It is only quite recently that the perithecium, ascocarp or sporocarp, as it has been variously called, of the Erysiphaceae has been shown to possess a regular dehiscence accompanied by the ejection of the ascospores. We find the fact first recorded, I believe, by Worthington G. Smith, in 1884. In 1903 I independently observed the phenomenon in the same species, viz. Erysiphe Graminis DC., which Smith had studied. Smith's observations seem to have been entirely overlooked until attention was called to them in my paper in 1903, and up to this date the perithecium of the Erysiphaceae is described in nearly all text-books as a cleistothedum or cleistocarp, since it was supposed to remain closed until it decayed and ruptured to permit of the escape of the ascospores.