THE COHESION THEORY (BEST ACCEPTED)

 

DEPENDS ON A CONTINUOUS COLUMN OF WATER FROM THE TIPS OF THE ROOTS TO THE MESOPHYLL CELLS OF THE LEAF

 

THE DRIVING FORCE

 

   EVAPORATION OF WATER FROM THE

   LEAF CREATING A NEGATIVE

   PRESSURE

 

   AS WATER EVAPORATES, MORE    WATER   IS PULLED TO THE LEAF    SURFACE.  WATER IS LITERALLY

   PULLED THROUGH THE PLANT

   CREATING A NEGATIVE PRESSURE.

 

EVIDENCE IN SUPPORT OF XYLEM

WATER TENSION

 

1.   YOU CAN HEAR THE WATER BEING

   DRAWN IN TO A WOUND WHEN THE

   WATER COLUMN IS SEVERED AND,   IF SEVERED BENEATH THE SURFACE

   OF A DYE SOLUTION, YOU CAN SEE

   THE DYE BEING DRAWN UP INTO

   TRACHEARY ELEMENTS

 

2.   THE STEM OF A RAPIDLY TRANS-

   PIRING PLANT DECREASES IS

   DIAMETER DURING THE DAY AND

   INCREASES IN DIAMETER AT NIGHT

 

   a.   THE STEM ALSO DECREASES

      IN DIAMETER AT THE TOP OF

      THE PLANT FIRST SINCE MORE

      TENSION IS NEEDED THE

      FURTHER THE TISSUE IS FROM

      THE WATER SOURCE

 

MAINTAINING A WATER COLUMN

 

THE ABILITY TO RESIST BREAKAGE IS A FUNCTION OF TENSILE STRENGTH. 

 

  

 

TENSILE STRENGTH = A MEASURE OF

   THE MAXIMUM TENSION A MATERIAL

   CAN WITHSTAND BEFORE BREAKING

 

PROPERTIES AFFECTING TENSILE

STRENGTH ARE:

 

   1.   DIAMETER OF THE CONDUIT

   2.   PROPERTIES OF THE CONDUIT

      WALL

   3.   PRESENCE OF DISSOLVED GASES

      OR SOLUTES

 

PURE WATER CAN WITHSTAND TENSIONS OF -25 TO -30 MPa AT

20OC

 

THE TENSION NEEDED TO PULL WATER

TO THE TOP OF A TALL TREE IS

-2.5 TO -3.0 Mpa

 

WHEN THE WATER COLUMN IS

INTERUPTED, WATER CAN NO LONGER BE PULLED TO THE TOP OF A PLANT.

 

  

   WHEN A BUBBLE IS INTRODUCED

   INTO A WATER COLUMN IT CAN    EITHER:

 

   1.   REDISSOLVE AND BE

      REABSORBED (CO2, OXYGEN,

      NITROGEN)

 

   2.   EXPAND AND FILL THE CELL

      VOLUME CAUSING CAVITATION

      FORMING AN EMBOLISM

 

WHAT CAUSES CAVITATION?

 

   1.   FREEZING

   2.   INCREASED TRANSPIRATION

   3.   WATER STRESS

 

REPAIR OF CAVITATED TISSUES

 

   1.   CONTAINMENT OF EMBOLISM

      WITHIN A SINGLE CELL

 

      a.   BORDERED PITS

 

  

2.   REABSORPTION OF GAS BUBBLE

   AT NIGHT

 

3.   DEVELOPMENT OF POSITIVE

   PRESSURE

 

ROOTS, SOIL AND UPTAKE OF WATER

 

SOIL-PLANT-ATMOSPHERE CONTINUUM

 

SOIL COMPOSITION

   1.   SOLID PHASE

      a.   INORGANIC ROCK PARTICLES

      b.   ORGANIC MATERIALS

 

   2.   SOIL SOLUTION

 

      a.   DISSOLVED SOLUTES

  

   3.   GAS PHASE

 

   4.   MISCELLANEOUS

 

      BACTERIA, ALGAE, FUNGI,

      EARTHWORMS, ETC.

 

SOIL STRUCTURE DETERMINED BY

   SAND, SILT AND CLAY CONTENT

 

SOIL STRUCTURE AFFECTS POROSITY

 

PORE SPACE OCCUPIES 40 TO 60% OF

   THE SOIL AND IS DEFINED AS THE

   AIR SPACES OR CHANNELS BETWEEN

   SOIL PARTICLES

 

TWO TYPES OF PORES:

 

   1.   LARGE - 10 TO 60 UM

   2.   CAPILLARY

 

WATER DRAINS FROM LARGE PORES BUT IS HELD BY CAPILLARY PORES

 

WATER THAT REMAINS IN SOIL AFTER LOSS BY GRAVITY IS CALLED FIELD CAPACITY

 

WHEN SOIL WATER CONTENT IS SO LOW THAT A PLANT CAN GET NO WATER IT IS SAID TO BE AT PERMANENT WILTING POINT

FIELD CAPACITY - PERMANENT WILTING POINT = AVAILABLE WATER

 

ABSORPTION OF WATER BY ROOTS

 

FUNCTIONS OF ROOTS:

   1.   ANCHOR THE PLANT

   2.   STORAGE OF CARBOHYDRATES

   3.   SITES OF SYNTHESIS

 * 4.   ABSORB AND TRANSPORT WATER

      AND MINERALS

 

ROOTS CAN MAKE UP MORE THAN HALF OF THE PLANT BODY

 

MOST WATER ABSORPTION TAKES PLACE AT THE TIPS OF ROOTS AT

THE ZONE OF MATURATION.  LITTLE ABSORPTION TAKES PLACE IN THE MERISTEMATIC REGION OR IN THE

ZONE OF ELONGATION.

 

THE ZONE OF MATURATION HAS ROOT HAIRS WHICH GREATLY INCREASE THE

SURFACE AREA OF ROOTS.  ROOT HAIRS CAN ALSO REACH IN TO SMALLER PORES TO FIND WATER.

 

RADIAL MOVEMENT OF WATER THROUGH ROOTS

 

WATER MOVEMENT IS APOPLASTIC OR SYMPLASTIC

 

APOPLAST - CELL WALL SPACES

SYMPLAST - LIVING CELLS - ACROSS

            MEMBRANES AND THROUGH

            PLASMADESMATA

 

 

 

 

 

 

 

 

 

 

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