Basidia are found on the surface of the hymenium, and the basidia ultimately produce spores. The paraphyses (sterile cells interspersed among the asci in the hymenium) are slender threadlike. The asci are present in the hymenium layer. The hyphae of the subhymenium (a layer of tissue directly under the hymenium) are interwoven. Further growth makes the hymenium convex with the asci facing towards the outer side. Similar structures called cystidia often occur within the hymenium of the Basidiomycota. In many instances, however, the fertile hymenium extends down the stipe some distance. The stipe (stem) is up to wide and long, with coloration similar to the hymenium. A hymenium is a layer of microscopic spore-bearing cells that covers the surface of gills. Cystidia are cells of the fertile hymenium that do not produce spores. Most polypores have a poroid hymenium, but not all species. In Ascomycete fungi, locules are chambers within the hymenium in which the perithecia develop. Their sides are covered with a spore-forming surface, the hymenium. Cystidia (large sterile cells on the hymenium) are absent. Cystidia are often present in the hymenium.