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Chapter 9: Plant Organization

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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.

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Organization of the plant bodyFig. 9.1

Vegetative organs are the leaf, stem and root

The body of a plant has a root system and a shoot system.

Sh

oo

t S

yst

em

Root System

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The Root System

Consists of a main root

(taproot) and many lateral

roots, which absorb water

and minerals from the soil

for the plant.

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Root systems

-Produce hormones

-Perennial plants often store the products of photosynthesis in their roots.

Fig. 9.2

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A stem is the main axis of the plant along with its lateral branches.

Transports water and minerals from roots to leaves, and products of photosynthesis in the other direction.

Shoot System - Stems

A cylindrical stem can expand in girth as well as in length.

Fig. 9.1

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A leaf attaches to a stem at a node; an internode is the region beween nodes.

Fig. 9.1

Shoot System - LeavesA leaf is a broad, thin organ (maximizes surface area) that carries on photosynthesis (some have other functions).

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Blade

Petiole

Attaches to the node here

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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.

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Monocot and dicot traitsFig. 9.3

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Fig. 9.3

Monocots:

Parallel veins (sugarcane, corn)

Flowers have 3 or multiples of 3 (6,9,12, etc.) parts

Dicots:

Veins form a net pattern (oak tree)

Flowers have 4 or 5 or multiples of 4 or 5 (8,10, etc.) parts

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Dicot leavesFrom Page 150

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Plant Tissues

1) Epidermal tissue – outer covering

2) Ground tissue – majority of plant tissue

3) Vascular tissue – transport

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Epidermal Tissue

Epidermal tissue forms the outer protective covering of a herbaceous (non-woody) plant.

Exposed epidermal cells are covered with waxy cuticle to minimize water loss.

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Root Hairs are Epidermal Tissue

Root hairs greatly increase the absorptive capacity of the root.

Fig. 9.4

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Stoma of leaves are part of the epidermal tissue Fig. 9.4

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Cork (an epidermal tissue) of anolder stem

Cork is a component of bark.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.

Fig. 9.4

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Ground Tissue

-thin-walled and capable of photosynthesis when they contain chloroplasts.

-have thicker walls for flexible support (celery strands).

-are hollow, nonliving support cells with secondary walls.

-forms the bulk of the plant.

Fig. 9.5

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Vascular TissueTwo types of vascular (transport) tissue:

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.

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Xylem structure

Xylem transports water and minerals from roots to leaves

Contains two types of conducting cells: tracheids and vessel elements.

Wat

er

Roots

Leaves

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Phloem structure

Transports organic nutrients from leaves to roots

Has sieve-tube elements with companion cells

Plasmodesmata extend between cells at sieve plates.

Leaves

Roots

Org

anic

nu

trie

nts

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Leaves (produce organic nutrients by photosynthesis)

Phloem Xylem

Carries Organic Nutrients Carries Water and Nutrients

Roots (absorb water and minerals from the soil)

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Organization of RootsWithin a root are zones where cells are in

various stages of differentiation.

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Dicot root tip

In the zone of maturation, mature cells are differentiated and epidermal cells have root hairs.

In the zone of elongation, cells become longer as they specialize.

The root apical meristem is in the zone of cell division; the root cap is a protective covering for the root tip.

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Dicot root tip Fig. 9.8

Epidermis – single layer of thin-walled, rectangular cells; root hairs present in zone of maturation

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Movement of materials into vascular cylinder of the root

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

Fig. 9.8

Layer of impermeable lignin and suberin

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In Dicot Roots Vascular Tissue is star-shaped; phloem in separate regions between arms of xylem

Fig. 9.8

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Branching and Taproot of dicots

Pericycle can start the development of branch roots.

Fig. 9.9

In some dicot plants, a primary root, or taproot, grows straight down and is the dominant root; it can be fleshy and stores food.

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Organization of Monocot Roots

See Fig. 9.10

Monocot roots have the same growth zones as a dicot root but they DO NOT undergo secondary growth (become woody).

In a monocot root’s centrally located pith, ground tissue is surrounded by a vascular ring.

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Monocot root

Monocots have a large number of slender roots, which make up a fibrous root system, and are known as adventitious roots.

Fig. 9.11

Adventitious roots that emerge from the surface to help anchor the plant are called prop roots.

-used for support

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Some plants 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.

Fig. 9.11

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Mycorrhizae - mutualistic association between roots (better water absorption) and fungi (receive sugars, etc.)

Peas and other legumes have root nodules in which nitrogen-fixing bacteria live.

Other Root Diversity

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Shoot tipOrganization of Stems

Fig. 9.12

produces new cells that elongate and add length to the stem.

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Primary tissues are new tissues formed each year from primary meristems right behind apical meristem.

Meristem – Embryonic Tissue (undifferentiated) that develops into specialized tissue.

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Fate of Primary Meristems

Protoderm gives rise to epidermis.Ground meristem produces parenchyma in the pith and cortex.Procambium produces primary xylem and primary phloem; later, vascular cambium occurs between xylem and phoem.

Fig. 9.12

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Herbaceous (nonwoody) Stems

Mature herbaceous stems exhibit only primary growth (not secondary).

The outermost tissue is the epidermis (not bark), which is covered by a waxy cuticle.

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Herbaceous Dicot Stem -vascular bundles are in a distinct ring

Fig. 9.13

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Monocot stem -vascular bundles are scattered throughout

Fig. 9.14

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Woody StemsA woody plant has both primary and

secondary tissues.

Secondary tissues develop during the second and subsequent years of growth from lateral meristems (vascular cambium and cork cambium).

Secondary growth, (annual growth) increases the girth of a plant.

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Dicot stems

The secondary tissues produced by the vascular cambium, are called secondary xylem and secondary phloem,

Fig. 9.15

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Secondary growth in a dicot stem Fig. 9.15

Pith rays are composed of living parenchyma cells that allow materials to move laterally.

Cork cambium replaces epidermis with cork cells impregnated with suberin.

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Section of woody stem

Fig. 9.15

The bark of a tree contains cork, cork cambium, and phloem.

A woody stem has three distinct areas: the pith, the wood, and the bark.

Spring wood followed by summer wood makes up one year’s growth or annual ring.

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Annual Rings

This tree had a pith date of 256 BC and an outer ring of about AD 1320, making this tree nearly 1,600 years old when it died (it's about 3 feet across)!

(photo © H.D. Grissino-Mayer and R.K. Adams).

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Tree trunk Fig. 9.16

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Modified stems Fig. 9.17

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Organization of Leaves

Photosynthesis and Increased area for gas exchange

Helps prevent water loss

gas exchange

Photosynthesis

Fig. 9.18

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The body of the leaf is composed of mesophyll.

Parenchyma cells of these mesophyll layers house chloroplasts.

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Classification of leavesFig. 9.19

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Leaf diversity

The leaves of a cactus are spines attached to a succulent stem.

Climbing leaves are modified into tendrils.

The leaves of a few plants are specialized for catching insects.

Fig. 9.20