PLANT TISSUES

Non Permanent Tissues -

Meristematic Tissue

• Undifferentiated plant cells that

are continually dividing by mitosis

• Large thin walled cells

• No vacuole

• Dense cytoplasm

• Large nucleus

• Found at growing tips of roots and stems to increase length of these structures – apical meristem

• Also as cambium which causes increase in width of stems and roots

• Function: to produce new cells for growth, repair and replacement of damaged or lost parts or organs

Meristems

• Apical meristems

At the tips of main and lateral shoots and in

tips of roots. Responsible for primary

growth and length.

• Lateral meristems

Parallel to the stem or the root e.g.

vascular cambium and cork cambium.

Responsible for increase in diameter

(secondary growth).

Permanent tissues

• Mature tissues of the plant

• Derived from meristematic tissue by cell

enlargement and differentiation

• Simple tissues: same cells that perform

the same function (parenchyma,

collenchyma, sclerenchyma)

• Complex tissues: different cells that

perform more than 1 function

(epidermis, xylem, phloem)

Permanent Tissues

• These tissues are grouped into three tissue systems

• 1. dermal tissue: covers the plant body

• 2. ground tissue: perform a variety of functions and make up the bulk of the plant body

• 3. vascular tissue:responsible for transport in the plant

Vascular tissue

• Conducts liquids around the plant

• Xylem transports water and inorganic

salts (strength and support)

• Phloem conducts food (sugar and

organic solutes)

Xylem • Four types of tissues:

– 1. tracheids

– 2. vessels

– 3. xylem fibres

– 4. xylem parenchyma

–Tracheids are elongated

dead cells with tapered ends

(not continuous)

–Cell walls are thickened with

lignin causing the protoplasm

to die leaving a lumen

–Water is transported from

tracheid to the next through

pits in the walls

Xylem

• Function: transport water and inorganic salts from the roots to the leaves

• Vessels are elongated tubes that extend sometimes meters in length (dead cells)

• Walls are lignified in different ways (strength for woody tissues) – Annular rings

– Spiral or helical

– Pitted

– Scalariform

– Reticulate

• Long, thin, perforated, hollow tubes stacked on top of each other

• This allows water to flow up stems to leaves as ascending sap – good water conduction

Xylem vessels

• Annular thickening:

2dry wall is laid over the

primary wall in rings. Allows for

stretching.

• Spiral/ helical thickening:

2dry wall laid down in the form

of spirals. Allows vessels to

stretch or bend.

• Pitted thickening:

2dry wall laid down everywhere

except small areas that form

pits. Pits allow movement of

water between cells.

• Scalariform thickening:

2dry wall covers the entire

primary wall except for

unthickened parts which are

left in parallel lines (looks like

a ladder).

• Reticulate thickening:

Like scalariform except

unthickened parts look like a

net.

Xylem

• Xylem fibres are

dead cells with

heavily thickened cell

walls. They do not

conduct water but

strengthen and

support the plant.

• Xylem Parenchyma

is the only living

tissue in mature

xylem. Walls are

thicker than ordinary

parenchyma cells.

Store starch and

water and can

conduct solutes for

short distances.

Phloem tissue • Phloem fibres and cells are

found in vascular bundles

in roots, stems and leaves

• Phloem transports

nutrients made in the

leaves to all parts of the

plant

• This is known as

descending sap

• 4 tissues:

– Sieve tube

– Companion cells

– Phloem fibres

– Phloem parenchyma

Phloem tissue• Consists of elongated cells

called sieve tubes

• The walls of the cells are

thickened with lignin

• Pores/ perforations in the walls

form a sieve-like structure

• Mature sieve tubes lack nuclei

and have thick slime in the

vacuole

• The conducting elements of

phloem

• Thin cellulose walls lined with

layer of cytoplasm. Few

organelles and no nuclei.

• Mass of tubules in the centre

called P-protein

• Cross walls between sieve

cells are perforated and are

called sieve plates and sieve

pores, respectively

Phloem

• Each sieve tube has a

companion cells.

• Smaller than sieve

cells and connected

to them by

plasmodesmata

• The companion cells

contain nuclei, which

controls the functions

of the sieve tubes

• Phloem fibres are

strengthening tissue

• Cell walls are lignified

• No water conduction

• Can form protective

cap

• Phloem parenchyma

are living cells that

store starch

Dermal Tissue• Epidermis is the outermost

layer of the plant

• Protection from drying out and mechanical injury

• These cells are:

• square shaped (bricks)

• closely packed with no air spaces

• Transparent (lack chlorophyll) –allow sunlight to penetrate inner tissues for photosynthesis

• Usually only a single layer of cells

• Thicker outer cell walls for mechanical protection

Modified Epidermis• Stem and leaf epidermal cells are modified to secrete a

waxy substance called cuticle

• The waxy cuticle reduces the amount of water lost by the aerial parts of the plant

Modified Epidermis

• Guard cells are bean-shaped cells with a

thick inner wall and thin outer wall

• They also contain chloroplasts allowing

them to regulate the opening and closing of the stoma

• This allows substances like water vapour, CO² and O² to

move in and out of leaves

Modified Epidermis

• Epidermis is different in roots

• No cuticle as roots need to absorb

water

• No guard cells and stomata

• Cells have microscopic extensions

called root hairs

Modified Epidermis

• Root hair cells have small outgrowths of the cell wall

• increases theabsorptive surface area

• The vacuole extends into the root hair to absorb more water

• Small enough to fitbetween soil particles

Epidermal tissues

Summary of epidermal functions

• Protects plants against injury and

desiccation

• Cuticle prevents water loss

• Guard cells regulate gas exchange

and transpiration

• Root hairs absorb water and

mineral salts from the soil

Parenchyma

• Most abundant plant tissue:

• Large thin walled living cells

• Centrally situated nucleus

• Large vacuole

• Many plastids i.e. chloroplasts and leucoplasts

• Parenchyma cells containing chloroplasts are referred to as chlorenchyma

• Large intercellular air spaces between cells which allows gases to circulate

• Found in roots, stems and leaves

• Main function: storage and photosynthesis, gas movement, packing tissue

Types of parenchyma

• Spongy parenchyma:

– Large round cells

– Loosely packed with many airspaces

– Chloroplasts and leucoplasts

– Found in roots, stems and leaves

– Store substances like starch and water in fleshy organs

Types of parenchyma

• Palisade parenchyma

• Elongated cells arranged vertically

• contain many chloroplast

• Cytoplasm streams to ensure all chloroplasts receive maximum sunlight

• Main photosynthesizing tissue in leaves

• Aerenchyma

• Large air spaces in plants growing in water–logged soils

Functions of parenchyma

• Packing tissue

• Stores water and nutrients

• Air spaces allows for water

movement, photosynthesis and

respiration

Collenchyma • Spherical cells with cell

walls thickened in the

corners

• Long living cells

• Larger than parenchyma

cells

• Contain chloroplasts

• Usually found in young

growing stems

• Support and strength but

allows stretching

• To prevent collapse of

vascular tissue

• to prevent collapse

Sclerenchyma

• Cells with very thick cell walls

• Thickening due to lignin and sometimes suberin

• This causes the contents of the cells to die off leaving a hollow cavity or lumen

• Occur as stone cells or fibres

• Provide support and protection around vascular bundles, fruit, seeds and pips, twine, rope, hemp, linen and other textiles

Sclerenchyma• Fibres

• Elongated cells with

pointed tips

• Thick cells walls

• Mostly found in

vascular tissue

• Tough, strong and

flexible

• Occur in groups with

interlocking ends

• Used to make various

textiles and linen

• Stone cells/ sclereids

• Do not interlock

• Lignified cell walls

• Provide rigidity and

hardness