Light micrograph of Dryopteris, a fern. Prothallus with young sporophyte developing from fertilized egg (zygote), showing first leaf and first root. Ferns occur worldwide in moist shady places such as woods, forests, and caves, and on rocky slopes and walls. There are many temperate forms, but ferns attain their greatest size (tree-ferns) and greatest number of species in the Tropics. They are not found in true deserts or marine habitats. The conspicuous fern plant is the sporophyte, producing spores in sporangia in a variety of sori on leaf surfaces. The sporophyte alternates with the small (1cm) free-living gametophyte, a prothallus which bears the sex organs (antheridia and archegonia).