Stomatal periodicity is the rate of opening and closing of stomata during day and night. Stomata usually open during day and close during night. There are three categories of stomata :
(a) Barley or cereal type : They are dumb-bell shaped.
The stomata usually open during the day for a few hours.
(b) Leuceme or alfalfa type: They open during day and close during night under mesophytic conditions. In succulents, the stomata remain closed during the day. They open only during night.
(c) Potato Type : Under mesophytic conditions, the stomata can remain open through day and night but close down for different periods and at various times under less water availability.