Transpiration is the process where water evaporates from the pores of
leaves. Because of the charges of water molecules (I don't know the
specifics), the evaporation of water molecules leads to others being
pulled up the xylem tubes towards the leaves - hence transpiration is a
necessary evil because it involves constant water loss, but is also
useful for providing water for photosynthesis, support of cells by
osmotic intake of water and cooling for optimum enzymatic efficiency.
Therefore as transpiration increases (which will be if temperature is high to cause more evaporation, windspeed is high to carry water vapour away and humidity is low so diffusion of vapour away from the leaf is more favourable), the intake of water will also increase because it is literally pulled up the xylem tubes from the roots to the leaves.
That's how I understand it anyway :)
I hope this helps
