Transpiration is important for any plant to survive. It keeps water moving in the plant, along with the dissolved mineral salts that the plant needs for nutrition and it helps cool the plant. The air in the inter cellular air space behind the stoma becomes saturated with water that evaporates to the atmosphere via the stoma. The definition of the process is that it is the evaporation from the inter cellular spaces to the atmosphere via stomata and you got this spot on- well done.
Water is needed for most metabolic processes in the plant, It also keep cells turgid – full of water so they maintain their structure and shape. The process is controlled by special , crescent shaped cells called guard cells that surround pores called stomata in the lower epidermis of the plant’s leaves. When the guard cells are turgid, they swell and the stoma opens, when they are flaccid, the stoma close.
The guard cells lose water and if no more water comes from the next cell, they become flaccid. They become turgid when full of water because, unlike most of the other cells on the lower surface of a leaf, they possess large amounts of chloroplasts and produce glucose which increases the concentration in the cell so more water is drawn in. Thus, in sunlight, the stoma are usually open as the cells actively photosynthesise.
The process is kept going by a continuous replacement for water from the soil. Water enters the root hairs by osmosis –the special diffusion from water in a region of high concentration to one of lower concentration via a semi permeable membrane (the cell wall in this case). Then, by osmosis it is passed across the cortex until it reaches the xylem vessels. It is passed across because as cells lose water, a water deficit is created in that cell, so it draws water from the next. This creates root pressure which is enough to take water up to around 30 feet, which is enough for most plants.
Another force – leaf suction- is created by the constant evaporation of water from the leaf cells and this will draw water from up to 30ft below. So, root pressure pushes water up 30ft, leaf suction draws water up to 30ft which is usually enough. However, when a plant reaches over 60 ft (as in some tall trees) other forces come into play – these are cohesion which is the attraction of water molecules to each other and adhesion, whichis the attraction of the water molecules to the walls of the xylem vessels. This means the water column remains intact unless the column is broken. This explains how tress can be taller than the otherwise limiting factors would seem to allow.
Factors like the humidity of the air surrounding the plant and temperature will affect the rate of transpiration and of course, there must be enough water available in the soil ( although some plants have adapted to obtain water from the air). Transpiration is an amazing and highly balanced process.
