Most fungi are made up of many cells (or multicellular)—with the exception of yeasts. The many cells make up a mass of intertwined filaments—also called branching tubes— known as hyphae (a mass of hyphae is called a mycelium). Some hypha contain internal crosswalls (septa) that divide the hyphae into separate cells, but some do not; for example, coenocytic hyphae lack septa. Cytoplasm in many species flows freely throughout the hyphae, passing through major pores present in the septa. Because of this streaming, proteins and other materials that are synthesized in the hyphae can be carried to their tips, which are actively growing. As a result, fungal hyphae may grow very rapidly when food and water are abundant and the temperature is optimum.