FACT
Turgidity, or turgor
pressure, refers to
the water content of
cells and how this
lends structural
support to the plant.
When cells absorb
water, the vacuoles
fill up and the
cytoplasm
increases, pushing
against the cell
membranes, which
in turn push against
the rigid cell walls.
This makes the cells
rigid, or turgid.Investigation: Observing annual tree rings to assess age and climatic conditionsAim:BackgroundEvery year a tree forms a new layer of xylem around the trunk. This forms tree rings, which
are visible as circles in a cross section of a tree that has been cut down. Each tree ring, or
wood layer, consists of two colours of wood; light wood that grows in spring and summer
and dark wood that grows in autumn and winter. Tree rings can be counted to give you
a rough estimate of the age of a tree. Occasionally a tree will form many rings in one
year or miss forming rings in a year. The width of the tree rings is greater in years where
good growing conditions occur. In years with droughts or low temperatures, the trees will
produce narrower rings. Therefore, by looking at the tree rings you can get an idea of the
weather affecting a tree in a particular year. Scientists can use this information to help
determine the weather patterns of the past as well as events such as forest fires, earthquakes,
and volcanic eruptions. The study of past events using the growth rings of trees is known as
dendrochronology ( dendros = tree, chronos = time).Method:- Examine a section of a tree trunk/stem provided by your teacher and count the tree
rings, starting with the innermost ring. Measure the width of each ring using a ruler, or
make a note of whether a ring is narrow or wide. Make a note of any scars caused by
events such as fires or pests. - Draw a bar graph showing the width of your tree rings for every year of the tree’s life.
- How old is this tree? What can you say about the climatic conditions throughout the
life of this tree?
6.3 Transpiration
Transpiration is a process that involves loss of water vapour through the stomata of plants.
Transpiration is thought to be a ’necessary cost or evil’ to allow the plant to absorb water
from the soil. It is an inevitable process.
Transpiration is important in plants for three major reasons:
1.Cooling of the plant: the loss of water vapour from the plant cools down the plant
when the weather is very hot.
2.The transpirational pull: when the plant loses water through transpiration from the
leaves, water from the stem and roots moves, or is ‘pulled’, upwards into the leaves.
Water is therefore taken up from the soil by osmosis and finally exits the plants through
the stomata.
3.Plant structure: young plants or plants without woody stems require water for struc-
tural support. Transpiration helps maintain the turgidity in plants.The two major features that cause xylem sap to flow within the plant aretranspirational pull
androot pressure.
Transpirational pull: results from the evaporation of water from the surfaces of the mesophyll
Chapter 6. Support and transport systems in plants 169