is thought to control the timing of stomatal initiation specification and FLP is thought to be involved in preventing the further division of the guard cells once they are formed. Environmental conditions affect the development of stomata, in particular, their density on the leaf surface. It is thought that plant hormones, such as ethylene and cytokines, control the stomatal developmental response to the environmental conditions. Accumulation of these hormones appears to cause increased stomatal density such as when the plants are kept in closed environments. See also Bark Cork cambium Periderm References External links Plant tissue systems Plant anatomy Tissues (biology)