Page 15 - LN-cH-11

Two of them are given below
Boot Pressure
It is believed that all plants absorb excess of water by an active process and tends to
build up a positive hydrostatic pressure within the root system known as root
pressure. Due to this activity the water is pushed upward along the length of the
stem to a small height. The pressure inside the xylem is caused due to the diffusion
pressure gradient and is maintained by the activity of living cells.
Demonstration of Root Pressure in Plants
At the start of the experiment, choose a plant having soft stem on a day when there
is plenty of moisture in the atmosphere. Early in the morning cut the stem
horizontally near the base, i.e., just above the soil with a sharp blade. The moment
cut is made, few drops of solution start oozing out of the cut stem, which is due to
the positive root pressure. Now in order to determine the rate of exudation
(substances that oozes out) and the composition of the exudates (like sap, germs,
latex, etc.), fix a vertical glass tuBe filled with water, with the help of a rubber tube to
the cut end the stem. A column of sap is seen to ripe in the tube, which will be the
measure of the root pressure.
Root pressure is inhibited or reduced during reduced aeration, low or high
temperature, drought, etc.
Root pressure is maximum during early morning of spring and – rainy season when
the level of evaporation is low or minimum and decreases with the advancement of
day.
The magnitude of root pressure is about two bars or atmospheres.
Guttation
* When the amount of root pressure is high and rate of itranspiration is low, many
herbaceous plants tend to loose small quantities of water or liquid in the form of
drops from the hydathodes (small pores) or water glands. These are present on the
margins of the leaves or where the main vein ends and near the tips of blades of
some vascular plants like i i grasses and small herbaceous plants (rose, strawberry,
tomato, etc).
* This phenomenon water loss in its liquid phase is called guttation. It takes place
usually in early morning. The water that oozes out contains organic-and inorganic
substances.
Contribution of Root Pressure
As root pressure contribute a modest push in the overall water transport. The
enormous tension developed by transpiration, breaks the continuous chains of water
molecules in the xylem which gets re-establish by the root pressure. In this way root
pressure provides a great contribution in the transport of water.
Limitations to Root Pressure
Root pressure cannot account for the translocation of water or ascent of sap due to
the following reasons
(i) It cannot lift sufficient amount of water upward to meet the requirement of water.
(ii) It fails to play a role in water movement in tall trees like gymnosperms, etc.
(iii) The amount of the fluid transported by the root pressure is not enough in
measuring the movement of water in xylem in many trees.
(iv) Root pressure seems to be absent in summer when the requirements of water
are high.
Although it plays a major role in the transport of water through xylem in some plants