ASCENT OF SAP

ASCENT OF SAP

(or Ascent of water)

In order to reach the topmost

parts of the plant, the water has to move upward through the

stem. This upward movement of

water is called as Ascent of Sap.

✹ When the water is absorbed by the roots, it is carried to various parts of the plant.

✹ The fluid that moves upward in the stem is not pure water.

✹ It is a dilute solution of mineral ions

absorbed from the soil and is referred to

as sap ^.

Giant Red-wood trees in California

The tallest tree in the world is the coast

redwoods

(Sequoia sempervirens)

More than 100 meters tall

Equivalent to 40-Storey Building

Redwoods trees in California

A towering example of Eucalyptus delegatensis in Tasmania (Australia)

114 m tall

One of the

world's tallest known tree

Eucalyptus regnans

in

Tasmania,

Australia.

Trees transport water by a

mechanism

unparalleled in engineering

science.

The upward movement of water in plants is called ascent of sap. This phenomenon

occurs through the xylem tissue. The water absorbed by hairs passes through the cortex, passage cells and pericycle enter the

tracheary elements of xylem. Water is lifted

up to leaves as a continuous column in the

tracheary elements of xylem by virtue one

or more forces like root pressure, cohesion

and adhesion of water and transpiration pull

acting concurrently.

This continuous and unbroken rising of water (along with

soluble inorganic mineral

salts) is called the ascent of

sap.

If a ring of bark from a cut twig is removed

from the stem and it is immersed in water.

Then after sometime, it is observed that the

leaves above the ringed

part of the stem remain

fresh and green. It is

because water is being

continuously supplied

to the upper part of the

twig through xylem.

Path of ascent of sap

It is well-believed that the ascent of sap takes place through xylem.

In herbaceous plants almost all the

tracheary elements participate in

the process, but in large woody

trees, the tracheary elements of

only sap wood are functional.

Capillary

water Root hairs Cortex

EndodermisPericycle

Stem xylem Leaves

Transpiration

Root xylem

Further, it has been proved

experimentally that sap moves up the stem through the lumen of

xylem vessels and tracheids and

not through their walls.

Theories of Ascent of Sap

A number of theories have been

proposed to explain the mechanism of ascent of sap from time to time.

Some of the important theories are

given as following.

Theories of Ascent of Sap

Vital Force Theories (a) Godlewski (1884)

By pumping activity

(b) Jansen supported Godlewski

(c) Strasburger (1893) Ascent of sap

continues even if cells are killed by poison

(d) J. C. Bose gave

‘Pulsation Theory’

Root Pressure Theory (a) Stephen Hales

(1727)

(b) Priestley (1727) the elastic walls of the cortical cells, exert pressure and force them towards the xylem vessels.

Physical Force Theories (a) Capillary Theory*

Involvement of tracheids & vessels By Boehm(1804)

(b) Imbibition Theory Unger & Sachs (1878) (c) Cohesion Theory/

Cohesion –Tension*

Theory / Transpiration Pull Theory*/ Dixon &

Jolly Theory (1895).

Further supported by Kramer and Kozlowski (1960).

Capillary action

Tracheids

Tracheids are elongate, hollow, dead cells with

highly lignified walls. The walls contain numerous pits—regions where

secondary wall is absent but primary wall remains.

Tracheids are present in all vascular plants.

Pits

Vessel elements

Vessels consist of a stack of two or more vessel elements.

Like tracheids, vessel

elements are dead cells and are connected to one another through perforation plates—

regions of the wall where pores or holes have

developed. Vessels are found

in most angiosperms and are

lacking in most gymnosperms.

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What are Adhesion and Cohesion?

• Adhesion occurs when water forms hydrogen bonds with xylem cell walls.

• Cohesion occurs when water

molecules hydrogen bond with each

other.

Figure 36.2-1

H₂O and

minerals

H₂O

Transpiration Pull

Water and minerals in the soil are absorbed by the roots.

Transpiration, the loss of water from leaves (mostly through

stomata), creates a

force within leaves

that pulls xylem sap

upward.

Criticism to Different Theories

Strasburger (1891) and Overton (1911) used poisons (like picric acid) and excessive heat to kill the living cells of the plant. When such twigs were dipped in water, ascent of sap could still occur uninterrupted. This definitely proved that no vital force is involved in ascent of sap.

Pulsation theory : Sir J.C. Bose (1923) said that living cells of innermost layer of cortex, just outside the endodermis are in rhythmatic pulsations. Such pulsations are

responsible for pumping the water in upward direction.

According to Bose, the pulsatory cells pump the water into vessels.

Criticism : Dixon failed to verify the results of Bose.

Transpiration pull-cohesive force theory

This theory was proposed by Henry H. Dixon and John H. Jolly in 1895 and it is the most widely accepted theory till date. It is based upon the

following considerations:

i) There is existence of continuity in water column right from the vein endings of leaves down to root hairs.

ii) There is great mutual attraction (due to hydrogen bonding) among the water molecules which is called cohesive force.

iii) There is a great adhesive force between the water column and the walls of xylem vessels (again due to hydrogen bonding) .

iv) The energy for the upward movement of water column is provided by transpiration pull from leaf cells. This creates a more negative water

potential in the leaf cells, causing water to move from xylem ducts to these cells.

Ascent of Xylem Sap

26

The End