Plant cells are organised into
tissues and these form plant organs such as leaves, roots and stems.
These organs must function together
in such way to work as an organ system - to ensure the plant gets
all its needs to survive and grow into a mature plant.
Transportation of e.g.
nutrients or waste products is one of the most important function of a
plant organ system.
Plant organs are made of
tissues
(A
summary of those
you must know are briefly described below and in more detail later where
necessary)
Epidermal tissue
Epidermal tissue covers the
whole surface of the plant - its the equivalent of our 'skin'!
Meristem tissue
Meristem cells are found in
the growing tips of shoots and roots.
They can differentiate into
all the different types of plant cell needed for growth and
reproduction.
Palisade mesophyll tissue
Most photosynthesis occurs in
the palisade mesophyll tissue, part of the leaves.
Spongy mesophyll tissue
Spongy mesophyll tissue forms
part of the leaf and contains lots of air spaces to let gases
diffuse in and out of the leaf structure.
Xylem and phloem
Xylem and phloem are tubular
cell networks that allow the transportation of mineral ions,
food e.g. sugars and water around the plant - the leaves, roots
and stems must be all connected together.
Both xylem and phloem tissue networks are held
together in vascular bundles which give strength and
protection to plant tissues.
Waxy cuticle
The cuticle is
a water repellent protective layer covering the epidermal cells
of leaves and other parts and limits water loss.
More on the
tissue structures of a leaf and their functions - adaptations of leaves
These descriptions apply to dicotyledonous plants.
So, starting from the top layers, and
all marked on the above diagram ...
The epidermal tissue on the
upper side of the leaf are covered with a waxy cuticle layer
which is water repellent - this helps water loss by evaporation.
The upper epidermal layer is transparent to visible light, so light can penetrate to the
palisade cell layer where it is needed for photosynthesis.
The palisade mesophyll layer
is made of the palisade cells which are packed with lots of chloroplasts - the
sites of photosynthesis - note that the palisade cells are near the
upper surface to receive the most light.
The xylem and phloem are
networks of vascular sheathed bundles of cells that are the backbone
veins of the
plant's transport system .
The details of how all these function is
described in detail in
Part 2.
The leaf tissues are adapted for
efficient gas exchange.
The broad flat green leaves of plants
exposed to light, provide a large surface area for the light
absorbing sites of photosynthesis - more than the thinner stem.
The leaves are thin so the
absorbed carbon dioxide has only a short distance to diffuse to the
photosynthesising cells.
Leaves have veins (vascular
bundles of xylem and phloem cells) that support the leaf and transport water and minerals to
the leaves and glucose away from the leaves.
The lower epidermal tissue
is full of tiny holes (stomata, pores) which allow carbon dioxide to
diffuse into the leaf for photosynthesis.
Reminder: carbon
dioxide + water == light/chlorophyll ==> glucose + oxygen
The opening and closing of
stomata is controlled by guard cells which respond to changes
in environmental (ambient) conditions including the movement of
water in and out of leaves.
The spongy mesophyll tissues
contain air spaces which increase the rate of diffusion gases in
(carbon dioxide) and out (oxygen).
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