H. Ascent of SAP• Once the water is absorbed by the root hairs, it is

translocated to various parts of the plant.

• The fluid that moves upward in the stem is notpure water.

• It is a dilute solution of mineral ions absorbedfrom the soil and is referred to as sap.from the soil and is referred to as sap.

• The process of translocation of sap from theroots to the tap of the plant is called ASCENT OFSAP. OR

• The upward movement of water with dissolvedminerals from the roots to the aerial parts of theplant through the stem is called ASCENT OF SAP.

H.1. Path of Conduction of Water

• The water is conducted upwards bytracheary elements of xylem i.e. tracheidsand vessels.

• In young herbaceous plants almost all thetracheary elements participate in thistracheary elements participate in thisprocess but in large woody trees thetracheary elements of only sap wood arefunctional.

• Following expt. shows that the ascent of saptakes place through tracheary elements.

H.1.1. Experiment to show that the water is

conducted through the tracheary elements of Xylem.• Girdling or Ringing: It was first introduced by Malpighi.

• Stephen Hales was the first to prove by ringing experimentthat xylem was sole path for ascent of sap from roots to theleaves.

• It consists of removing a ring of bark i.e. all the tissuesoutside vascular cambium including the vital vascular tissueof phloem.of phloem.

• It is done as follows: 2 small twigs or branches are taken.Girdle, or a ring of bark is removed from one of thesebranches by a sharp knife.

• In the second twig, xylem is removed by carefulmanipulation without causing much injury to the bark, orthe xylem is blocked with wax or grease.

• Thus, the girdled part of the first twig contains only xylemand that of the second twig has all tissues except xylem.

• Both the twigs are placed in separate beakers containingwater.

• After some time leaves on the first twig appear turgidwhile on the second twig with no xylem, the leaves droop.

• The leaves appear turgid in the first case because thesecontinue to receive water in the presence of xylem.

• In the second twig, leaves wilt because there is noconduction of water due to absence or blocked xylemelements.

• Thus, this expt. shows that water is conducted throughtracheary elements of Xylem.

Expt. Showing path of ascent of sap using

Eosin. • A dilute solution of Eosin (red dye) is taken in a beaker

in which a Balsam twig is placed.

• After some time the veins of the leaves appear redand some parts of stem becomes pink.

• A section of the stem or a petiole shows that only thexylem elements have taken the stain indicating thatxylem elements have taken the stain indicating thatthe solution (water) moves through xylem.

• It is thus estd that the water is conducted throughtracheary elements of xylem i.e. trachea and vessels.

• These are dead cells with lignified secondary wall.

• The lumen of these elements allows an easy upwardflow of water.

H.2. Mechanism of Ascent of SAP

• To explain mechanism of Ascent of Sap

various theories has been put forward

like:

1). Root Pressure Theory1). Root Pressure Theory

2). Capillary Theory

3). Transpirational Pull Theory

H.2.1. Root Pressure Theory

• According to this theory, the root

pressure developed in xylem vessels

pushes up water to leaves and branches

in herbs.in herbs.

• This theory is not applicable in case of

tall plants.

H.2.2. Capillary Theory

• This theory explains that the xylem and

tracheids occur in the form of thin capillaries,

thus xylem capillaries apply a physical force

which helps in ascent of sap.

• This theory may be possible up to some

extent in small plants.

• The Ascent of Sap in tall plants is not

possible by this force.

H.2.3. Transpirational Pull Theory

• This theory was proposed by DIXON and JOLY (1894).

• Also termed as ‘Cohesion-Tension Theory’ because of

the main forces behind the ascent of sap are cohesion

and tension of water molecules.

• The main characters of this theory are:• The main characters of this theory are:

1). Continuous Water Column:

A continuous column of water exists from root to

leaves through stem mainly in tracheary elements.

Even if a few tracheary elements get blocked, the

other maintain the continuity of the column.

2). Cohesion:

Water molecules stick together by a strongforce of attraction called cohesion or cohesiveforce. This mutual attraction is due to thepresence of H-bonds between adjacent watermolecules.

3). Adhesion:

Water molecules also stick to walls of XylemWater molecules also stick to walls of Xylemvessels due to attraction. This is calledadhesion or adhesive force.

CohesionCohesion andand AdhesionAdhesion ensuresensures thatthat thethe waterwater

columncolumn isis notnot brokenbroken..

4). Development of Transpirational Pull:

Mesophyll tissue is present in leaves. The intercellularspaces amongst mesophyll cells of leaves is always saturatedwith water vapour.

These spaces come in contact with outside air throughstomatal opening.

Since the outside air is rarely saturated with water vapour.The vapour from the intercellular spaces diffuses out byTranspiration.

The mesophyll cells then absorb moisture from deeperThe mesophyll cells then absorb moisture from deepercells to overcome this loss of moisture. This creates the pull(tension) in the water column through stem to the roots.

Since the column is continuous it is pulled up like a ropedue to this tension.

Thus, when water transpires from leaves it pulls the watermolecules of the column upwards and water is able to rise upinside the plant. This pull is referred to as Transpirational pull.

Facts

• Root pressure is absent in

gymnosperms (some of the tallest

trees are gymnosperms).

• Maximum root pressure recorded in• Maximum root pressure recorded in

plants is 2 to 3 bars which is

sufficient to raise the water column

to a height of 21 metre only.