Transcript The Plant Body – Tissues and Organs

The Plant Body – Tissues and Organs
Cooksonia – 408 MYA
Plant Tissues
• Meristematic tissue - site of growth in plant; origin of the
other tissue types: apical meristems - site of primary
growth; lateral meristems - site of secondary growth
• Dermal tissue system - the outer protective covering of
the plant
• Vascular tissue system - comprises the xylem and phloem
- it is embedded within the ground tissue system
• Ground tissue system - the inner supportive tissues of the
plant - pith
Plant Meristems and Growth
• Primary growth is an increase in length – it
occurs at apical meristems
• Secondary growth is an increase in plant
diameter – it occurs at lateral meristems – in
particular the vascular cambium and cork
cambium
Dermal Tissues
• Make up outermost tissue layer of plants
• In young plants, it consists of a single layer
of cells – the epidermis – that may secrete
cutin to make protective wax layer of
cuticle
• May have hairs or trichomes
Trichomes
Stomata
• Scattered through the leaf
epidermis are openings
called stomata that allow
the plant to breathe
• The opening (pore) is
surrounded by two guard
cells
Periderm
• In woody plants the epidermis cracks and
splits and is replaced by periderm which is
formed by the cork cambium
• the periderm consists of cork cambium,
cork cells and some other cells – it is the
bark of mature trees • cork is mostly dead cells with cell walls
containing much suberin
Periderm
Ground Tissues
• Ground tissue makes up most of the tissues in herbaceous
plants
• There are several ground cell types which perform a
variety of functions
• Parenchyma cells – very diverse – often loosely arranged,
main location of photosynthesis and storage in leaves
• Collenchyma cells – main support tissue in young plant
stems – found in leaves, stems and petals – usually with
thickened corners of cell walls
• Sclerenchyma cells – can be either fibers or sclerids –
fibers provide support but are dead at maturity – thick
secondary cell walls; sclerids provide support as well
Tissues in an herbaceous stem
Mesophyll cells
are parenchyma
Parenchyma and Collenchyma
Sclerenchyma - Sclerids
Yucca leaf basket –
sclerenchyma fibers
Vascular Tissues
• Vascular tissues are responsible for
transporting material through the plant body
• Xylem cells move water and nutrients from
roots to rest of plant
• Phloem cells move carbohydrates and other
photosynthetic products from leaves to rest
of plant
Xylem
• Xylem is dead at maturity and transports water
essentially through a hollow tube - angiosperms
have tracheary cells are called vessels which tend
to have flattened ends, angiosperms also have
tracheids
• in gymnosperms the tracheary cells are called
tracheids and are usually sharply tapered
• eventually the xylem becomes full of sap and is no
longer used for water transport, then functions in
support and forms heartwood
Xylem –
Vessels
and
Tracheids
Phloem
• Phloem cells are called sieve tube elements
because of the sieve like plates at the end of
the cells - they are alive at maturity but are
crushed as the plant grows in diameter and
must be continually replaced
• Some sieve cells have companion cells
which govern transport of material through
the sieve
Phloem – sieve elements
The Roots
• Roots make up most of the underground
portion of the plant
• Roots anchor plant in soil
• Roots absorb water and nutrients
• Roots serve as storage organs – especially
storage of starch
Hornbeam Roots
Carrots – Storage Root
Carrot Flowers – Wild Carrot
aka Queen Anne’s Lace
Fibrous
Tap
Extent of Root Systems
Size of Root Systems
• The most well studied root system was for a
4 month old rye plant - its roots occupied a
volume of 6 liters
• When measured the total surface area of the
root system, including root hairs was 639
m2, or 130 times the surface of the shoot
• It had approximately 14 billion root hairs
with an absorbing surface of 401 m2 - if laid
end to end, they would extend over 10,000
km
Size of Root Systems
• Deepest known roots –
desert mesquite shrub
roots down to 53.3 m
• Tamarisk and Acacia
trees – roots to 30 m
deep
• Herbaceous Alfalfa –
roots to 6 m deep
Root Growth
• The growth of most roots is continuous
process that only stops under adverse
conditions such as drought or low
temperature
• During their growth through the soil, roots
follow the path of least resistance and
frequently follow spaces left by earlier roots
which have died and decayed
Mango Tree Root System
More Root Growth
• The tip of the root is covered by a root cap - a
mass of cells which protects the apical meristem
as it pushes through the soil
• As the root pushes through the soil, cells of the
root cap are sloughed off from the margins - they
are replaced by new growth of cells at the center
of the root cap
• The sloughed off cells and growing root tip are
covered by a slimy sheath called the mucigel
which lubricates the root as it passes through the
soil
Rhizosphere and Mucigel
• The mucigel provides an environment
favorable for growth of beneficial bacteria
• The rhizosphere is formed of the mucigel,
root hairs, sloughed off root cap cells and
various microorganisms
Mucigel Sheath
Root Absorption
• Epidermis in young roots absorbs water and minerals and
this is facilitated by root hairs - tubular extensions of the
epidermal cells
• Some roots have a thin cuticle covering the epidermis;
other roots have epidermal cells containing suberin - even
so, the epidermal cells offer little resistence to the entrance
of water and nutrients
• The innermost layer of the cortex is compact and lacks air
spaces - this is the endodermis - it has Casparian strips
which are bands of suberin between cells that prevent the
passage of water and air - thus in endodermis, all
substances must pass through cells