The wall of pollen grains and spores is built up by structurally distinct layers. These are formed in a strict order, either by the sporangial tapetal cells or by the plasma membrane of the gametophyte. In modern plants the taxonomical affinity can be inferred from their ultrastructure. For instance, several differences exist between the sporoderm in extant Isoetales and Selaginellales. In Isoetales the pre-aperture is formed by an exospore thinning, in Selaginellales by a fold. Isoetales possess multi-laminated zones near the pre-apertural thinning, while most Selaginellales do not.