The Columbia Encyclopedia, Sixth Edition. Columbia Encyclopedia World Factbook American Heritage Coll.
Various factors influence the transpiration rate.
Photosynthesis, induced by light, has the effect of increasing the water pressure in the guard cells that border each stomate and that, in expanding, pull apart to widen the stomate aperture and thereby increase water loss. Low humidity promotes the diffusion of water vapor from the air passages inside the leaf into the outside air. A lack of water in the soil cuts down the water supply to the cells, thus limiting expansion of the guard cells. Therefore the rate is highest on a bright, dry day and lowest at night or in drought conditions. Plants also lose some water by guttation, a process whereby water is exuded directly through pores called hydathodes.