1. Chapter 9: Plant Organization

2. Plant Organs
The flowering plants, or angiosperms, have characteristic organs and tissues.
An organ is a structure that contains different types of tissues and performs one or more specific functions.
The vegetative organs of a flowering plant – the root, stem, and leaf – allow the plant to live and grow.
The body of a plant has a root system and a shoot system.
Flowering plants are extremely diverse because they are adapted to living in varied environments. Despite their great diversity in size and shape, flowering plants usually have three vegetative organs – roots, stems, and leaves.

3. Organization of the plant body
The body of a plant consists of a root system and a shoot system. Roots are the only type of plant organ in the root system. The shoot system contains stems and leaves, two other types of plant organs. The root system is connected to the shoot system by vascular tissues that extends from the roots to the leaves. Axillary buds can develop into branches of stems or flowers, the reproductive structures of a plant.

4. Roots
The root system of a plant has a main root, or taproot, and many branch or lateral roots.
The root system absorbs water and minerals from the soil for the plant.
The cylindrical shape of the root allows it to penetrate the soil.
Root hairs greatly increase the absorptive capacity of the root.
The absorptive capacity of a root is greatly increased by its many root hairs located in a special zone near the root tip. Root hairs, which are projections from root-hair cells, are especially responsible for the absorption of water and minerals. Root hairs are so numerous that they increase the absorptive capacity of a root tremendously. Root-hair cells are constantly being replaced.

5. Roots can also have other functions.
Roots produce hormones that stimulate the growth of stems and coordinate their size with the size of the root.
Generally the root system is equivalent in size and extent to the shoot system.
Perennial plants often store the products of photosynthesis in their roots.
Perennial plants, which die back and then regrow the next season, must store the products of photosynthesis in their roots.
Examples of other plants that store the products of photosynthesis in their roots are carrots and sweet potatoes.

6. Root system
The root system anchors the plant and absorbs water and minerals.

7. Stems The shoot system of a plant includes both stems and leaves.
A stem is the main axis of the plant along with its lateral branches.
At the tip of the stem is tissue that allows the stem to elongate and produce leaves.
A leaf attached to a stem at a node; and internode is the region beween nodes.
The presence of nodes and internodes is used to identify a stem even if it is underground. In some plants the nodes of horizontal stems asexually produce new plants.

8. Shoot system
The shoot system contains the stem and its branches, which support the leaves and transport water and organic nutrients.

9. A cylindrical stem can expand in girth as well as in length.
Stems also contain vascular tissue that transports water and minerals from roots to leaves, and also transports the products of photosynthesis in the opposite direction.
A cylindrical stem can expand in girth as well as in length.
Some stems have functions other than transport; some are specialized for storage.
Nonliving cells form a continuous pipeline for water and mineral transport, while living cells join end to end for organic nutrient transport.
As trees grow taller each year, they accumulate nonfunctional woody tissue that adds to the strength of the stems.

10. Leaves A leaf is a broad, thin organ that carries on photosynthesis.
This shape maximizes the surface area for collection of solar energy and absorption of carbon dioxide.
The wide portion of a leaf is the blade, a petiole is the stalk of the leaf, and axillary buds are found at the leaf axil.
Some leaves have other functions.
The upper and acute angle between the petiole and the stem is designated the leaf axil, and this is where an axillary (lateral) bud, which may become a branch or a flower, originates.
Not all leaves make up foliage. Some leaves are specialized to protect buds, attach to objects (tendrils), store food (bulbs), or even capture insects.

11. Leaves
Leaves, such as these from a tomato plant, are often broad and thin. They carry on photosynthesis. A tomato plant has a compound blade with several leaflets.

12. Monocot Versus Dicot Plants
Flowering plants are divided into two groups depending on their number of cotyledons (seed leaves).
Monocots (monocotyledons) have one cotyledon; dicots (dicotyledons) have two.
Cotyledons provide nutrients for seedlings before true leaves begin photosynthesizing.

13. Dicot stems have vascular bundles in a ring.
The vascular (transport) tissue is organized differently in monocots and dicots.
Monocot roots have vascular tissue in a ring; in stems, vascular bundles are scattered.
Dicot roots have vascular tissue in a star shape with phloem located between arms of xylem.
Dicot stems have vascular bundles in a ring.
Vascular bundles contain vascular tissue surrounded by a bundle sheath. Phloem is vascular tissue that transports organic nutrients; xylem transports water and minerals.

14. Monocot and dicot traits
The number of cotyledons, and the arrangement of vascular tissue in roots and stems distinguishes monocots from dicots.

15. Leaf veins are vascular bundles within a leaf.
Monocots usually have parallel venation.
Dicots exhibit netted venation, which may be either pinnate or palmate.
Adult monocots and dicots differ in the number of flower parts, and dicot pollen grains have three apertures while monocot pollen grains have one aperture.
Important monocots are rice, wheat and corn; oak trees and dandelions are dicots.

16. Monocots and dicots differ in the venation pattern of leaves and in number of flower parts.

17. Dicot leaves

18. Plant Tissues
A plant grows throughout its lifespan because of meristem (embryonic tissue) in stem and root tips (apexes).
Three specialized tissues are in plants:
Epidermal tissue – forms the outer protective covering
Ground tissue – fills interior of a plant
Vascular tissue – transports water and nutrients and provides support.
Three types of meristem continually produce three types of specialized tissue in the body of the plant: protoderm, the outermost primary meristem gives rise to epidermis; ground meristem produces ground tissue; and procambium produces vascular tissue.

19. Epidermal Tissue
Epidermal tissue forms the outer protective covering of a herbaceous plant and is modified in roots, stems, and leaves.
Exposed epidermal cells are covered with waxy cuticle to minimize water loss.
Epidermal cells in roots have root hairs.
Lower leaf epidermal cells have guard cells and stomata.
Root hairs increase the surface area of a root for absorption of water and minerals and help anchor the plant firmly in place.
Protective hairs of a different nature are produced by epidermal cells of stems and leaves. Epidermal cells may also be modified as glands that secrete protective substances of various types.
In leaves, the lower epidermis in particular contains specialized cells called guard cells. The guard cells, which unlike epidermal cells have chloroplasts, surround microscopic pores called stomata (sing., stoma). When the stomata are open, gas exchange occurs.

20. Root hairs
Epidermal modifications include root hairs to absorb water.

21. Stoma of leaf
Epidermal modifications include stomata in leaf epidermis that functions in gas exchange. The stoma (pore) is opened or closed by guard cells.

22. In older woody plants, the epidermis of the stem is replaced by cork tissue.
Cork, a component of bark, is made up of dead cells that may be sloughed off.
New cork cells are made by a meristem called cork cambium.
As cork cells mature, they fill with suberin, a lipid that makes them waterproof and chemically inert.
Cork protects woody plants and helps them resist fungi, bacteria, and animals.

23. Cork of older stem
Cork replaces epidermis in older woody stems.

24. Ground Tissue Ground tissue forms the bulk of the plant.
Ground tissue contains parenchyma cells, which are thin-walled and capable of photosynthesis when they contain chloroplasts.
Collenchyma cells have thicker walls for flexible support.
Sclerenchyma cells are hollow, nonliving support cells with secondary walls.
Parenchyma cells can also divide and give rise to more specialized cells, such as when roots develop from stem cuttings placed in water.
Collenchyma cells often form bundles just beneath the epidermis and give flexible support to immature regions of the plant. Stringy strands from celery stalks are made of collenchyma cells.
Sclerenchyma cells have thick secondary cell walls impregnated with lignin that makes the walls tough and hard. Most sclerenchyma cells are nonliving; their primary function is to support mature regions of the plant. Two types of sclerenchyma cells are fibers and sclereids. Fibers are mostly found in vascular tissue and may be long and slender; hemp fibers are used to make rope, and flax fibers are made into linen. Flax fibers are not lignified, which is why linen is soft. Sclereids, which are shorter than fibers and more varied in shape, are found in seed coats and nutshells, and also giver pears their gritty texture.

25. Ground tissue cells
Parenchyma cells have thin walls and intercellular air spaces. The walls of collenchyma cells are much thicker than those of parenchyma cells. Sclerenchyma cells have very thick walls and are nonliving – their only function is to give strong support.

26. Vascular Tissue
There are two types of vascular (transport) tissue that extend from roots to leaves.
Xylem transports water and minerals from roots to leaves and contains two types of conducting cells: tracheids and vessel elements.
Phloem transports organic nutrients from leaves to roots and has sieve-tube elements with companion cells; plasmodesmata extend between cells at sieve plates.
Both tracheids and vessel elements are hollow and nonliving, but vessel elements are larger, lack transverse end walls, and are arranged to form a continuous pipeline for water and minerals. The elongated tracheids, with tapered ends, form a less obvious means of transport, but water can move across the end walls and side walls through pits where the secondary wall does not form. Xylem also contains parenchyma cells that store various substances, and fibers and sclerenchyma cells that lend support.
The conducting cells of phloem are sieve-tube elements each of which has a companion cell. Sieve-tube elements contain cytoplasm but no nuclei. These elements have channels in their end walls that in cross section make them resemble a sieve. Plasmodesmata (sing., plasmodesma), which are strands of cytoplasm, extend from one cell to another through this sieve plate. The smaller companion cells have a nucleus in addition to cytoplasm; the nucleus of the companion cell controls both the companion cell and the sieve-tube element.
Vascular tissue is located in the vascular cylinder in dicot roots, in vascular bundles within stems, and in leaf veins in leaves.

27. Xylem structure
This drawing shows the general organization of xylem tissue.

28. Phloem structure
This drawing shows the general organization of phloem tissue.

29. Organization of Roots
Within a root are zones where cells are in various stages of differentiation.
The root apical meristem is in the zone of cell division; the root cap is a protective covering for the root tip.
In the zone of elongation, cells become longer as they specialize.
In the zone of maturation, mature cells are differentiated and epidermal cells have root hairs.

30. Dicot root tip
The root tip is divided into three zones and a root cap, best seen in a longitudinal section such as this. The cells in the root cap have to be continually replaced because they are ground off as the root pushes through rough soil particles. In the zone of elongation, the cells become longer as they become specialized. In the zone of maturation, the cells are mature and fully differentiated. This zone is recognizable even in a whole root because root cells are borne by many of the epidermal cells.

31. Tissues of a Dicot Root
Epidermis – single layer of thin-walled, rectangular cells; root hairs present in zone of maturation
Cortex – thin-walled, loosely-packed parenchyma; starch granules store food
Endodermis – between cortex and vascular cylinder, single layer of endodermal cells bordered by the Casparian strip; regulates entrance of minerals into the vascular cylinder
The Casparian strip is a layer of impermeable lignin and suberin that does not permit water and mineral ions to pass between adjacent cell walls. The only access to the vascular cylinder is through the endodermal cells themselves.

32. Movement of materials into vascular cylinder
Because of the Casparian strip, water and minerals must pass through the cytoplasm of the endodermal cells in order to enter the vascular cylinder. In this way, endodermal cells regulate the passage of minerals into the vascular cylinder.

33. Vascular Tissue – has star-shaped xylem in dicots with phloem in separate regions between arms of xylem; the pericycle gives rise to lateral roots
The pericycle is the first layer of cells within the vascular cylinder.

34. Branching of a dicot root
This cross section of a willow shows the origination of a branch root from the pericycle.

35. Organization of Monocot Roots
Monocot roots have the same growth zones as a dicot root but they do not undergo secondary growth.
In a monocot root’s centrally located pith, ground tissue is surrounded by a vascular ring composed of alternating xylem and phloem bundles.
Monocot roots also have pericycle, endodermis, cortex, and epidermis.

36. Monocot root
This cross section enlargement of a monocot root shows the exact placement of various tissues. Note the vascular ring around a central pith.

37. Root Diversity
Roots have special adaptations and associations to better perform the functions of anchorage, absorption of water and minerals, and carbohydrate storage.
In some dicot plants, a primary root, or taproot, grows straight down and is the dominant root; it can be fleshy and stores food.
Examples of taproots that store food are found in carrots, beets, turnips, and radishes.

38. Taproot
A taproot may have branch roots in addition to a main root.

39. Monocots have no single main root but instead have a large number of slender roots.
These slender roots and their lateral branches make up a fibrous root system.
Fibrous roots develop from the lower nodes of the stem rather than from the root system, and are known as adventitious roots.
Adventitious roots that emerge from the surface to help anchor the plant are called prop roots.
Grasses have fibrous roots, for example.
Corn plants have prop roots, as do mangrove trees. Mangroves, which roots submerged in water, also have pneumatophores on their prop roots that allow the plant to acquire oxygen from the air for cellular respiration.

40. Fibrous roots and prop roots
A fibrous root system has many slender roots with no main root. Prop roots are specialized for support.

41. Some plants (dodders and broomrapes) are parasitic on other plants.
Their stems have rootlike projections called haustoria that grow into the host plant and extract water and nutrients from the host.
Mycorrhizae are a mutualistic association between roots and fungi that aid the plant in extraction of nutrients and water from soil.
Peas and other legumes have root nodules in which nitrogen-fixing bacteria live.
Nitrogen-fixing bacteria remove nitrogen from the atmosphere and make it available to the plants. Otherwise, plants have to rely on soil nitrogen which is often in limited supply.

42. Dodder
Dodder is a parasitic plant consisting mainly of orange-brown twining stems. The green in the photograph is the host plant. Haustoria are rootlike projections of a stem that tap into the host’s vascular system.

43. Organization of Stems
During primary growth, the shoot apical meristem at the shoot tip produces new cells that elongate and add length to the stem.
The shoot apical meristem is protected within a terminal bud where leaf primordia envelope it.
In the temperate zone, a terminal bud stops growing in winter and is protected by bud scales.
The growth of a stem can be compared to the growth of a root. Leaf primordia are immature leaves. In the spring, when growth resumes, bud scales fall off and leave a scar. The age of the stem can be determined by counting bud scale scars.

44. Leaf primordia are produced at nodes; the stem between two nodes is called an internode.
Internodes increase in length as the stem grows.
Axillary buds, which are dormant but may develop into branch shoots or flowers, form at the axes of leaf primordia.

45. Shoot tip
The shoot apical meristem within a terminal bud is surrounded by leaf primordia.

46. In addition to leaf primordia, three specialized types of primary meristem that contribute to shoot length develop from shoot apical meristem.
Protoderm gives rise to epidermis.
Ground meristem produces parenchyma in the pith and parenchyma in the cortex.
Procambium produces primary xylem and primary phloem; later, vascular cambium occurs between xylem and phoem.

47. Fate of primary meristems
The shoot apical meristem produces the primary meristems: protoderm gives rise to epidermis, ground meristem gives rise to pith and cortex, and procambium gives rise to vascular tissue, including primary xylem, primary phloem, and vascular cambium.

48. Herbaceous Stems
Mature nonwoody stems, or herbaceous stems, exhibit only primary growth.
The outermost tissue is the epidermis, which is covered by waxy cuticle.
These stems have distinctive vascular bundles, with xylem toward the inside and phloem toward the outside.
In dicot stems, vascular bundles are in a distinct ring; monocot vascular bundles are scattered throughout.
The waxy cuticle of herbaceous stems helps to prevent water loss.
The distinct ring of herbaceous dicot stems separates the cortex from the central pith, which stores water and the products of photosynthesis. The cortex is sometimes green and carries on photosynthesis, and the pith may function as a storage site. Monocot stems have no well-defined cortex or pith.

49. Herbaceous dicot stem

50. Monocot stem

51. Woody Stems A woody plant has both primary and secondary tissues.
Primary tissues are new tissues formed each year from primary meristems right behind apical meristem.
Secondary tissues develop during the second and subsequent years of growth from lateral meristems (vascular cambium and cork cambium).

52. Primary growth, which occurs in all plants, increases the length of the plant.
Secondary growth, which occurs in conifers and some dicots, increases the girth of a plant.
Trees undergo secondary growth because of a change in vascular cambium.
The secondary tissues produced by the vascular cambium, called secondary xylem and secondary phloem, add to the girth of trunks, stems, branches, and roots.

53. Dicot stems
The drawing in the upper left shows a dicot stem with no secondary growth. The other diagram shows a dicot stem with some secondary growth. Cork has replaced the epidermis, and vascular tissue produces secondary xylem and secondary phloem.

54. Secondary growth in a dicot stem
This shows a three-year-old stem in which cork cambium produces new cork. The primary phloem and cortex will eventually disappear, and only the secondary phloem (within thet bark), produced by vascular cambium, will be active that year. The secondary xylem, also produced by vascular cambium, builds up to become annual growth rings.

55. The bark of a tree contains cork, cork cambium, and phloem.
As a result of secondary growth, a woody dicot stem has an entirely different type of organization.
A woody stem now has three distinct areas: the pith, the wood, and the bark.
Pith rays are composed of living parenchyma cells that allow materials to move laterally.
The bark of a tree contains cork, cork cambium, and phloem.
Cork cambium replaces epidermis with cork cells impregnated with suberin.
Although secondary phloem is produced each year by vascular cambium, phloem does not build up for many seasons. The phloem tissue is soft, making it possible to remove the bark of a tree; however, this is harmful to the tree because without phloem organic nutrients cannot be transported.
Cork cambium is meristem located beneathe the epidermis. Cork cells are impregnated with suberin, a waxy layer that makes them waterproof but also causes them to die. This is protective because now the stem is less edible. Gas exchange is now impeded, however, except at lenticels, which are pockets of loosely arranged cork cells not impregnated with suberin.

56. Section of woody stem
This section of a woody stem shows tissues at a higher magnification.

57. Annual Rings
In trees that have a growing season, vascular cambium is dormant during winter.
In spring, with plentiful moisture, xylem contains wide vessels with thin walls in spring wood; summer wood has a lower proportion of vessels.
Spring wood followed by summer wood makes up one year’s growth or annual ring.
It is possible to tell the age of a tree by counting annual rings.

58. Tree trunk
The relationship of bark (cork and phloem), vascular cambium, and wood is retained in a mature stem. The pith has disappeared.
In older trees, the inner annual rings, called heartwood, no longer function in water transport. The cells become plugged with deposits, such as resins, gums, and other substances that inhibit the growth of bacteria and fungi. Heartwood may help support a tree, although some trees live for many years after the heartwood has rotted away.

59. Stem Diversity
Plants use stems for such functions as reproduction, climbing, survival, and food storage.
Modified stems aid adaptation to different environments.
Examples of stem modifications include:
Stolons
Rhizomes
Tubers
Corms
Stolons are aboveground horizontal stems, or runners, that can produce new plants.
Underground horizontal stems, or rhizomes, survive the winter and contribute to asexual reproduction. Some rhizomes have enlarged portions called tubers.
Corms are bulbous underground stems that lie dormant during winter.
Humans use stems in many ways.

60. Stolons and rhizomes
A strawberry plant has aboveground, horizontal stems called stolons. Every other node produces a new shoot system. The underground stem of an iris is a fleshy rhizome.

61. Tubers and corms
The underground stem of a potato plant has enlargements called tubers. We call the tubers potatoes. The corm of a gladiolus is a stem covered by papery leaves.

62. Organization of Leaves
Leaves are the organs of photosynthesis in vascular plants.
Leaves have a flattened blade, that may be single or composed of leaflets, attached to a petiole.
The epidermal layers may bear protective hairs or glands that produce irritating substances; a waxy cuticle reduces water loss and permits gas exchange.

63. The body of the leaf is composed of mesophyll.
Guard cells surrounding stomata in the lower epidermis allow gases to enter and exit the leaf.
The body of the leaf is composed of mesophyll.
Palisade mesophyll has elongated cells, and spongy mesophyll has irregular cells surrounded by air spaces.
Parenchyma cells of these mesophyll layers house chloroplasts.
The loosely packed arrangement of the cells in the spongy layer increases the amount of surface area for gas exchange.

64. Leaf structure
Photosynthesis takes place in the mesophyll tissue of leaves. The leaf is enclosed by epidermal cells covered with a waxy layer, the cuticle. Leaf hairs are also protective. The veins contain xylem and phloem for the transport of water and solutes. A stoma is an opening in the epidermis that permits the exchange of gases.

65. Leaves are adapted to environmental conditions and may be broad and wide or reduced with sunken stomata.
The leaves of a cactus are spines attached to a succulent stem.
Climbing leaves, such as those of peas, are modified into tendrils.
The leaves of a few plants are specialized for catching insects.

66. Classification of leaves
The cottonwood tree has a simple leaf. The shagbark hickory has a pinnately compound leaf. The honey locust has a twice pinnately compound leaf. The buckeye has a palmately compound leaf.

67. Leaf diversity
The spines of a cactus plant are leaves modified to protect the fleshy stem from animal consumption and limit the loss of water. The tendrils of a cucumber are leaves modified to attach the plant to a physical support. The leaves of the Venus’s flytrap are modified to serve as a trap for insect prey. When triggered by an insect, the leaf snaps shut. Once shut, the leaf secretes digestive juices, which break down the soft parts of the prey’s body, allowing nutrients such as nitrogen to be absorbed by the plant body.

68. Chapter Summary
A flowering plant (angiosperm) has three vegetative organs: the root absorbs water and minerals, the stem supports and services leaves, and the leaf carries on photosynthesis.
Flowering plants can be divided into monocots and dicots based on structural differences.

69. Three types of meristem divide and produce specialized tissues.
Dermal tissue consists of epidermis composed of epidermal cells.
Ground tissue contains parenchyma, collenchyma, and sclerenchyma cells.
Vascular tissue consists of xylem (vessel elements and tracheids) that transports water and minerals, and phloem (sieve-tube elements and companion cells) that transports organic nutrients.

70. A root tip has three zones: the zone of cell division, the zone of elongation, and the zone of maturation.
A herbaceous root has epidermis, cortex, endodermis, and a vascular cylinder.
The dicot vascular cylinder is star-shaped, while the monocot root has a ring of vascular tissue with alternating bundles of xylem and phloem surrounding pith.
Roots have diverse structures, including taproots, adventitious roots, and prop roots.

71. Primary growth of a stem is due to the shoot apical meristem, which is protected within a terminal bud.
Stems have nodes and internodes.
In cross section, a nonwoody dicot has epidermis, cortex, vascular bundles in a ring, and an inner pith.
Monocots have scattered vascular bundles.
Secondary growth is due to vascular cambium; wood contains annual rings of xylem.

72. Stems are diverse; humans find many uses for stems.
A leaf has an upper and a lower epidermis, and mesophyll tissue forms the bulk of the leaf.
Stomata regulate the passage of gases in and out of leaves.
Leaves are diverse and are adapted to environmental conditions.