Most of the hyphae forming the medulla of the stroma of the brown rot fungi are 4–7 μ in diameter and contain food reserves in large vacuoles and lipid bodies. Some stromatal hyphae have very thick walls and perform a protective function. Smaller hyphae (1–2 μ in diameter) form a network through the medulla and their structure suggests that they initiate the growth of vegetative hyphae and spores after the stroma has passed through a period of rest.