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