What is the shape of guard cells found in the stomata of dicots and monocot?

What is the shape of guard cells found in the stomata of dicots and monocot? Which special property help the guard cells in opening and closing?

Shape of guard cells found in
(i)Dicots Bean/kidney are shaped.
(ii) Monocots Dumb-bell are shaped.
Guard cells have special elastic property which help them to contract and stretch during the opening and closing of stoma.