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Plant structure
Leaf structure
The leaf has three main sections to it.
1. Cuticle.
2. Epidermis.
3. Mesophyll.
1. Cuticle
2. Epidermis
3. Palisade
mesophyll
4. Spongy mesophyll
5. Epidermis
6. Cuticle
The cuticle:
Consists of a waxy material which improves
impermeability. It is produced by the epidermis cells.
Epidermis:
Consists of a thin layer of transparent cells that
allow for as much light transfer as possible towards
the palisade cells. It is also a protection.
Mesophyll:
Consists of two layers of cells with different
characteristics and functions.
The palisade cells (top layer):
Consists of one or several layers of elongated, narrow cells
with their long axes (perpendicular to the axis of the leaf)
and situated below the epidermis.
Spongy cells (bottom layer):
Consists of irregularly shaped cells which are located under
the palisade cells. They contain less chloroplasts.
There may be many intercellular air spaces between the
spongy cells which are interconnected and communicate
with the stomata of the lower epidermis. It allows food
producing cells from the leaf to access the gases needed
for photosynthesis and respiration.
Guard cells:
These are cells surrounding each stoma. They help to
regulate the rate of respiration by opening and closing
the stomata.
When the presence of potassium increases in the
guard cells, water is sent to them to dilute the
potassium. In doing so, the guard cells get “Fat”
(turgid) and allow the stomata to open.
As soon as the guard cells lose water, the cells
become flaccid and the stomatal opening is then
closed.
This usually occurs when light levels drop and the use
of CO2 in photosynthesis decreases.
Adaptation of the leaves
1. Their elongated shape allows for a wider surface area
of sunlight absorption.
2. They are thin, so gas exchange can happen quickly
because there is light space to cover.
3. The large spaces between cells inside the leaf provide
an easy passage of diffusion.
4. Many stomata in the lower surface of the leaf allows
the exchange of CO2 and O2.
5. Higher concentration of chloroplast on the upper layer
to insure photosynthesis.
6. Branching network of veins allows for a good water
supply
Plant nutrition:
- Transport system
- Transpiration
- Photosynthesis
Transport system in plants
The transport of nutrients (water, minerals and other
molecules) happens by means of two specialised
cell tissues, Xylem and Phloem.
Vital substances are moved from the source
(wherever it is taken from or is produced) to sinks
(where they are used or stored).
The xylem and phloem are both grouped into
vascular bundles. These bundles can be found in
several places in leaves and stem.
Xylem
Is a tissue that consists of tubes and transport cells
(vessels) that circulates water and dissolved minerals.
These vessels are elongated tubes, joined end to end, with
no content when mature. Their cell walls are thicker and
contain lignin. Lignin helps improve the impermeability and
strength of the tubes.
In essence these cells are dead (the lignin does not allow
for water diffusion into the cell).
Phloem
Consists of sieve tubes which form vertical columns. The
ends of each cell is perforated to allow for products of
photosynthesis to pass through. The walls are NOT
lignified
It transports food substances such as sugars away from
the leaf to other parts of the plant.
Vascular bundles
Water travels up the stem in the xylem from the roots to the
leaves.
However, food travels in any direction, up or down the stem
to where it is needed.
In the leaves, they are arranged in a cylinder a little way in
from the epidermis. Helps against wind movement
In the roots, the vascular bundles are in the centre to resist
against
wind.
Cortex and
pith.
The tissue between the vascular bundles and the epidermis
is called the cortex.
The central tissue in the Stem is called the pith.
Xylem
Phloem
Cortex
Endodermis
Cross section of the root
Roots
The root has no distinct epidermis. The root tip (root cap)
has cells that are continuously being replaced because
they get damaged as they push through the soil.
The root also has root hairs which increase the surface
area for absorption. These are also renewed continuously.
There are two types of roots.
Tap root
Fibrous root
A single root grows
vertically down into the
soil.
Several roots grow at the
same time. There is not
distinguishable root.