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ABSORPTION, TRANSLOCATION AND TRANSPIRATION OF WATER
TERMINOLOGY
absorption - uptake of water by roots.

translocation - movement of water through plants, mainly through xylem
.
transpiration - loss of water vapor from leaves and other above ground plant parts; 
                           mainly occurs through the  stomata.

guttation - loss of liquid water from leaves;
                     occurs through hydathodes (similar to stomata, but they do not close).
 

SITE OF WATER ABSORPTION
1) young roots - most absorption, mainly through root hairs due to:
    a) very numerous - 14 billion on a typical rye plant.
    b) large surface area -14,000 ft2 (1310 m2) on a typical rye plant
    c) rapidly and constantly produced - 975 linear ft (300 m) per day on a squash plant

2) older roots - little absorption due to:
    a) suberization of endodermis
    b) periderm (bark) formation
 

COHESION THEORY OF TRANSLOCATION IN THE XYLEM
1) Transpiration occurs and is driving force

2) Causes negative pressure in leaves

3) Column of water is pulled up in the xylem and translocated due to:
     a) H-bonding (hydrogen-bonding)
     b) small size of xylem pores
   c) negative charges on xylem walls
 

FUNCTIONS OF TRANSPIRATION
1) driving force for translocation: transpiration causes a negative pressure in leaves, 
      which "pulls" the water up the xylem.

2) evaporative cooling of leaves:  540 cal of heat energy is dissipated for every gram 
     of water that evaporates from leaves, which is a major contributor to the cooling of
leaves.

Transpiration is usually much greater than is needed to satisfy these two functions. Thus, many horticultural practices attempt to minimize excessive transpiration.