transpiration

• transpiration is the loss of water from the plant
• transpiration is caused by the evaporation and diffusion of water from a plant's surface - most transpiration happens at the leaves
• the evaporation creates a slight shortage of water in the leaf, and so more water is drawn up from the rest of the plant through the xylem vessels to replace this water
• as the water is evaporated more water is drawn up from the roots, and so there is a constant transpiration stream of water through the plant
• transpiration is just a side-effect of the way leaves are adapted for photosynthesis
• leaves have to have stomata in them so that gases can be exchanged easily - because there is more water inside the plant than in the air outside
• the water escapes from the leaves through the stomata by diffusion
• light intensity

  • the brighter the light, the greater the rate of transpiration
  • stomata begin to close as it gets darker - photosynthesis can't happen in the dark, so they don't need to be open to let CO2 in
  • when stomata are closed very little water can escape
• temperature:

  • the warmer it is, the faster the rate of transpiration
  • when its warm the water particles have more energy to evaporate and diffuse out of the stomata
• air flow

  • the better the air flow around the leaf (e.g. stronger wind), the greater the transpiration rate.
  • if air flow around a leaf is poor, the water vapour just surrounds the leaf and doesn't move away - this means there is a high concentration of water particles outside the leaf as well as inside it, so diffusion doesn't happens quickly
• air flow

  • if there is good air flow, the water vapour is swept away, maintaining a low concentration of water in the air outside of the leaf.
  • diffusion then happens quickly, from an are of higher concentration to an area of lower concentration
• humidity

  • the drier the air around a leaf, the faster transpiration happens
  • if the air is humid, there is a lot of water in it already, so there is not much of a difference between the inside and the outside of the leaf
• humidity

  • diffusion happens fastest is there's a really high concentration in one place, and a really low concentration in the other