The size and shape of fern fronds vary greatly by species, but most are quite similar at the tissue level. Leaflets have an upper and a lower epidermis, the latter of which contains numerous microscopic pores. Between the layers is the mesophyll, generally composed of cells with large intercellular spaces. However, in many ferns the mesophyll was eliminated at some point during evolution and, therefore, photosynthesis primarily takes place in the epidermal layers. Veins of ferns range widely in size, from massive midribs to delicate capillaries.