Perithecium
(from peri, around, and thekion, a receptacle; inasmuch as it surrounds the receptacle for the spores). The name given to certain bodies formed, in the processes of reproduction, in a large group of Fungi, the Pyrenomycetes (which see). The Perithecia vary a good deal in form, and in mode of grouping together; but they are always small, and are usually flask-shaped. They may be almost free from the mycelium, or may be imbedded in it, and, in the latter case, the wall is hardly distinguishable from the mass in which it lies. When free, the wall is often rather brittle, but moderately strong, and is formed of squarish or rounded cells, usually dark brown or black. The Perithecia may be scattered about singly, or crowded on specialised bodies. When mature, each usually contains several long, transparent cells of uniform width, or windened upwards (asci), and in each ascus there are generally eight spores, less often from two to six, or an indefinite number, for reproduction. These escape through a narrow neck, in which is a circular pore, or the aperture may be a long slit; or there may not be an aperture, the Perithecia bursting ultimately. Intermixed with the asci in many are numerous slender filaments, as long as the asci. They are the paraphyses, and their presence or absence is of consequence in distinguishing the genera in the group.