312 CLASS ASCOMYCETEAE 



or the nucleus only may divide, producing a binucleate oogone one of 

 whose nuclei eventually disintegrates. An opening is formed from the 

 antherid into the oogone and through this the single male nucleus passes, 

 or the antherid nucleus divides and one nucleus passes into the oogone, the 

 other remaining in the antherid. According to Harper, Hein (1927), and 

 several other observers the male and female nuclei fuse in the oogone but 

 according to Dangeard (1907) and his students there is no passage of a 

 male nucleus into the oogone hence no nuclear fusion at this stage. Berg- 

 man (1941) reinvestigating the development in the species Sphaerotheca 

 castagnei Lev. confirms the passage of the antherid nucleus into the 

 oogone, where it passes by the larger oogone nucleus and takes up a basal 

 position. According to him there is no union of the nuclei at this stage. In 

 the meantime from the cell basal to the oogone there grow out branching 

 hyphae which push upward and around the oogone with the attached 

 antherid, thus forming a layer of 16 to 20 cells of which the antherid stalk 

 is one. An additional layer of hyphae arises in a similar manner from the 

 basal cell, within the first layer, and finally a third layer is produced. The 

 innermost layer consists of thin- walled cells, called by Hein "nurse cells," 

 richly filled with food while the two outer layers form the cortex. This 

 eventually becomes dark and the cells become more or less polygonal in 

 outline. The zygote nucleus of the oogone divides rapidly according to 

 Harper and the oogone elongates, becoming an elongated plurinucleate 

 structure which is soon divided by septa into a row of from five to eight 

 cells, all uninucleate except the penultimate cell. Hein finds in his studies 

 that only three cells are produced, the middle one with two nuclei. Berg- 

 man describes the development in a different manner. The male and 

 female nuclei in the oogone do not unite immediately but each divides, 

 producing thus four nuclei in the somewhat elongated and curved oogone. 

 Two septa divide this into three cells of which the middle one is binu- 

 cleate. In Sphaerotheca and Podosphaera the two nuclei unite and the cell 

 enlarges and becomes the single ascus. The fusion nucleus divides suc- 

 cessively until eight nuclei are formed. Around each nucleus part of the 

 cytoplasm of the ascus is cut out by the formation of spore walls, thus 

 producing eight ascospores or, by degeneration of some of the nuclei or 

 partly formed spores, a smaller number. The ascospores are hyaline and 

 broadly ellipsoid. The ascus is obovoid. Miss Allen (1936) reports that in 

 Erysiphe polygoni DC. the formation of recognizable antherids is the 

 exception. Usually when any two suitable hyphae come in contact, 

 whether end to end or otherwise, the intervening walls dissolve and 

 nuclear transfer occurs. From the nearby hyphae the perithecium arises 

 containing a maze of interconnecting cells with varying numbers of 

 nuclei. Eventually the outer, colored cortex develops and several of the 

 contained cells enlarge to become the asci, two nuclei uniting in each. 



