The opening and closing of the stomata is the result of variation in the turgidity 01 their guard cells, which is immediately affected by the condition of turgidity of the cells of the epidermis contiguous to them.
The only way in which their turgidity is modified is by the entry of water into them from the contiguous cells of the general epidermis and its subsequent withdrawai through the same channel.
The great turgidity which is thus caused exerts a considerable hydrostatic pressure on the stele of the root, the vessels of the wood of which are sometimes filled with water, but at other times contain air, and this often under a pressure less than the ordinary atmospheric pressure.
There is set up at once a certain hydrostatic pressure, due to the turgidity which ensues upon such absorption, and the extensible cell wall stretches, at first in all directions.
The increase in surface of the cell wall is thus duefirstly to the stretching caused by turgidity, and secondly to the formation and deposition of new substance upon the old.