The body of most fungi is divided into a network of filaments called hyphae, which increase the surface area for absorption of nutrients. Some species of fungi have walls called septa that partition the hyphae into cells. Fungal cells have cell walls comprised of chitin. Similar to plants, fungi store energy in the form of glycogen. Fungi tend to be nonmotile, although some do contain motile cells with flagella or cilia. Fungi reproduce both sexually and asexually, with some species only reproducing asexually at certain points in their life cycle. Cells produced during the asexual phase are called spores, which contain only one set of chromosomes. The mycelium is the vegetative part of the fungus, consisting of a mass of hyphae.