Chapter 46 - Plant Structure

Monocots Vs Dicots

Roots and Shoots

Roots anchor, absorb H₂O and minerals, and store starch.

The shoot is the above-ground portion of the plant. The stem transports water and minerals to the leaves and sugar to the roots.

Tissue types

Plants have four tissue types.

Vascular tissue transports, dermal tissue protects, meristematic tissue grows (cells divide), and ground tissue forms the rest of the plant.

Vascular tissue

Xylem

transports water and minerals from roots to leaves

composed of hollow, nonliving cells

tracheids- elongated with tapered ends, pits or depressions

vessel elements- larger, forms a continuous pipeline

Phloem

Transports organic nutrients, usually from leaves to roots

Phloem cells are living.

sieve-tube cells- no nucleus; connected to each other by plasmodesmata

Sieve plates are found at the ends of the cells.

Companion cells contain a nucleus and are located in close proximity to sieve-tube cells. They are connected to sieve tube cells by plasmodesmata.

Meristematic tissue

Areas within the plant that are capable of growth (cell division) are called meristems.

Primary Growth

Primary growth occurs only at the shoot and root tips in areas called apical meristems. Primary growth is responsible for elongating the plant. In areas that contain only primary growth, stem thickness increases by cell enlargement, not by the production of new cells.

Secondary Growth

Lateral meristems produce new cells that make the stems and roots thicker. This type of growth is called secondary growth. Secondary growth occurs only during the second and subsequent years and only in woody species.

There are two kinds of lateral meristems, the vascular cambium and the cork cambium. These lateral meristems form as rings within the plant body as the stem increases in thickness. The diagrams below illustrate how the vascular cambium divides to produce new xylem cells toward the inside of the vascular cambium and new phloem cells toward the outside.

	The vascular cambium cells divide longitudinally.
One of the new cells remains vascular cambium and the other becomes xylem. The cells can be seen enlarging in this diagram.
	Occasionally, the inner cells remain vascular cambium and the outer ones become phloem.

Dermal tissue

outer covering of plant

It consists of closely packed cells that function to protect.

The epidermis covers the plant but is replaced by cork (periderm) in the stems and roots of woody plants.

The epidermis has a waxy covering called a cuticle that protects the plant from desiccation.

The periderm is the outer part of the bark.

Ground tissue

Ground tissue fills interior of plant. It contains parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma

thin-walled

least specialized of the three cell types

found in all organs

usually functions in photosynthesis or storage

photosynthetic parenchyma have chloroplasts

parenchyma that function for storage have colorless plastids

Parenchyma can divide to produce more specialized types of cells.

Collenchyma

Collenchyma cells have thicker primary cell walls, especially at the corners.

A primary cell wall is one that is produced while the cell is growing.

Collenchyma often forms bundles just beneath the epidermis for flexible support of immature parts of the plant body.

Sclerenchyma

Sclerenchyma cells have thick secondary cell walls, usually toughened with lignin.

A secondary cell wall is one that is produced after the cell is mature. It is produced inside the primary cell wall.

Most sclerenchyma cells are nonliving. They function to support mature regions and produce hard parts (example: nut shells).

omit: contain fibers (long and slender) and sclereids (shorter, varied shape)

example: sclereids make nut shells hard

Primary growth of stem

Primary growth occurs only in apical meristems which are located at the tips of the stems and roots.

Meristems in stems are protected by newly formed leaves within a bud.

Axillary buds

usually dormant

in the axes of mature leaves

develop into branches

Herbaceous stems (nonwoody

Herbaceous stems are produced by primary growth.

The outermost tissue is epidermis and is covered by waxy cuticle to prevent water loss.

The vascular tissue is found in bundles that are arranged in a ring (dicots) or scattered (monocots).

In dicots, the xylem is toward inside; the phloem is toward the outside.

cortex- In dicot stems, the cortex is located in the area between the vascular bundles and the epidermis. In monocot stems, it occupies the area surrounding the vascular bundles.

The center of the stem is pith and may function as storage.

Secondary Growth of Stems

Secondary growth occurs in plants that live > 1 year.

primary growth occurs for a short distance behind the apical meristem, then secondary growth occurs.

It begins with the formation of a vascular cambium and a cork cambium.

ebVascular cambium

Initially, vascular cambium is found between the xylem and phloem in the vascular bundles of dicots.

After one years growth, it joins to form a continuous ring.

Cell division toward the inside and outside form xylem and phloem.

Seasonal climates produces growth rings because cells grow faster and are larger in the spring than later in the growing season.

Cork cambium

Cortex cells beneath the epidermis produce the cork cambium.

The cork cambium produces cork.

Cork is waterproof because the cell walls are impregnated with of suberin.

Pockets of cells lack suberin. These are called lenticels and function to allow gas exchange.

Cork replaces the epidermis on woody stems and roots.

Bark

The bark of trees consists of cork, cork cambium, cortex, and phloem.

Summary of Stem Growth

Stem External Structure

Stems support, conduct, store water and photosynthate (products of photosynthesis).

nodes- where leaves attach

internodes - between nodes

bud - contains apical meristem and newly-forming leaves

leaves - photosynthetic organs

Types of stems

stolons (runners)- horizontal, aboveground- strawberries

rhizomes- horizontal, underground; responsible for rapid spread of many weeds

tubers- enlarged tips of rhizomes; food storage- potato

corms- underground, short, thick, vertical; food storage- gladiolus

bulbs- underground with thick, fleshy leaves- onion

tendrils- assist plant in climbing

Leaves

Leaves usually function in photosynthesis, so they are flattened to increase the surface exposed to light.

blade, petiole

simple, compound

pinnate, palmate

opposite,alternate, whorled

vary according to environmental conditions

broad in shade

reduced in dry areas (ex: spines in cacti)

succulent leaves hold water

can be adapted for food storage (onions)

climbing leaves can be modified as tendrils

Monocot leaves have parallel veins; dicot leaves have a net-like pattern.

The top layer is the epidermis, a type of dermal tissue. It often has protective hairs and/or glands that produce irritants.

always a waxy cuticle

mesophyll: parenchyma cells w/ chloroplasts

Stomata

Stomata (sing. stoma) are openings in the epidermis of leaves and stems that allow gas exchange.  Guard cells surround the opening and function to open or close it.  Guard cells that contain chloroplasts, other epidermal cells do not contain chloroplasts

When K⁺ is pumped into the guard cell by active transport (requires ATP), water follows by osmosis. This causes the cells to bend and open. When K⁺ (and water) leaves the guard cells, they close.

C₃ plants

palisade layer and spongy layer

The loss of water from the leaves by evaporation is called transpiration. It accounts for more than 90% of water taken up by the roots.

Dicot roots

zone of cell division- root apical meristem; just behind the root cap

zone of elongation- area where cells elongate; become more specialized

zone of maturation (differentiation)

cells mature and become fully differentiated (specialized)

The epidermal cells form root hairs in this zone. Root hairs increase the absorptive surface area.

Specialized tissues of roots

Epidermis

outer layer consisting of rectangular-shaped cells

Root hairs are extensions of epidermal cells that project 5-8 mm into the soil. They increase the surface area of the root for absorption.

Cortex

interior to epidermis

large thin-walled parenchyma; loosely packed

water can move through cortex without entering cells

starch granules in cortex function for storage

Vascular tissue

xylem- star-shaped

phloem- between rays of xylem

Endodermis

The endodermis is a single layer of cells that forms a boundary between the cortex and the inner vascular cylinder.

The endodermis is lined on 4 sides by the Casparian strip. The casparian strip is a coating that prevents water from seeping between the cells and thus forces water to enter the endodermal cells before passing through to the vascular cylinder.

The Casparian strip surrounds cells of the endodermis and prevents water and minerals from seeping between the cells. In order to get to the vascular cylinder, water and minerals must pass through the cell membrane.		Without the casparian strip, water and minerals would be able to enter the vascular cylinder by going between cells.

Pericycle

The pericycle is the layer just inside the endodermis.

It retains the capacity to divide and form branch roots.

Monocot roots

Unlike monocot stems, the vascular tissue in monocot roots is arranged in a ring.

Monocot roots are like dicot roots in that they contain pericycle, endododermis, cortex (outside of vascular tissue), and an epidermis.

The central portion of the root is called pith. It is composed of parenchyma and functions in storage.

Monocot roots typically have no secondary growth.

Secondary growth in roots

Secondary growth in roots is similar to stems; annual growth rings are formed.

Vascular cambium forms between xylem and phloem.

The pericycle produces the cork cambium.

Primary Growth in a dicot root.
As the root increases in diameter, the vascular cambium becomes circular.

Summary of Root Growth

Root Systems

Dicots

the primary (first) root grows straight down; called a taproot

often fleshy and stores food; ex: carrots, beets, turnips, radishes

Monocots

fibrous root system, no main root

Adventitious roots- new roots that arise from an aboveground structure; example- prop roots on corn