Turgidity is the state of a cell when its vacuole is full of water so its contents are pushed against the cellwalls. Because cellulose which makes the cell wall is rigid, there is little give and the cell become turgid or rigid in its ultimate swollen shape. This helps keep the stem upright , leaves at the best angle for photosynthesis and flowers fresh and inviting.
Transpiration is important for the plant. It keeps water moving with its dissolved mineral salts that the plant needs for nutrition and it helps cool the plant. The air in the intercellular air space behind the stoma becomes saturated with water that evaporates to the atmosphere via the stoma.
Water first 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 (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. So long as a steady stream of water keeps coming, the cells remain turgid.
However, if the guard cells lose water and no more water comes from the next cell, they become flaccid. Then other cells lose water and so on until all the cells in that organ and eventually th eentire plant become flaccid. They lose strength because they are no longer pushing against each other and the plant wilts. The stems bend over, the leaves droop and flowers wither.
For some plants, wilting is actually used to prevent excessive water loss during periods of intense heat. In others, wilting can be a problem because they keel over and may be vulnerable to soil borne diseases and not get pollinated.
Wilting is not necessarily the end for a plant. Although during wilting the cell contents can disperse in the cell and lose integrity, they will recover if water is added to restore turgidity.
Water itself displays some strange qualities. The water molecules are attracted to each other by a force known as cohesion. They are also attracted to the walls of the tubes through which they travel (xylem vessels). Becasue of these properties, so long as the water column remains unbroken, it will stay intact. This can be proved by watching flowers in water. Even thgough separated from their parent plant, solong as you keep water being taken up, they will remain turgid until the flowers drop and water is no longer drawn up the stems.
However, if they are so short of water that cell’s organelles and contents begin to break down, even adding water will not restore the plant. This is called the permanent wiling point.
