1. SEED VASCULAR PLANTS Division Cycadophyta: palms (naked seeds)
Division Ginkgophyta: ginkgo (naked seeds)
Division Gnetophyta: only few species (naked seeds)
Division Coniferophyta: cone plants: pine, spruce, fir, larch, yew (naked seeds)
Division Anthophyta: flowering plants (seeds in fruit)

2. DIVISION ANTHOPHYTA Flowering plants Sex organs in flowers
Seeds in ovary that ripens into fruit
Two classes:
Monocotyledoneae (Monocot)
Dicotyledoneae (Dicot)

4. Roots, Stems, and Leaves
Cacti leaves are modified into thin, sharp spines
The reduced-leaf surface area prevents excess water loss

5. Roots, Stems, and Leaves

6. Specialized Tissues in Plants
If you look deep inside a living plant, that first impression of inactivity vanishes
Instead, you will find a busy and complex organism packed with specialized systems and subsystems
Materials move throughout the plant, and growth and repair take place continuously
Plants may act at a pace that seems slow to us, but their cells work together in remarkably effective ways to ensure the plant's survival

7. Seed Plant Structure
The cells of a seed plant are organized into different tissues and organs, as shown in the figure
Three of the principal organs of seed plants are roots, stems, and leaves
These organs are linked together by systems and subsystems that run the length of the plant, performing functions such as transport and protection and coordinating plant activities

8. Tissues in a Vascular Plant
Vascular plants consist of roots, stems, and leaves
Each of these organs contains dermal tissue, vascular tissue, and ground tissue, as shown by the cross sections of the leaf, stem, and root

9. Tissues in a Vascular Plant

10. Roots The root system of a plant absorbs water and dissolved nutrients
Roots anchor plants in the ground, holding soil in place and preventing erosion
Root systems also protect the plant from harmful soil bacteria and fungi, transport water and nutrients to the rest of the plant, and hold plants upright against forces such as wind and rain

11. Stems
A stem has a support system for the plant body, a transport system that carries nutrients, and a defense system that protects the plant against predators and disease
Stems can be as short as a few millimeters or as tall as 100 meters
Whatever its size, the support system of a stem must be strong enough to hold up its leaves and branches
Similarly, the stem's transport system must contain subsystems that can lift water from roots up to the leaves and carry the products of photosynthesis from the leaves back down to the roots

12. Leaves Leaves are the plant's main photosynthetic systems
The broad, flat surfaces of many leaves help increase the amount of sunlight plants absorb
Leaves also expose a great deal of tissue to the dryness of the air and, therefore, must contain subsystems to protect against water loss
Adjustable pores in leaves help conserve water while letting oxygen and carbon dioxide enter and exit the leaf

13. Plant Tissue Systems
Within the roots, stems, and leaves of plants are specialized tissue systems
Plants consist of three main tissue systems:
Dermal tissue: is the skin of a plant in that it is the outmost layer of cells
Vascular tissue: is the plant's bloodstream, transporting water and nutrients throughout the plant
Ground tissue: and ground tissue is everything else

14. Dermal Tissue
The outer covering of a plant consists of dermal tissue, which typically consists of a single layer of epidermal cells
The outer surfaces of these are often covered with a thick waxy layer that protects against water loss and injury
The thick waxy coating of the epidermal cells is known as the cuticle
Some epidermal cells have tiny projections known as trichomes, which help protect the leaf and also give it a fuzzy appearance
In roots, dermal tissue includes root hair cells that provide a large amount of surface area and aid in water absorption
On the underside of leaves, dermal tissue contains guard cells, which regulate water loss and gas exchange

15. Vascular Tissue
Vascular tissue forms a transport system that moves water and nutrients throughout the plant
The principal subsystems in vascular tissue are xylem, a water-conducting tissue, and phloem, a food-conducting tissue
Vascular tissue contains several types of specialized cells:
Xylem consists of tracheids and vessel elements
Phloem consists of sieve tube elements and companion cells
As you can see in the figure, both xylem and phloem are made up of networks of hollow connected cells that carry fluids throughout the plant

16. Vascular Tissue in a Stem
Vascular tissue is made up of several different types of cells
Xylem consists of tracheids and vessel elements
Phloem consists of sieve tube elements and companion cells
Xylem tissue (left) conducts water from the roots to the rest of the plant
Phloem tissue (right) conducts a variety of materials, mostly carbohydrates, throughout a plant

17. Vascular Tissue in a Stem

18. Xylem All seed plants have a type of xylem cell called a tracheid
Recall that tracheids are long, narrow cells with walls that are impermeable to water
These walls, however, are pierced by openings that connect neighboring cells to one another
When tracheids mature, they die, and their cytoplasm disintegrates

19. Xylem
Angiosperms have another kind of xylem cell that is called a vessel element
Vessel elements are much wider than tracheids
Like tracheids, they mature and die before they conduct water
Vessel elements are arranged end to end on top of one another like a stack of tin cans
The cell walls at both ends are lost when the cells die, transforming the stack of vessel elements into a continuous tube through which water can move freely

20. Phloem The main phloem cells are sieve tube elements
These cells are arranged end to end, like vessel elements, to form sieve tubes
The end walls of sieve tube elements have many small holes in them
Materials can move through these holes from one adjacent cell to another
As sieve tube elements mature, they lose their nuclei and most of the other organelles in their cytoplasm
The remaining organelles hug the inside of the cell wall
The rest of the space is a pipeline through which sugars and other foods are carried in a watery stream

21. Phloem
Companion cells are phloem cells that surround sieve tube elements
Companion cells keep their nuclei and other organelles through their lifetime
Companion cells support the phloem cells and aid in the movement of substances in and out of the phloem stream

22. Ground Tissue
The cells that lie between dermal and vascular tissues make up the ground tissues, shown in figure
In most plants, ground tissue consists mainly of parenchyma
Parenchyma cells have thin cell walls and large central vacuoles surround by a thin layer of cytoplasm
In leaves, these cells are packed with chloroplasts and are the site of most of a plant's photosynthesis
Ground tissue may also contain two types of cells with thicker cell walls:
Collenchyma cells have strong, flexible cell walls that help support larger plants
Collenchyma cells make up the familiar “strings” of a stalk of celery
Sclerenchyma cells have extremely thick, rigid cell walls that make ground tissue tough and strong

23. Ground Tissues 
Ground tissue is made of cells whose cell walls have different thicknesses
Parenchyma cells have thin walls and function mainly in storage and photosynthesis
The root cells shown are filled with purple-staining starch grains
Collenchyma and sclerenchyma cells both function in support
Collenchyma cells have irregularly shaped walls, but the walls of sclerenchyma cells are much thicker and harder

24. Ground Tissues 

25. Plant Growth and Meristematic Tissue
Most plants have a method of development that involves an open, or indeterminate, type of growth
Indeterminate growth means that they grow and produce new cells at the tips of their roots and stems for as long as they live
These cells are produced in meristems, clusters of tissue that are responsible for continuing growth throughout a plant's lifetime
The new cells produced in meristematic tissue are undifferentiated—that is, they have not yet become specialized for specific functions, such as transport

26. Plant Growth and Meristematic Tissue
Near the end, or tip, of each growing stem and root is an apical meristem
An apical meristem is a group of undifferentiated cells that divide to produce increased length of stems and roots
The figure at right shows examples of root and shoot apical meristems
Meristematic tissue is the only plant tissue that produces new cells by mitosis

27. Plant Growth and Meristematic Tissue
Meristematic tissue produces new cells by mitosis
Apical meristems, which consist of many actively dividing cells, are located at the tips of shoots (left) and roots (right)
The apical meristem of a root is surrounded by a root cap that protects the root as it grows through the soil

28. Plant Growth and Meristematic Tissue

29. Plant Growth and Meristematic Tissue
At first, the cells that originate in meristems look very much alike: They divide rapidly and have thin cell walls
Gradually, these cells develop into mature cells with specialized structures and functions, a process called differentiation
As these cells differentiate, they produce each of the tissue systems of the plant, including dermal, ground, and vascular tissue

30. Plant Growth and Meristematic Tissue
The highly specialized cells found in flowers, which make up the reproductive systems of flowering plants, are also produced in meristems
Flower development begins when certain genes are turned on in a shoot apical meristem
The actions of these genes transform the apical meristem into a floral meristem, producing the modified leaves that become the flower's colorful petals, as well as the reproductive tissues of the flower
Many plants also grow in width as a result of meristematic tissue that lines the stems and roots of a plant

31. Roots
As soon as a seed begins to grow, it puts out its first root to draw water and nutrients from the soil
Other roots soon branch out from this first root, adding length and surface area to the root system
The overall size of a plant's root system can be astonishing: The total surface area of the root system of a rye plant was measured at more than 600 square meters—130 times greater than the combined surface areas of both the stems and leaves

32. ROOT Absorbs water and dissolved mineral for the plant
Anchors the plant in the soil
Types:
Fibrous:
Greatly branched
Tap:
Single large root

33. Types of Roots The two main types of roots are:
Taproots, which are found mainly in dicots
Fibrous roots, which are found mainly in monocots
In some plants, the primary root grows long and thick while the secondary roots remain small
This type of primary root is called a taproot, shown in figure
Taproots of oak and hickory trees grow so long that they can reach water far below Earth's surface
Carrots, dandelions, beets, and radishes have short, thick taproots that store sugars or starches

35. Types of Roots Plants have taproots, fibrous roots, or both
Taproots have a central primary root and generally grow deep into the soil
Fibrous roots are usually shallow and consist of many thin roots.

36. Types of Roots

37. Types of Roots
In other plants, such as grasses, fibrous roots branch to such an extent that no single root grows larger than the rest
The extensive fibrous root systems produced by many plants help prevent topsoil from being washed away by heavy rain

39. Root Structure and Growth
Roots contain cells from the three tissue systems—dermal, vascular, and ground tissue
A mature root has an outside layer, the epidermis, and a central cylinder of vascular tissue
Between these two tissues lies a large area of ground tissue
The root system plays a key role in water and mineral transport
Its cells and tissues, as shown in the figure, contain a number of subsystems that carry out these functions
The root's epidermal subsystem performs the dual functions of protection and absorption
Its surface is covered with tiny cellular projections called root hairs
These hairs penetrate the spaces between soil particles and produce a large surface area through which water can enter the plant
Just inside the epidermis is a spongy layer of ground tissue called the cortex
This layer extends to another layer of cells, the endodermis
The endodermis completely encloses the root's vascular subsystem in a region called the vascular cylinder

40. Root Structure and Growth
A root consists of a central vascular cylinder surrounded by ground tissue and the epidermis
Compare how cells in different regions of the root are structurally specialized for different functions
Root hairs along the surface of the root aid in water absorption
Only the cells in the root tip divide
In the area just behind the root tip, the newly divided cells increase in length, pushing the root tip farther into the soil
The root cap, located just ahead of the root tip, protects the dividing cells as they are pushed forward
Dicot roots, such as the one shown in the cross section, have a central column of xylem cells arranged in a radiating pattern
How are root hairs structurally specialized?

41. Root Structure and Growth

42. Root Structure and Growth
Roots grow in length as their apical meristem produces new cells near the root tip
These fragile new cells are covered by a tough root cap that protects the root as it forces its way through the soil
As the root grows, the root cap secretes a slippery substance that lubricates the progress of the root through the soil
Cells at the very tip of the root cap are constantly being scraped away, and new root cap cells are continually added by the meristem
Most of the increase in root length occurs immediately behind the meristem, where cells are growing longer
At a later stage, these cells mature and take on specialized functions
The process by which unspecialized cells change to become specialized in structure and function is known as cell differentiation

43. ROOT GROWTH Meristem: Located just behind the root cap
Zone of rapidly dividing cells (become cells in the zone of elongation or root cap)
Root cap:
Tip of root
Protects meristem
As root grows through the soil, cells are rubbed off and replaced by meristematic cells
Zone of elongation:
Cells cease to divide but enlarge
Enlargement of cells pushes the root deeper into the soil
Zone of maturation:
Cells differentiate

45. Root Functions
Roots anchor a plant in the ground and absorb water and dissolved nutrients from the soil
How does a root go about the job of absorbing water and minerals from the soil?
Although it might seem to, water does not just “soak” into the root from soil
It takes energy on the part of the plant to absorb water
Our explanation of this process begins with a description of soil and plant nutrients

46. ROOT FUNCTION Anchors the plant in the soil
Absorption of water and dissolved mineral
Root hair
Fingerlike extension of a single epidermal cell
Greatly increase surface area of the root enabling the root to absorb water and dissolved mineral from the soil
Contains many dissolved substances such as minerals, sugars, and amino acids
Soil contains fewer dissolved substances
Concentration gradient develops between the soil and root hair
Water moves into the root hair by osmosis
Absorbs macronutrients (Nitrogen/Potassium) in large amount and micronutrients (Manganese) in small amounts by active transport
Food storage

47. Uptake of Plant Nutrients
An understanding of soil helps explain how plants function
Soil is a complex mixture of sand, silt, clay, air, and bits of decaying animal and plant tissue
Soil in different places and at different depths contains varying amounts of these ingredients
Sandy soil, for example, is made of large particles that retain few nutrients, whereas the finely textured silt and clay soils of the Midwest and southeastern United States are high in nutrients
The ingredients define the soil and determine, to a large extent, the kinds of plants that can grow in it

48. Uptake of Plant Nutrients
To grow, flower, and produce seeds, plants require a variety of inorganic nutrients in addition to carbon dioxide and water
The most important of these nutrients are nitrogen, phosphorus, potassium, magnesium, and calcium
The functions of these essential nutrients within a plant are described in the table
These nutrients are located in varying amounts in the soil and are drawn up by the roots of a plant In addition to these essential nutrients, trace elements are required in small quantities to maintain proper plant growth
Trace elements include sulfur, iron, zinc, molybdenum, boron, copper, manganese, and chlorine
Large amounts of trace elements in the soil can be poisonous

49. Essential Plant Nutrients
Soil contains several nutrients that are essential for plant growth
Each nutrient plays a different role in plant functioning and development, and produces distinct effects when deficient in the soil
If you notice that a plant is becoming paler and more yellow, what nutrient might need to be added?

50. Essential Plant Nutrients

51. Active Transport of Minerals
The cell membranes of root hairs and other cells in the root epidermis contain active transport proteins
These proteins use ATP (an energy source) to pump mineral ions from the soil into the plant
The high concentration of mineral ions in the plant cells causes water molecules to move into the plant by osmosis, as shown in the figure at right

52. Active Transport and Osmosis in a Root
Roots absorb water and dissolved nutrients from the soil
Most water and minerals enter a plant through the tiny root hairs
Water moves into the cortex, through the cells of the endodermis, and into the vascular cylinder
Finally, water reaches the xylem, where it is transported throughout the plant
Cells in the endodermis are made waterproof by the Casparian strip
The Casparian strip is another example of how cells are specialized to perform a particular function—in this case, preventing the backflow of water out of the vascular cylinder into the root cortex

53. Active Transport and Osmosis in a Root

54. Active Transport of Minerals
You may recall that osmosis is the movement of water across a membrane toward an area where the concentration of dissolved material is higher
By using active transport to accumulate ions from the soil, cells of the root epidermis create conditions under which osmosis causes water to “follow” those ions and flow into the root
Note that the root does not actually pump water
But by pumping dissolved minerals into its own cells, the end result is almost the same—the water moves from the epidermis through the cortex into the vascular cylinder

55. Movement Into the Vascular Cylinder
Both osmosis and active transport cause water and minerals to move from the root epidermis into the cortex
From there, the water and dissolved minerals pass the inner boundary of the cortex and enter the endodermis
This process is shown in the figure Active Transport and Osmosis in a Root

56. Movement Into the Vascular Cylinder
The endodermis encloses the vascular cylinder and stretches up and down the entire length of the root, like a cylinder
It is composed of many individual cells, each shaped a bit like a brick
Each of these cells is surrounded on four sides by a waterproof strip called a Casparian strip
To imagine what the Casparian strip looks like, think of a brick with a thick rubber band stretched around it
The rubber bands stick together like mortar between the bricks
Imagine many of these bricks placed edge to edge to build a cylinder
When a root is viewed in cross section, the endodermis forms a circle

57. Movement Into the Vascular Cylinder
Recall that water moves into the vascular cylinder by osmosis
Because water and minerals cannot pass through the waxy Casparian strip, once they pass through the endodermis, they are trapped in the vascular cylinder
As a result, there is a one-way passage of materials into the vascular cylinder in plant roots

58. Root Pressure 
Why do plants “need” a system that ensures the one-way movement of water and minerals?
That system is how the plant generates enough pressure to move water out of the soil and up into the body of the plant
As minerals are pumped into the vascular cylinder, more and more water follows by osmosis, producing a strong pressure
If the pressure were not contained, roots would expand as they filled with water

59. Root Pressure 
Instead, contained within the Casparian strip, the water has just one place to go—up
Root pressure, produced within the cylinder by active transport, forces water through the vascular cylinder and into the xylem
As more water moves from the cortex into the vascular cylinder, more water in the xylem is forced upward through the root into the stem
In the figure, you can see a demonstration of root pressure in a carrot root
Root pressure is the starting point for the movement of water through the vascular system of the entire plant
But it is just the beginning
Once you have learned about stems and leaves, you will see how water and other materials are transported within an entire plant

61. Demonstration of Root Pressure
As a carrot root absorbs water, root pressure forces water upward into the glass tube, which takes the place of the stem and leaves of the carrot in this demonstration

62. Demonstration of Root Pressure

63. Stems
What do a barrel cactus, a tree trunk, a dandelion stem, and a potato have in common?
They are all types of stems
Stems vary in size, shape, and method of development
Some grow entirely underground; others reach high into the air
Stems also vary in structure and internal arrangement of cells

64. Stem Structure and Function
In general, stems have three important functions:
They produce leaves, branches, and flowers
They hold leaves up to the sunlight
They transport substances between roots and leaves
Stems make up an essential part of the water and mineral transport systems of the plant
The vascular tissue in stems conducts water, nutrients, and other compounds throughout the plant
Xylem and phloem, the major subsystems of the transport system, form continuous tubes from the roots through the stems to the leaves
These vascular tissues link all parts of the plant, allowing water and nutrients to be carried throughout the plant
In many plants, stems also function as storage systems and in the process of photosynthesis

65. Stem Structure and Function
Like the rest of the plant, the stem is composed of three tissue systems: dermal, vascular, and ground tissue
Stems are surrounded by a layer of epidermal cells that have thick cell walls and a waxy protective coating

66. STEM FUNCTION
Three functions:
Support
Transport
Storage

67. Stem Structure and Function
In most plants, stems contain distinct nodes, where leaves are attached, and internode regions between the nodes, as shown in figure
Small buds are found where leaves attach to the nodes
Buds contain undeveloped tissue that can produce new stems and leaves
In larger plants, stems develop woody tissue that helps support leaves and flowers

68. Buds, Nodes, and Internodes
Stems produce leaves and branches and hold leaves up to the sunlight, where they carry out photosynthesis
Leaves are attached to a stem at structures called nodes
These nodes are separated by regions of the stem called internodes

69. Buds, Nodes, and Internodes

70. Stems Adapted for Storage and Dormancy
For example, many kinds of plants have modified stems that store food
Tubers, rhizomes, bulbs, and corms can remain dormant during cold or dry periods until favorable conditions for growth return
Examples of these are shown in the figure at right

71. Stems Adapted for Storage and Dormancy
Many kinds of plants have modified stems that store food
Tubers, rhizomes, bulbs, and corms can remain dormant during cold or dry periods until favorable conditions for growth return

72. Stems Adapted for Storage and Dormancy

73. STEM TYPES Different types: Upright: herbaceous (green) / woody
Stolon: grow along ground surface (strawberry)
Tuber: food storage (potato)
Fleshy: water storage and photosynthesis (cactus)
Corms: food storage (gladiolus/crocus/some begonias)
Bulb: food storage (tulip/onion/daffodil)
Rhizome: grow horizontally underground (iris/water hyacinth/some types of blueberries)

75. Monocot and Dicot Stems
The arrangement of tissues in a stem differs among seed plants
In monocots, vascular bundles are scattered throughout the stem
In dicots and most gymnosperms, vascular bundles are arranged in a cylinder
Recall that monocots and dicots are two types of flowering plants, or angiosperms
For a comparison of monocot and dicot stems, look at the figure

76. Monocot and Dicot Stems
The arrangement of vascular bundles in the stem of a monocot differs from that in the stem of a dicot
In a monocot, vascular bundles are scattered throughout the stem
In a dicot, vascular bundles are arranged in a ring

77. Monocot and Dicot Stems

79. Monocot Stems 
The cross section of a young monocot stem shows all three tissue systems clearly
The stem has a distinct epidermis, which encloses a series of vascular bundles, each of which contains xylem and phloem tissue
Phloem faces the outside of the stem, and xylem faces the center
In monocots, these bundles are scattered throughout the ground tissue
The ground tissue is fairly uniform, consisting mainly of parenchyma cells

80. Dicot Stems 
Young dicot stems have vascular bundles, but they are generally arranged in an organized, ringlike pattern
The parenchyma cells inside the ring of vascular tissue are known as pith, while those outside form the cortex of the stem
In dicots, these relatively simple tissue patterns become more complex as the plant grows larger and the stem increases in diameter

81. STEM STRUCTURE Terminal bud:
Tip of a twig (develops into leaves or flowers)
Contains meristem tissue which adds to the length of the stem
Lateral bud:
Above leaf scars (scars left by vascular bundle of dead leaves)
Adds lateral branches
Lenticels: raised areas (blisters) that allow the exchange of gases between the atmosphere and the inner tissues of the stem
Vascular bundles:
Monocot: scattered
Dicot: arranged in a ring

83. Primary Growth of Stems
Plants grow in ways that are distinctly different from other organisms
For their entire life, new cells are produced at the tips of roots and shoots
This method of growth, occurring only at the ends of a plant, is called primary growth
The increase in length produced by primary growth from year to year is shown in the figure
Primary growth of stems is produced by cell divisions in the apical meristem
It takes place in all seed plants

84. Primary Growth in a Stem
All seed plants undergo primary growth, which is an increase in length
Every year, apical meristems, shown in red, divide to produce new growth
The primary growth for one season consists of a stem and several leaves

85. Primary Growth in a Stem

87. STEM GROWTH Vertical growth (height)only at the tip
Vertical/horizontal growth at buds
Horizontal growth (circumference) at the lateral meristem
Vascular Cambium:
Between the xylem and phloem
Zone of cell division adding cells to the phloem (outside) and xylem (inside)
Spring: cells wide and thin
Summer: cells small and thick
Density difference creates annual rings

88. Secondary Growth of Stems
If a plant is to grow larger year after year, its stems must increase in thickness as well as in length
They have more mass to support and more fluid to move through their vascular tissues
Yet, only meristematic tissue can produce new cells for growth
Some monocots, such as palm trees, produce thick stems from a meristem that becomes wider as the plant grows
However, most monocots, such as grasses, produce only fleshy growth and do not grow very tall
Many dicots grow extremely tall and also grow in width to support this extra weight
This growth occurs as a result of meristems other than the apical meristem

89. Secondary Growth of Stems
The method of growth in which stems increase in width is called secondary growth
In the figure at right, you can see the pattern of secondary growth in a dicot stem
In conifers and dicots, secondary growth takes place in lateral meristematic tissues called the vascular cambium and cork cambium
The type of lateral meristematic tissue called vascular cambium produces vascular tissues and increases the thickness of stems over time
Cork cambium produces the outer covering of stems
Another kind of cambium enables roots to grow thicker and branch
The addition of new tissue in these cambium layers increases the thickness of the stem

90. Secondary Growth in a Dicot Stem
Dicots produce secondary growth from meristematic tissue called vascular cambium
This tissue forms between the xylem and phloem of the individual vascular bundles, as shown in A
Once the tissue forms, as shown in B, it divides to produce xylem cells toward the center of the stem and phloem cells toward the outside
These different tissues form the bark and wood of a mature stem, shown in C

91. Secondary Growth in a Dicot Stem

92. Formation of the Vascular Cambium
In a young dicot stem produced by primary growth, bundles of xylem and phloem are arranged in a ring
Once secondary growth begins, the vascular cambium appears as a thin layer situated between clusters of vascular tissue
This new meristematic tissue forms between the xylem and phloem of each vascular bundle
Divisions in the vascular cambium give rise to new layers of xylem and phloem
As a result, the stem becomes wider
The cambium continues to produce new layers of vascular tissue, causing the stem to become thicker and thicker

93. Formation of Wood 
Most of what we call “wood” is actually layers of xylem
These cells build up year after year, layer on layer
As woody stems grow thicker, the older xylem near the center of the stem no longer conducts water and instead becomes what is known as heartwood
Heartwood usually darkens with age because it accumulates impurities that cannot be removed
Heartwood is surrounded by sapwood, which is active in fluid transport and therefore usually lighter in color
Both heartwood and sapwood are shown in the figure at left

94. Layers in a Mature Tree
In a mature tree that has undergone several years of secondary growth, the vascular cambium lies between layers of xylem to the inside, and layers of phloem to the outside
The youngest xylem, called sapwood, transports water and minerals
Which layer contains meristematic cells?

95. Layers in a Mature Tree

96. Formation of Wood
In most of the temperate zone, tree growth is seasonal
When growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, light-colored xylem cells with thin cell walls
The result is a light-colored layer of wood called early wood
As the growing season continues, the cells become smaller and have thicker cell walls, forming a layer of dark wood
This darker wood is called late wood

98. Formation of Wood
This alternation of dark and light wood produces what we commonly call tree rings
Each ring is composed of a band of light wood and a band of dark wood
Thus, a ring corresponds to a year of growth
By counting the rings in a cross section of a tree, you can estimate its age
The size of the rings may even provide information about weather conditions, such as wet or dry years
Thick rings indicate that weather conditions were favorable for tree growth, whereas thin rings indicate less favorable conditions

99. Formation of Bark 
On most trees, bark includes all of the tissues outside the vascular cambium, as shown in the figure at right
These tissues include phloem, the cork cambium, and cork
How does bark form?
Picture a tree as new xylem is being laid down
It is expanding in width, or girth
Recall that the phloem tissue lies to the outside of this xylem
Phloem must grow to accommodate the larger size of the tree
As the vascular cambium increases in diameter, it forces the phloem tissue outward
This expansion causes the oldest tissues to split and fragment as they are stretched by the expanding stem
Were this expansion left unchecked, the outer covering of the stem might eventually split and break

100. Formation of Bark 
Another layer of growing tissue, the cork cambium, solves this potential problem
The cork cambium surrounds the cortex and produces a thick protective layer of cork
Cork consists of cells that have thick walls and usually contain fats, oils, or waxes
These waterproof substances help prevent the loss of water from the stem
The outermost cork cells are usually dead
As the stem increases in size, this dead bark often cracks and flakes off in strips or patches

102. Leaves The leaves of a plant are its main organs of photosynthesis
In a sense, plant leaves are the world's most important manufacturers of food
Sugars, starches, and oils manufactured by plants in their leaves are sources of food for virtually all land animals

103. Leaves
Recall from Chapter 8 that photosynthesis uses carbon dioxide and water to produce sugars and oxygen
Leaves, therefore, must have a way of obtaining the materials needed for photosynthesis as well as distributing its end products
Much of the internal structure of leaves can be understood in terms of their functions in carrying out photosynthesis

104. LEAF Main organ of photosynthesis
Uses carbon dioxide and water in the presence of sunlight, enzymes, and chlorophyll to produce sugar and oxygen

105. Leaf Structure
The structure of a leaf is optimized for absorbing light and carrying out photosynthesis
As you can see in the figure below, leaves may differ greatly in shape, yet share certain structural features
To collect sunlight, most leaves have thin, flattened sections called blades
The blade is attached to the stem by a thin stalk called a petiole
Like roots and stems, leaves have an outer covering of dermal tissue and inner regions of ground and vascular tissues
As shown in the figure at right, leaves are covered on the top and bottom by epidermis made of a layer of tough, irregularly shaped cells
The epidermis of many leaves is also covered by the cuticle
Together, the cuticle and epidermal cells form a waterproof barrier that protects tissues and limits the loss of water through evaporation

106. Tissues in a Leaf
Leaves absorb light and carry out most of the photosynthesis in a plant
Some of the most important manufacturing sites on Earth are found in the leaves of plants
The cells in plant leaves are able to use light energy to make carbohydrates
Compare the structure of the different kinds of cells in a leaf

107. Tissues in a Leaf

108. LEAF PARTS Node: region of the stem where the leaf is attached Leaf:
Two parts:
Blade: flattened portion
Petiole: stemlike portion that connects the leaf to the stem
Bud: at the base of each petiole
Veins: vascular bundles (xylem/phloem)
Monocot: parallel
Dicot: net

110. Leaf Shapes 
Most of a leaf consists of a blade attached to the stem by a petiole. The blade of a simple leaf (left) can be different shapes
In a compound leaf (right), the blade is divided into many separate leaflets

111. Leaf Shapes 

112. Leaf Shapes
The vascular tissues of leaves are connected directly to the vascular tissues of stems, making them part of the plant's transport system
In leaves, xylem and phloem tissues are gathered together into bundles that run from the stem into the petiole
Once they are in the leaf blade, the vascular bundles are surrounded by parenchyma and sclerenchyma cells

113. LEAF SHAPE
Toothed
Smooth
Lobed

115. LEAF ARRANGEMENT
Opposite
Whorled
Alternate

117. SIMPLE/COMPOUND LEAF Simple Pinnately compound Palmately compound
Bipinnately compound

119. Leaf Functions
A leaf can be considered a system specialized for photosynthesis
Subsystems of the leaf include tissues that bring gases, water, and nutrients to the cells that carry out photosynthesis

120. LEAF CROSS SECTION Epidermis:
Protects the leaf from injury and desiccation
Most cells do not contain contain chloroplast
Waxy covering (cuticle) prevents water evaporation
Amount varies from species to species
Some contain epidermal hairs (extension of the cell) which slow the rate of water evaporation
Upper: thicker cuticle layer
Lower: contain guard cells and stoma

122. Photosynthesis
The bulk of most leaves consists of a specialized ground tissue known as mesophyll, shown in the figure at right
Photosynthesis in most plants occurs in the mesophyll
The carbohydrates produced move into phloem vessels of the transport system, which carry them to the rest of the plant

123. Tissues in a Leaf

124. Photosynthesis
A leaf has specialized cells that enable it to carry out photosynthesis
Just under the epidermis is a layer of mesophyll cells called the palisade mesophyll
These closely packed cells absorb light that enters the leaf
Beneath the palisade layer is the spongy mesophyll, a loose tissue with many air spaces between its cells
These air spaces connect with the exterior through stomata (singular: stoma), porelike openings in the underside of the leaf that allow carbon dioxide and oxygen to diffuse into and out of the leaf
Each stoma consists of two guard cells, the specialized cells in the epidermis that control the opening and closing of stomata by responding to changes in water pressure

125. LEAF CROSS SECTION Lower epidermis:
Pair of guard cells ( containing chloroplast) surrounds a pore (stoma) that opens and closes
Size of opening depends on the shape of the guard cells (when open gases move in and out)
Guard Cell:
Cell wall next to stoma is thick and inflexible
Cell wall next to epidermis is thin and elastic
During photosynthesis (sunny day) the guard cells become swollen with water (turgid) pushing the thinner wall into the neighboring epidermal cell and pulling the thicker walls away from each other thus opening the stoma
Carbon dioxide enters
Oxygen and water vapor exit
When it is dark (no photosynthesis), the guard cells are less turgid (loss water) and the stoma closes
No oxygen nor carbon dioxide exchange
Water is conserved

126. LEAF CROSS SECTION Palisade Mesophyll: Spongy Mesophyll:
Cells upright and parallel
Maximum exposure to sunlight
Main region of photosynthesis
Spongy Mesophyll:
Loosely packed
Air pockets
Gases exchange

128. Transpiration
The surfaces of spongy mesophyll cells are kept moist so that gases can enter and leave the cells easily
This also means that water evaporates from these surfaces and is lost to the atmosphere
Transpiration is the loss of water through its leaves
This lost water is replaced by water drawn into the leaf through xylem vessels in the vascular tissue

129. Gas Exchange 
Leaves take in carbon dioxide and give off oxygen during photosynthesis
When plant cells use the food they make, the cells respire, taking in oxygen and giving off carbon dioxide (just as animals do)
Plant leaves allow gas exchange between air spaces in the spongy mesophyll and the exterior by opening their stomata

131. Gas Exchange 
It might seem that stomata should be open all the time, allowing gas exchange to take place and photosynthesis to occur at top speed
This is not what happens!
If stomata were kept open all the time, water loss due to transpiration would be so great that few plants would be able to take in enough water to survive
So, plants maintain a kind of balance
Plants keep their stomata open just enough to allow photosynthesis to take place but not so much that they lose an excessive amount of water

132. Opening and Closing of Stomata
Plants regulate the opening and closing of their stomata to balance water loss with rates of photosynthesis
A stoma opens or closes in response to the changes in pressure within the guard cells that surround the opening
When the guard cells are swollen with water (left), the stoma is open
When the guard cells lose water (right), the opening closes, limiting further water loss from the leaf

133. Opening and Closing of Stomata

134. Gas Exchange 
Guard cells are epidermal cells found on the undersides of leaves
They are structurally specialized to control stomata and thus regulate the movement of gases, especially water vapor, into and out of leaf tissues
The stomata open and close in response to changes in water pressure within the guard cells, as shown in the figure at right
When water pressure within the guard cells is high, the thin outer walls of the cells are forced into a curved shape
This pulls the thick inner walls of the guard cells away from one another, opening the stoma
When water pressure within the guard cells decreases, the inner walls pull together and the stoma closes
Guard cells respond to conditions in the environment, such as wind and temperature, helping to maintain homeostasis within a leaf
Notice how the structure of guard cells, which is quite different from the structure of other epidermal cells, helps them to carry out this task

136. Gas Exchange 
In general, stomata are open during the daytime, when photosynthesis is active, and closed at night, when open stomata would only lead to water loss
However, stomata may be closed even in bright sunlight under hot, dry conditions in which water conservation is a matter of life and death

137. CHEMICAL ACTIVITY PHOTOSYNTHESIS RESPIRATION Only during sunlight
24 hours a day

138. Transport in Plants
The pressure created by water entering the tissues of a root can push water upward in a plant stem
This creates more than enough pressure to force water into the vascular system and out of the root
However, root pressure does not exert enough pressure to lift water up into trees, such as the topmost needles of a redwood tree 90 meters above the ground
To draw water to such great heights, plants take advantage of some of water's most interesting physical properties

139. Water Transport
Recall that xylem tissue forms a continuous set of tubes that stretch from roots through stems and out into the spongy mesophyll of leaves
This set of tubes forms a complex transport system within a plant
The transport is carried out by a subsystem of cells and tissues
Active transport and root pressure cause water to move from soil into plant roots
Root pressure alone, however, cannot account for the movement of water and dissolved materials throughout an entire plant
Obviously, other forces are at work
These include capillary action and transpiration
The combination of root pressure, capillary action, and transpiration provides enough force to move water through the xylem tissue of even the tallest plant
As you will learn, transpiration is the most powerful of these forces

140. STEM TRANSPORT Transpiration (xylem: dead cells at maturity)
Transpiration-cohesion theory (xylem: dead cells at maturity)
Translocation: Pressure-flow hypothesis (phloem: living cells)

141. Capillary Action 
Water molecules are attracted to one another by a force called cohesion
Recall from Chapter 2 that cohesion is the attraction of molecules of the same substance to each other
Because of cohesion, water molecules have a tendency to form hydrogen bonds with each other
Water molecules can also form hydrogen bonds with other substances
This results from a force called adhesion, which is attraction between unlike molecules
Place empty glass tubes of various widths into a dish of water, as shown below, and you will see both forces at work
The tendency of water to rise in a thin tube is called capillary action
Water is attracted to the walls of the tube, and water molecules are attracted to one another
The thinner the tube, the higher the water will rise inside it

142. Capillary Action 
Capillary action—the result of water molecules' ability to stick to one another and to the walls of a tube—contributes to the movement of water up the cells of xylem tissue
As shown here, capillary action causes water to move much higher in a narrow tube than in a wide tube

143. Capillary Action 

144. Capillary Action
What does capillary action have to do with water movement through xylem?
Recall that there are two main types of xylem tissue in flowering plants: tracheids and vessel element
Both tracheids and vessel elements form hollow connected tubes similar to a thin, glass capillary tube
Capillary action in the tubelike structures formed by both types of cells causes water to rise well above the level of the ground

145. Transpiration
For trees and other tall plants, the combination of root pressure and capillary action does not provide enough force to lift water to the topmost branches and leaves
The major force in water transport is provided by the evaporation of water from leaves during transpiration
When water is lost through transpiration, osmotic pressure moves water out of the vascular tissue of the leaf, as shown in the figure
Then, like a locomotive pulling a train with hundreds of cars, the movement of water out of the leaf “pulls” water upward through the vascular system all the way from the roots
This process is known as transpirational pull

146. Movement of Water Through Plant
Root pressure, capillary action, and transpiration contribute to the movement of water within a plant
Transpiration is the movement of water molecules out of leaves
The faster water evaporates from a plant, shown in A, the stronger the pull of water upward from the roots, shown in B

147. Movement of Water Through Plant

148. Transpiration How important is transpirational pull?
On a hot day, even a small tree may lose as much as 100 liters of water to transpiration
The hotter and drier the air, and the windier the day, the greater the amount of water lost
As a result of this water loss, the plant draws up even more water from the roots

149. TRANSPIRATION
Water eventually enters the leaf from the stem, where it is used in photosynthesis and cell metabolism
Water evaporates through the stomata openings of the lower epidermis of the leaf:
As water evaporates from the stoma, cells close to the stoma absorb water from neighboring cells by osmosis. Those cells, in turn, absorb water from more distant cells, until eventually water is absorbed from the xylem in the veins (vascular bundles) of the leaf
Water is pulled up the xylem
Evaporated water molecules pull other water molecules after them
Molecules of water move upward through the xylem
Water column extends from the root to the stoma
Column is held together by adhesion and cohesion
Water molecules in the soil replace those that leave the plant through its leaves (xylem vessels are never empty)

153. TRANSPIRATION-COHESION THEORY
Water is pulled up the xylem in a continuous column that stretches from the roots to the leaves
The forces of adhesion and cohesion make the thin column of water in the xylem behave like a dense, tightly-pulled wire
Cohesion: attraction between molecules of the same kind
Water molecules are polar
Positive (H) of one water molecule attracts the negative (O) of the neighboring water molecule and so on
A long chain of water molecules is formed
Water molecules throughout the entire xylem system are held together by these cohesion forces
Adhesion: attraction of unlike molecule
Water molecules adhere to the surface of the xylem vessels (like water adhering to the glass/plastic of a graduated cylinder)

154. Controlling Transpiration
The leaf's gas exchange subsystem helps to maintain homeostasis by keeping the water content of the leaf relatively constant
For example:
When water is abundant, it flows into the leaf, raising water pressure in the guard cells, which then open the stomata
Excess water is then lost through the open stomata by transpiration
When water is scarce, the opposite occurs
Water pressure in the leaf falls, and the guard cells respond by closing the stomata
This reduces further water loss by limiting transpiration

155. Transpiration and Wilting
Osmotic pressure keeps a plant's leaves and stems rigid, or stiff
High transpiration rates can lead to wilting
Wilting results from the loss of water—and therefore of the pressure in a plant's cells
Without this internal pressure to support them, the plant's cell walls bend inward, and the plant's leaves and stems wilt
When a leaf wilts, its stomata close
As a result, transpiration slows down significantly
Thus, wilting helps a plant to conserve water

156. Nutrient Transport Functions of Phloem
Many plants pump sugars into their fruits
This action often requires moving sugars out of leaves or roots into stems, and then through stems to the fruits
All of this movement takes place in the phloem
In cold climates, many plants pump food down into their roots for winter storage
This stored food must be moved back into the trunk and branches of the plant before growth begins again in the spring
Phloem carries out this seasonal movement of sugars within a plant

157. Nutrient Transport Movement From Source to Sink
A process of phloem transport moves sugars through a plant from a source to a sink
The source can be any cell in which sugars are produced by photosynthesis
The sink is a cell where the sugars are used or stored
How does phloem transport take place?

158. Nutrient Transport Movement From Source to Sink
One idea put forward by many plant scientists is called the pressure-flow hypothesis
As you can see in the figure at right, sugars are pumped into the phloem at one point, called the source
For example, sugars produced by photosynthesis may move from a leaf
As concentrations of sugar increase in the phloem, water from the xylem moves in by osmosis
This movement causes an increase in pressure at that point, forcing nutrient-rich fluid to move through the phloem away from nutrient-producing regions and toward a region that uses these nutrients, called the sink

159. The Pressure-Flow Hypothesis
The diagram shows the movement of sugars and water throughout the phloem and xylem as explained by the pressure-flow hypothesis
Materials move from a source cell, where photosynthesis produces a high concentration of sugars, to a sink cell, where sugars are lower in concentration
What is the source of the water that forces nutrients through phloem tissue?

160. The Pressure-Flow Hypothesis

161. TRANSLOCATION
Movement of food (dissolved sugar) from one part of a plant to another
Pressure-flow hypothesis:
Food is transported through the phloem (sieve tubes/sieve tube elements/companion cells) as a result of differences in pressure
When sugar molecules produced in a leaf enter a particular sieve element, the concentration of water in that element is lowered
Water enters from neighboring cells by osmosis increasing the pressure in the cell pushing the contents of the element into the next element
Energy is required

163. Nutrient Transport Movement From Source to Sink
Conversely, if part of a plant actively absorbs nutrients from the phloem, osmosis causes water to follow
This movement of water decreases pressure and causes a movement of fluid in the phloem toward the sink
When nutrients are pumped into or removed from the phloem system, the change in concentration causes a movement of fluid in that same direction
As a result, phloem is able to move nutrients in either direction to meet the nutritional needs of the plant