Ch#14 |Transportation | Lec #08 | ASCENT OF SAP (C) Imbibition | Bleeding in Plants (FSc Bio part 1)

Imbibition:
The cell wall components (especially cellulose, pectin and lignin) take up water, increase in volume but do not dissolve in water.
This is called imbibition.
It is another important force in the ascent of sap.
Sacks in 1874 suggested that the water molecules move along the cell walls of xylem vessels due to imbibition.


Imbibition forces (like dry cell wall and thicker protoplasm) may develop in the plant body. The dry cell walls can attract and absorb large amount of water.
The amount of water in the protoplasm is also increased.
The root cell walls imbibe by taking water from the soil.
This water moves by apoplast
pathway.

Reversible Process:
Imbibition is a reversible process. When water is lost, the original volume of the cell wall and of protoplasm is restored.
Imbibition In The Germinating Seeds:
The uptake of water by imbibition is very important in germinating seeds.
The volume of dry seed may increase up to 200 times by imbibition.
As a result the seed coat ruptures and the germination of seed effective.


Bleeding:
Flow of sap from the cut ends or surfaces of plants with considerable force is called bleeding.
It can be observed in certain plants when they are cut, pruned, tapped or wounded.
It can be observed in many land plants (mainly grape wine, some palms, sugar maple etc.) in the spring.
Although the flow of sap is usually slow a large quantity of the sap within 24hours comes out of the plant.
In some palms when tapped, there may be a flow of 10-15 litres of sap per day.
The sap in such plants contains sugar and water in addition to organic and inorganic substances (e.g. cells).

Factors Responsible For Bleeding:
There are two main factors responsible for bleeding:
(i) The hydrostatic pressure in xylem and phloem elements.
(ii) The root pressure which is exerted by the xylem tissues of roots.
The evaporation of water from the aerial parts of the plant especially through stomata of leaves is a process called transpiration.
Transpiration has very important role in the ascent of sap.

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