<P> Because stomata play such an important role in the plants survival, collecting information on their differentiation is difficult by the traditional means of genetic manipulation, as stomatal mutants tend to be unable to survive . Thus the control of the process is not well understood . Some genes have been identified . TMM is thought to control the timing of stomatal initiation specification and FLP is thought to be involved in preventing further division of the guard cells once they are formed . </P> <P> 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 . </P>

Which of the following structures is not a part of the epidermis of a leaf