The development into the two mesophyll types occurred step by step during evolution. Both spring from the same ground tissue and numerous transitions prove that the typical palisade and spongy parenchyma cells are merely extremes of a completed differentiation. With few exceptions are all cells of algae able to perform photosynthesis. No distinction can be made as to epidermis and mesophyll. Upon a layer of voluminous parenchyma cells sits an assimilation tissue of numerous small, plump cells gathered together in groups and interspersed with large, air-filled cavities.
Liverworts have neither stomata nor guard mechanisms.
Instead is the upper surface mingled with characteristic pores that cannot be closed. Many aquatic and marsh plants have a homogeneous aerenchym with very large intercellular cavities. They are not structured into palisade parenchyma and spongy parenchyma. Neither can it be distinguished between mesophyll and epidermis. The vascular bundles of nearly all vascular plants are surrounded by a bundle sheath , a more or less extensive layer of parenchyma cells.