Light micrograph of the transverse section of the stem of a moss known as Mnium. Notice the well differentiated cortex and medulla. The medulla is important as showing the possible beginnings of a vascular system in plant evolution. The leafy plant is the haploid gametophyte, bearing male and female sex organs at the apex. The diploid sporophyte is the seta (stalk) and capsule which grow attached to the gametophyte after fertilization. The young capsule is covered by a papery calyptra. The mature capsule opens at its tip by a peristome to shed spores. Moss species are important markers of habitat types.