4 Stems
In stems, secondary growth is initiated by the differentiation of the vascular cambium
this is a thin layer of actively dividing cells located between the bark and the main stem in woody plants, running between the xylem and the phloem
cells that divide from the vascular cambium outwardly become secondary phloem
cells that divide from the vascular cambium inwardly become secondary xylem

33. 33.4 Stems
While the vascular cambium is being established, a second kind of lateral cambium develops in the stem’s outer layer
the cork cambium consists of plates of dividing cells that move deeper and deeper into the stem as they divide
outwardly, this cambium divides to form densely packed cork cells
inwardly, this cambium divides to produce a layer of parenchyma cells

34. 33.4 Stems
The cork, the cork cambium, and the parenchyma cells collectively make up a layer called the periderm
the periderm is the plant’s outside protective covering
The term bark refers to all of the tissues of a mature stem or root outside of the vascular cambium
Wood is accumulated secondary xylem

35. Figure 33.13 Vascular cambium and secondary growth

36. 33.4 Stems
Because of the way it is accumulated, wood often displays rings
the vascular cambium divides more actively in the spring and the summer than in the fall and winter
The growth rate differences are reflected in alternating rings of growth of different thickness

37. Figure 33.14 Annual rings in a section of pine

38. 33.5 Leaves
Leaves are usually the most prominent shoot organ and are structurally diverse
growth occurs by means of marginal meristems
the marginal meristems grow outward and ultimately form the blade (the flattened portion) of the leaf
once a leaf is fully expanded, its marginal meristems cease to grow

39. 33.5 Leaves Additional leaf structures include
a slender stalk called a petiole
two leaflike organs, called stipules, may flank the base of the petiole where it joins the stem
veins, comprised of xylem and phloem, run through the leaf
in most dicots, the veins have a net or reticulate venation
in most monocots, the veins are parallel

40. Figure 33.16 Dicot and monocot leaves

41. 33.5 Leaves Leaf blades come in a variety of forms
simple leaves have a single, undivided blade
compound leaves have a blade divided into leaflets
pinnately compound describes leaflets that are arranged in pairs along a central vein
palmately compound describes leaflets that are radiate out from a common point at the blade end of the petiole

42. Figure 33.17 A leaf in cross section

43. 33.5 Leaves Leaves can be arranged in different patterns
alternate leaves spiral around a shoot
opposite occur on opposite sides of a shoot
whorl circle the stem as a group

44. 33.5 Leaves
A typical leaf contains masses of parenchyma, called mesophyll, through which the vascular bundles, or veins, run
a closely packed, columnlike layer or layers of parenchyma cells are found underneath the upper epidermis of a leaf
this is called the palisade mesophyll
it is packed with chloroplasts
the rest of the leaf interior, except for the veins, consists of spongy mesophyll
it has lots of interior spaces for gas exchange

45. Figure 33.17 A leaf in cross section

46. 33.6 Water Movement
Several factors are at work to move water up the height of a plant
the initial movement of water into the roots of a plant involves osmosis
water moves into the cells of the root because the fluid in the xylem contains more solutes than the surroundings
this osmotic force is called root pressure but, by itself, is not sufficient to “push” water up a plant’s stem

47. 33.6 Water Movement
In addition to root pressure, capillary action adds “pull” to the movement of water up a plant stem
capillary action results from the tiny electrical attractions of polar water molecules to surfaces that carry electrical charge
this attraction is called adhesion
but capillary action, by itself, is not strong enough to “pull” water up the plant stem

48. Figure 33.18 Capillary action

49. 33.6 Water Movement
A final “pull” to the process of moving water up a plant shoot is provided by transpiration
water evaporating from the top (leaf) of the tube pulls the column of water from the bottom (root)
the column of water does not collapse because water molecules are attracted to each other
this process is called cohesion
the narrower the diameter of the tube, the more tensile strength, or resistance to separation, of the water column

50. 33.6 Water Movement
The combination of gravity, tensile strength, and cohesion affects water movement
the whole process is explained by the cohesion-adhesion-tension theory

51. 33.6 Water Movement
Transpiration is the process by which water leaves a plant
more than 90% of the water taken in by a plant is lost to the atmosphere, mostly through the leaves
water first passes into the pockets of air in the spongy mesophyll and then evaporates through the stomata
high humidity and low temperatures increase transpiration rates

52. Figure 33.19 How transpiration works

53. 33.6 Water Movement
The only way that plants can control water loss on a short-term basis is to close their stomata
but plants need to balance closing their stomata with keeping them open for providing access to carbon dioxide
the stomata open and close because of changes in the water pressure of their guard cells

54. 33.6 Water Movement
When the guard cells are plump and swollen with water, they are said to be turgid and the stoma is open
When the guard cells lose water, the stoma closes
Figure How guard cells regulate the opening and closing of stomata

55. 33.6 Water Movement
Root hairs greatly increase the surface area of roots
root hairs are turgid because they contain a higher concentration of dissolved solutes than the soil
minerals also enter the root hairs because they contain a variety of ion transport channels that transport specific ions
this may involve active transport
the minerals are transported by the xylem while dissolved in water

56. Figure Root hairs

57. Figure 33.22 The flow of materials into, out of, and within a plant

58. 33.7 Carbohydrate Transport
Translocation is the process by which most of the carbohydrates manufactured in plants are moved through the phloem
the movement is a passive process
the mass flow of materials transported occurs because of water pressure generated by osmosis
an area where sucrose is made is called a source and an area where sucrose is delivered from the sieve tubes is called a sink
sucrose moves from a source to a sink by a process described by the pressure-flow hypothesis

59. Figure 33.23 How translocation works

60. 33.8 Essential Plant Nutrients
Minerals are involved in plant metabolism in many ways
nitrogen (N) is an essential part of proteins and nucleic acids
potassium (K) ions are used to regulate turgor pressure in guard cells
calcium (Ca) is an essential part of cell walls
magnesium (Mg) is a part of the chlorophyll molecule
phosphorous (P) is a part of ATP and nucleic acids
sulfur (S) is a key component of the amino acid, cysteine

61. 33.8 Essential Plant Nutrients
Other essential minerals for plant health include chlorine (Cl), iron (Fe), boron (B), manganese (Mn), zinc (Zn), copper (Cu), and molybdenum (Mb)
Most plants acquire minerals from the soil, although some carnivorous plants are able to use other organisms directly as sources of nitrogen, just as animals do

62. Figure 33.24 A carnivorous plant

63. Inquiry & Analysis
In the 23-cm section, is more 42K found in xylem or phloem?
Above and below the 23-cm section, is more 42K found in xylem or phloem?
Graph of Movement of 42K Through a Stem