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TISSUES WORKING TOGETHER
• The primary function of the leaves of most plants is to perform photosynthesis.
• In order for photosynthesis to occur, plants need
– energy in the form of sunlight
– carbon dioxide
– water
• Photosynthesis uses the above items to produce glucose and oxygen. Glucose may
then be converted into carbohydrates, including complex sugars. The carbohydrates, in
the form of starch, are used as a source of stored chemical energy.
• Sugar is needed by all parts of a plant. Plants must transport sugar and oxygen from their
leaves to the other plant parts for cellular respiration.
• Leaves produce more oxygen than a plant can use, so the excess is released into the air.
4.4
TISSUES WORKING TOGETHER
• Most leaves are green and thin, making them ideal for
absorbing light. Wide, thin leaves have a large surface
area for absorbing light. The green colour is produced
by chlorophyll, which absorbs light to begin the
photosynthesis process.
• Chlorophyll is contained in cell organelles called chloroplasts. Chloroplasts
conduct the chemical process of photosynthesis. Chloroplasts are located mostly in
the palisade layer and the spongy mesophyll. Review the diagram to see where
the cells are located.
• Notice that some of the cells are tightly
packed and others are not. Why do you
think these structures have developed
differently?
4.4
palisade layer
spongy mesophyll
• The leaf epidermis contains many tiny openings called
stomata. (stomate = one)
The stomata allow gas exchange and the release of
water vapour.
• Stomata are usually located on the lower surface of a leaf,
which
– reduces water loss
– provides more surface area for photosynthesis
– reduces the chance of airborne viruses, bacteria, and fungal
spores entering the leaf.
• When stomata are open, carbon dioxide enters and oxygen leaves the leaf.
Water vapour also escapes when stomata are open.
TISSUES WORKING TOGETHER 4.4
stomata
cuticle
guard cells
TISSUES WORKING TOGETHER
• The cuticle is a layer of wax on the upper and lower
surface of a leaf. It prevents the release of too much
water vapour, which would cause the leaf to dry out.
The cuticle also prevents gases from entering the leaf
by diffusion through the surface cells.
• Each stomate is surrounded by a pair of special
epidermal guard cells that control its opening
• and closing.
• Guard cells change their shape to respond to water
• levels in a plant. They also close at night when
• carbon dioxide is not needed for photosynthesis.
4.4
stomata
cuticle
guard cells
TISSUES WORKING TOGETHER
• Long, thin epidermal cells on the roots of a plant (called root hairs) absorb water
from the soil into the plant by a process called osmosis. Water is then
transported by the xylem (vascular tissue) from the roots, up the stem, to the
leaves.
• Although plants and animals are very different organisms, they have
similarities at the cellular level. Plant and animal cells need to perform some
of the same processes, such as respiration. In fact, both plant and animal cells
use sugar and oxygen in the process of respiration.
• Animals and plants both possess systems that must work together to
accomplish complex tasks.
4.4
PLANT GROWTH
• Cell division occurs only in certain parts of a plant.
Most differentiated plant cells cannot divide further.
However, plants will continue to grow for as long as they live.
• Apical meristems are undifferentiated cells located at the
tips of plant roots and shoots. These cells divide and
enable the plant to grow longer and develop specialized
tissue.
• The diagram shows three distinct regions of the
growing tips of roots on a plant.
4.6
apical meristems
PLANT GROWTH
• When meristem cells divide, they become elongated. This makes roots longer
and enables them to push their way through the soil. Cells in root tips may grow
10 times longer than their original size.
• In the region of maturation, elongated cells differentiate into specialized cells
of the dermal, ground, and vascular tissue systems. After this process is
complete, most cells can no longer grow or divide.
• Apical meristems also occur in buds at the very tip
of growing stems. They also occur along stems,
giving plants the ability to grow side branches off
main stems.
4.6
PLANT GROWTH
• Lateral meristems are undifferentiated cells located under
the bark in the stems and roots of woody plants, such as
trees. These cells divide and enable the plant to grow wider
and develop specialized tissue in the stem.
• Lateral meristems form two cylinders—one inside the other—that run the full length of
roots and shoots. As the tree grows in diameter, the outer lateral meristem produces new
dermal tissue called cork. The cork replaces the
old epidermal cells.
• The inner lateral meristem produces new
phloem tissue on its outer surface and new
xylem tissue toward its centre.
• The phloem and cork form the bark of the
tree. The rings of xylem form the interior of the
tree trunk. The accumulation of xylem produces
a ring, which is used to determine the age of a tree.
4.6
lateral meristems
Heart woodHeartwood ... in older trees, as new growth rings of sapwood are formed from the outside, inner rings of older sapwood, blocked with resins, become heartwood. Heartwood can no longer carry fluids, but its stiffness helps support the tree at the centre. Heartwood is generally darker than sapwood.
PLANT GROWTH
• Clones are individuals that are genetically identical
to each other. Many plants produce clones of
themselves. This is called vegetative reproduction.
Strawberry plants produce clones when they send
out shoots called runners across the surface of the
soil.
• The natural cloning ability of plants has been used for years by farmers and
gardeners to produce new plants.
• Agricultural scientists use a process called tissue culture propagation to grow
identical plant offspring. The scientists obtain individual plant cells from one
parent plant and then grow them into calluses (clumps of cells), and finally
into whole plants. Reproduced plants are genetically identical to their parent.
4.6
vegetative
reproduction
tissue culture
propagation