Plant Structure and Organization 1 Ch. 25 – Plant Structure & Organization

1Plant Structure anPlant Structure and Organizationd Organization

Ch. 25 – Plant Structure & Organization

2Plant Structure anPlant Structure and Organizationd Organization

Plant Organs

Plants possess two types of systems:

Root System

­Simply consists of roots

Shoot System

­Consists of stems & leaves

Vegetative Organs

­Roots, stems & leaves

Reproductive Organs

­Flowers, seeds & fruits

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Plant Organs

Roots

Generally, the root system is at least equivalent in size and extent to the shoot system

­Anchors plant in soil; gives support

­Absorbs water and minerals

­Produces hormones that stimulate growth of stems

Root hairs:

­Projections from epidermal root hair cells

­Greatly increase absorptive capacity of root

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Plant Organs

Roots (cont’d)

Perennial plants

­Vegetative structures live year to year

Herbaceous Perennial plants

­Upper parts die back in winter

­Store energy in roots

­Examples: carrots, sweet potatoes

5Organization­of­Plant­Body

6Vegetative­Organs­of­Several­Eudicots

7Plant Structure anPlant Structure and Organizationd Organization

Stems

Shoot system of a plant is composed of the stem, branches, and leaves

Stem is the main axis of a plant that elongates and produces leaves

­Nodes occur where leaves are attached to the stem

­ Internode is region between nodes

Stem also has vascular tissue that transports water and minerals from roots through stem to leaves & transports products of photosynthesis

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Stems

Other functions of stems:

Primary photosynthetic organ in cactus

Water reservoir in succulents

Store nutrients

­Tubers are horizontal or underground stems

­ i.e. Potatoes

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Leaves

Leaves are the major part of the plant that carries on photosynthesis

- Size, shape, color & texture of leaves vary.

- Used for identification of plants

Two main types of plants based on leaves:

- Evergreens – retain leaves for 2-7 years

- Deciduous – lose leaves every year

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Leaves

Foliage leaves are usually broad and thin for maximum surface area for absorption

­Blade - Wide portion of foliage leaf

­Petiole - Stalk attaches blade to stem

­Leaf Axil is the upper acute angle where axillary bud originates. May become leaf or flower.

Tendrils - Leaves that attach to objects

Bulbs - Leaves that store food

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Monocots versus Eudicots

Flowering plants are divided into 2 groups depending on number of cotyledons, or seed leaves in the embryonic plant.

Monocots

- One cotyledon

Eudicots

- Two cotyledons

12Flowering­Plants:Monocots­or­Eudicots

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Monocot vs. Eudicot

Monocots (grasses, palm trees, rice, wheat, corn)

Cotyledons act as transfer tissue; nutrients are derived from endosperm prior to the development of true leaves

Root vascular tissue occurs in a ring

Leaf veins are vascular bundles

Parallel leaf venation

Flower parts arranged in multiples of three

14Flowering­Plants:Monocots­or­Eudicots

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Monocot vs. Eudicot

Eudicots (dandelions to oak trees)

Cotyledons supply nutrients to seedlings

Root phloem located between xylem arms

- Pattern looks like a star

Netted leaf venation

- Pinnate – major veins originate from central vein

- Palmate – major veins originate from point of attachment of blade to petiole

16Flowering­Plants:Monocots­or­Eudicots

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Plant Tissues

Plants have ability to grow during entire lives because they possess meristematic tissue.

Apical meristems - located at or near tips of stems & roots

- increase the length of stems & roots

- Increase in length is called primary growth

- produce 3 types of tissues:

epidermal, ground & vascular

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Plant Tissues

Epidermal Tissues Form protective outer covering called epidermis

Contain closely packed epidermal cells

­ Walls of epidermal cells exposed to air are covered with a waxy cuticle to minimize water loss

­ Cuticle also protects against bacteria & disease

Roots contain root hairs

Leaves can have trichomes (hairs) to protect from sun & to conserve water. These can inject toxins into animals.

Lower leaf surface contain stomata surrounded by guard cells. These regulate gas exchange & water loss.

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Plant Tissues

Epidermal tissues (cont’d)

In woody plants, the epidermis of stem is replaced by periderm.

- Major component is cork

- Mature cork cells can be sloughed off

- New cork cells are made by a meristem called the cork cambium

20Modifications­of­Epidermal­Tissue

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Ground Tissue

Ground tissue forms bulk of interior of plant

Formed of three types of cells:

parenchyma, collenchyma & sclerenchyma

Parenchyma cells:

­Most abundant & alive at maturity

­Least specialized and are found in all organs of plant. Can divide and give rise to more specialized cells

­Functions: photosynthesis, food storage, respiration

22Ground­Tissue­

Cells

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Ground Tissue

●Collenchyma cells:

­Alive at maturity

­Have thicker primary walls

­Form bundles underneath epidermis

­Flexible support to immature regions of the plant

­Strands of celery are composed of these cells

24Ground­Tissue­

Cells

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Ground Tissue

Sclerenchyma cells:

Have thick secondary walls impregnated with lignin, a highly resistant substance that makes walls tough and hard

Most are nonliving; dead at maturity

Primary function is to support mature regions of the plant

­Two types of cells: fibers & sclereids

26Ground­Tissue­

Cells

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Vascular Tissue

There are two types of vascular tissues:

Xylem

­Transports water & minerals from roots to leaves

Phloem

­Transports sucrose & other organic compounds, including hormones, usually from leaves to roots

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Vascular Tissue

Both xylem & phloem are considered complex tissues (those made of 2 or more kinds of cells)

Xylem is made of:

Tracheids (hollow non-living cells)

­Long, with tapered ends

Pits in end walls help water move

Vessel Elements (hollow non-living cells)

­Larger, with perforated plates in their end walls

29Xylem­Structure

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Vascular Tissue - Phloem

Phloem is composed of living cells:

Sieve-tube members are conducting cells

­Form a continuous sieve tube; have pores in end walls

­Contain cytoplasm but no nuclei

●Companion cells attached to each sieve-tube

­Plasmodesmata extend between 2 types of cells. Companion cell’s nucleus might control sieve-tube.

31Phloem­Structure

32Plant Structure anPlant Structure and Organizationd OrganizationOrganization of Roots -

Longitudinal section

Apical meristemLocated in the root tipProtected by root cap

Zones of Root:

Zone of cell division - primary meristems

Zone of elongation - cells lengthen

Zone of maturation - contains fully differentiated (specialized)cells. Cells on epidermis have root hairs.

33Eudicot­Roots

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Tissues of Eudicot Root - Cross Section

Epidermis - single layer of cells

Cortex

- Large thin-walled parenchyma cells

- Loosely packed

- Possible for water to move without entering cells

- Functions in food storage.

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Tissues of Eudicot Root - Cross Section

Endodermis -

- Single layer of rectangular cells

- Between cortex and inner vascular cylinder

- Bordered on four sides by Casparian Strip. • This is a layer of impermeable lignin & suberin.

• Prevents passage of water & mineral ions between adjacent cell walls.

access to vascular cylinder is through endodermal cells themselves

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Tissues of Eudicot Root - Cross Section

Vascular Tissue Pericycle - First layer of cells within vascular cylinder

- Retains capacity to divide to start development of branch, or lateral, roots.

•Xylem - Appears star-shaped in eudicots

•Phloem - found between arms of xylem

37Branching­of­Eudicot­Root

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Organization of Monocots Roots

Monocot roots have same growth zones as eudicot roots, but do not undergo secondary growth.Ground tissue of root’s pith is surrounded by vascular ring

- Ring composed of alternating bundles of xylem and phloem

Have pericycle, endodermis, cortex & epidermis

39Monocot­Root

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Root Diversity

Primary root (taproot) - Fleshy, long single root, that grows straight downOften stores food Examples: carrots, beets, turnips, radishes

Fibrous root system - Large # of slender roots and lateral branchesAnchors plant to soilFound in grasses

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Root Specializations

Adventitous roots - Roots that develop from organs of the shoot system instead of root system

Prop roots - emerge above soil line & function to help anchor the plant

Others are found on horizontal stems or at nodes & help vines to climb (English Ivy)

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Root Specializations

Haustoria:Parasitic rootlike projections that grow into host plant

Make contact with vascular tissue and extract water and nutrients

Mycorrhizae:Associations between roots and fungiExtract water and minerals from soil better than roots alone

• Example of mutualism (plants/fungus benefit)

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Root Specializations

Root nodules - Contain nitrogen-fixing bacteria

• Found in peas, beans, and other legumes

• Bacteria can take up atmospheric nitrogen & make it available to plants

• Thus, plants aren’t dependent on supplies of nitrogen in the soil

• Legumes often planted to bolster N supply of soil.

44Root­Diversity

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Organization of Stems

Shoot apical meristemProduces new cells that elongate and increase length of stem

Protected by terminal bud

­Enveloped by leaf primordia (immature leaves)

46Woody­Twig

47Shoot­tip­and­Primary­Meristems

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Organization of Stems

● Specialized primary meristemsProtoderm – gives rise to epidermisGround Meristem – produces 2 tissues composed of parenchyma cells:

- pith (center of stem)

- cortex (between epidermis & vascular tissue)

49Shoot­tip­and­Primary­Meristems

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Organization of Stems

●Procambium

Produces first xylem cells, primary xylem & first phloem cells, primary phloem.

Differentiation occurs which produces mature xylem, phloem and a lateral meristem called vascular cambium. This tissue will lead to secondary growth (growth in width)

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Herbaceous Stems

Mature nonwoody stems that exhibit only primary growth

Outermost tissue is epidermis covered with waxy cuticle

Stems have distinctive vascular bundles

­Herbaceous eudicots - Vascular bundles arranged in distinct ring

­Monocots - Vascular bundles scattered throughout stem

52Herbaceous­Eudicot­Stem

53Monocot­Stem

54Monocot­Stem

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Woody Stems

Woody plants have both primary and secondary tissuesPrimary tissues formed each year from primary meristems

Secondary tissues develop during first and subsequent years from lateral meristems

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Woody Stems

Woody stems have no distinct vascular bundles, and instead have three distinct regions:

●BarkWoodPith

Vascular cambium occurs between the bark and the wood

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Secondary Growth

Secondary growth adds cells on either side of the vascular cambium:

● Secondary xylem – inner side

● Secondary phloem – outer side

Each year new phloem is added to the outside of the trunk of a tree. This creates bark.

Each year new xylem is added to the inside of the trunk of the tree. This creates wood.

58Secondary­Growth­of­Stems

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Wood

Wood is secondary xylem that builds up year after year In trees that have a growing season, vascular cambium is dormant during winter

- In spring, when moisture is plentiful secondary xylem contains wide vessel elements with

thin walls.

- This creates the spring wood.

- Later in season, moisture is scarce, vessels are small and are many thick-walled tracheids

- This creates the summer wood.

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Wood

When the trunk of a tree has spring wood followed by summer wood, the two together make up one year’s growth, or an annual ring.

You can tell the age of a tree by counting the annual rings.

Two Types of Wood:

The outer annual rings, where transport occurs, are called sapwood.

In older trees, inner annual rings, heartwood, no longer function in water transport.

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Bark

Bark of a tree contains periderm (cork & cork cambium) and phloem.

Phloem does not build up season to season like wood. Instead it dies and is shed over time.

Bark can be removed, but it is harmful to the plant due to lack of organic nutrient transport

Cork cells are impregnated with suberin, a waxy layer that makes them waterproof but causes them to die. This makes them harder and less edible & thus protects the tree.

62Three-year-old­Woody­Twig

63Tree­Trunk

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Stem Diversity

Stolons:Above-ground horizontal stemsProduce new plants where nodes touch the ground. This is called vegetative reproduction.

Example: strawberry plantsOther Modifications:

•Succulent stems on cacti store water •Stems that become tendrils to climb

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Stem Diversity

Rhizomes:Underground horizontal stemsContribute to asexual reproduction because each node bears a bud.

Variations:­Tubers - Enlarged portions functioning in food storage

Example: potato eyes are buds­Corms - Underground stems that produce new plants during the next season

Example:­gladiolus­“bulbs”

66Stem­Diversity

67Leaf­Structure

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Leaf Structure

Parts of a leaf:

•Epidermis

•Outer waxy cuticle

•Stomata & guard cells

•Body of leaf composed of mesophyll tissue in two types:

- palisade mesophyll - elongated cells

- spongy mesophyll - irregular cells bounded by air spaces

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Leaf Diversity

Blade of a leaf can be simple or compound:

• Simple leaf - single blade

• Compound leaf - divided into leaflets

- Pinnately compound leaves have leaflets occurring in pairs

- Palmately compound leaves have all of the leaflets attached to a single point

- Bipinnately compound leaves have leaflets subdivided into even smaller leaflets

70Classification­of­Leaves

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Leaf Diversity

Leaves can be arranged in stem in 3 ways:

• Alternate

• Opposite

• Whorled

Leaves are adapted to environmental conditions.Shade plants - broad, wide leavesDesert plants have reduced leaves, sunken stomata. Cactus spines are leaves.

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Leaf Diversity

Other specialized leaves:

• Onion bulbs

• Head of cabbage

• Large petiole (celery & rhubarb)

• Tendrils

• Specialized for catching insects:

- Sundews, Venus flytrap, pitcher plants

- These plants commonly live in marshy regions with low supply of soil nitrogen.

73Leaf­Diversity

Sundew

Pitcher

plant