Transpiration

Each stoma is bounded by two guard cells; the stomata are the main sites of transpiration and of gaseous exchange.
When the guard cells become turgid, their thin outer walls bulge out and pull the thick inner walls into a crescent shape, so the stomatal pore opens; when they lose turgor, the pore closes.
By opening and closing the stomata, the guard cells regulate the loss of water vapour, i.e., transpiration.
Guttation occurs through hydathodes, which are permanently open pores without regulating guard cells.
Hence guard cells help in transpiration.

A potometer is an apparatus used to measure the rate of transpiration of a leafy shoot.
The cut shoot is fixed in an air-tight water-filled apparatus with a graduated capillary tube; an air bubble introduced into the tube moves as the shoot takes up water.
What the potometer actually measures is the rate of absorption (uptake) of water by the shoot.
It is assumed that the amount of water absorbed is equal to the amount of water lost by transpiration.
So the potometer works on the principle that the amount of water absorbed equals the amount transpired.
(In reality a small part of the absorbed water is used in photosynthesis and growth, so the value is only an approximation.)