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Biology Olympiad Training Program 2006 1


PLANT STRUCTURE


Basic plant organs Roots anchor the plant in the soil, absorb nutrients

and water, and store food.

Stems support and bear the leaves and buds of the plant.

Some green stems can photosynthesise.

Leaves are the main photosynthetic organs of most

plants.


Modified shoots

Stolons – asexual reproduction. e.g. strawberries

Rhizomes – horizontal underground stems. e.g. grass

Tubers – food storage. e.g. potatoes

Bulbs – food storage. e.g. onion Biology Olympiad Training Program 2006 5

Modified leaves

Tendrils – clinging supports. e.g. peas

Spines – self defence. e.g. cacti

Succulent – water storage. Bright leaves – attractant.


Angiosperms - Monocots and dicots

Monocots

Dicots

Seeds/ Embryos

Leaf venation

Stems Roots Flowers


Plant tissues

Dermal – generally single celled layer that covers and protects young plant

parts.

Vascular – conducting tissue, composed of xylem and phloem.

Ground – all tissues found between the dermal and vascular systems.

All plant organs are made of 3 tissue types:



Dermal tissue

Dermal tissue, or the epidermis, is generally a single layer of tightly packed cells.

Functions in:

Protection of young parts of plant. Absorption of water and nutrients (root hairs).

Water retention (secretion of waxy cuticle).


Vascular tissue - xylem Xylem is made of tracheids and vessel elements. They are dead at functional

maturity.

They transport water and dissolved minerals upwards from the roots into

the shoots.

Tracheids are long, thin cells with tapered ends, found mainly in simple

plants

Vessel elements are wider, shorter, thinner walled and less tapered, mainly

in complex plants


Vascular tissue - phloem Phloem is made of sieve-tube

members. They are alive at functional maturity.

It transports sucrose, organic compounds, and some mineral ions

up and down the plant.

One companion cell accompanies each sieve-tube members.

Sieve plates are porous walls between sieve-tube members.


Ground tissue Ground tissue is tissue that is neither dermal nor vascular.

It is more than just filler. It also functions in photosynthesis,

storage, and support.

In dicot stems, ground tissue is divided into the pith, internal to the vascular tissue, and cortex, external to the vascular tissue.


Cell types

All plant tissues are made of 3 basic cell types:

1.  Parenchyma – ‘typical’ cells.

2.  Collenchyma – growth support.

3.  Sclerenchyma – support.

A major difference is in their cell wall structure.


Plant cell review Plants have chloroplasts

(sites of photosynthesis), a central vacuole (storage of

substances) surrounded by the tonoplast, and a cell wall (maintaining cell and plant

shape).

The protoplasm is the part of the cell interior to the cell

wall, i.e. the plasma membrane, the cytoplasm,

and the nucleus.



Parenchyma Parenchyma have thin, flexible primary

walls and no secondary walls.

They are the least specialised of the 3 cell types. Their protoplast typically has a large

vacuole.

Parenchyma perform most of the metabolic functions of the plant – eg

photosynthesis, storage of organic compounds, etc.


Collenchyma Collenchyma have unevenly thickened primary walls, and lack secondary walls.

Mature collenchyma cells are living, flexible, and elongate with the stems and

leaves they support.

They act as support for young parts of the plant shoot, that

are actively growing


Sclerenchyma Sclerenchyma have thick secondary

walls hardened by lignin.

Mature sclerenchyma cells are dead at functional maturity and cannot elongate.

Tracheids and vessel elements (of xylem) are made of sclerenchyma.

Fibers are long, slender, and tapered, and usually occur in groups.

Sclereids are short and irregular in shape.


PLANT GROWTH


Growth and development

Growth – irreversible increase in mass resulting from cell division and cell expansion.

Development – sum of all the changes that progressively elaborate an organism’s body.


Meristems Meristems are tissues that are

perpetually embryonic (i.e. they are totipotent cells)

Apical meristems supply cells for growth in length – primary growth.

Lateral meristems supply cells for growth in girth – secondary growth.

Found mainly in woody dicots.



Primary growth of roots

Root cap

Zone of cell division

Zone of elongation

Zone of maturation

Apical meristem Procambium Ground meristem Protoderm

Cortex Stele

Root hairs

Epidermis


Primary tissues of roots - monocots Epidermis

Cortex

Endodermis

Pericycle

Pith

Xylem

Phloem

Stele


Primary tissues of roots - dicots Epidermis

Cortex

Endodermis

Pericycle

Xylem

Phloem

Stele


Primary tissues of stems - monocot

Epidermis

Ground tissue

Xylem

Phloem

Vascular bundle


Primary tissues of stems - dicot Epidermis

Cortex

Pith

Sclerenchyma fibers

Xylem

Phloem Ray

Vascular bundle


Tissue organisation in leaves Cuticle Collenchyma

Upper epidermis

Spongy parenchyma

Palisade parenchyma

Lower epidermis

Cuticle Vein

Stoma Phloem Xylem

Guard cells

Air spaces



Secondary growth of stems

Vascular cambium Secondary phloem

Secondary xylem

Growth ring

Vascular cambium gives rise to secondary xylem and phloem for lateral growth


Woody stems

Heartwood

Sapwood

Vascular cambium

Phloem

Cork cambium Cork Periderm

Bark


Mechanisms of growth

Growing plant cells expand mainly through water uptake into the central

vacuole.

Cell growth is mainly orientated in the plane perpendicular to the

orientation of cellulose microfibrils in the cell wall.


PLANT REPRODUCTION


Flower structure Petal

Stigma

Style

Ovary

Sepal

Carpel

Anther Filament Stamen

Ovule


Types of flowers Complete flower – has all 4 floral organs.

Incomplete flower – missing one or more of the organs.

Bisexual (perfect) flower – contains both stamens and carpels.

Unisexual (imperfect) flower – contains either stamens or carpels. Flowers are either staminate or carpellate.

Monoecious plant species have staminate and carpellate flowers on the same plant.

Dioecious plant species have staminate and carpellate flowers on separate male and female plants.



Different structures of flowers

Composite flower e.g. sunflowers

Inflorescence


Angiosperm sexual life cycle Sporophyte

(2n)

MEIOSIS of ovule cells

MEIOSIS of anther cells

Pollen grain (n)

POLLINATION

GROWTH of POLLEN TUBE

FERTILISATION

SEED and FRUIT FORMATION

GERMINATION

Egg (n)


Pollination

Self-pollinated

Cross-pollinated

Cross pollination may be favourable because it

increases genetic diversity and cuts down on

potentially hazardous inbreeding which may

result from self-pollination.

Plants counter self-pollination by:

Structure of flower. Being dioecious.

Sexes maturing at different times. Self-incompatibility.


Seeds One seed develops from one ovule after it is fertilised.

Dicot seed

Monocot seed

Embryo Food supply

Seed coat

Seeds are resistant structures that helps in

plant dispersal.

They can resist harsh environments, allowing the young plant to wait

for favourable conditions before germinating.


Fruit Whilst seeds are developing, the ovary forms the fruit.

Dry fruit

Fleshy fruit

Function in dispersal and protection of enclosed seeds.

Many types of fruit – made of one or more

mature ovaries.


Germination Resumption of growth after a period of dormancy of the

embryonic plant inside the seed.

Requires:

•  Correct temperature. •  Imbibition (taking in) of water. •  Oxygen. •  Preceding period of dormancy

(sometimes). •  Proper photoperiod (amount of light

per day) – almost all seeds

After breaking to the surface, the young leaves turn green and start to photosynthesise, making food and continuing growth.



Asexual reproduction

Also known as vegetative reproduction.

Offspring are genetically identical clones of the parent plant – due to

formation of offspring through mitosis only.

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