Yet more plant shiz. (:

Transport up the stem, due to:

• Root Pressure- generated by the active transport of ions in the endodermis, creating a water potenital gradient.
  
• Cohesion Tension.

Xylem Vessels are thinner when not transpiring.

Water molecules are polar, form hydrogen bonds. Water molecules form hydrogen bonds to make continuous stream. Water is constantly lost by transpiration. When one molecule is lost, another is pulled along. Transpiration pull is the main cause of water movement, putting the water under tension.

Water attraction = COHESION.

Cohesion Tension:

• Water lost from respiritory surface in leaves during transpiration is replaced by water from xylem in leaves.
  
• This causes a negative pressure in the xylem of the leaves which pulls up a column of water with dissolved ions up with xylem.
  
• Column of water does not break as, 1) Hydrogen bonds hold water together, and 2) water molecules are attrated to the xylem vessels.
  

Transpiration:

• Water evaporates from the mesophyll cells into the air spaces in the leaf.
  
• It diffuses out of the stomata to the atmosphere. (Air has a low water potential, as it usually has a low % of water vapor.)

Rate of transpiration is affected by four main factors:

• TEMPERATURE: temperature rises and increases the kinetic energy, and therefore, the particles move faster, and the diffusion of water vapor is quicker.
• HUMIDITY: Air spaces in the leaf are normally saturated with water, whereas the air outside is much less humid. The greater the difference in humidity, the faster the water will diffuse out.
• AIR MOVEMENTS: The more rapidly the air is moved away from the leaf surface, the more rapid the rate of transpiration, as it takes away the layer of moisture that forms, and maintains a constant water potential gradient.
  
• LIGHT: This is an indirect effect, stomata are usually open during daylight hours, to allow carbon dioxide in, but it also lets water out.