ERYSIPHALES 



85 



III] 



The antheridial branch is much narrower; it applies itself to the side of 

 the oogonium and when first cut off contains a single nucleus (fig. 41 a). 

 It is clearly differentiated from the hyphae of the sheath not only by its 

 form and behaviour but by its much earlier appearance and definite relation 

 to the oogonium. Its nucleus soon divides ; one of the daughter nuclei 

 passes to the apex of the branch and a wall is formed cutting off the uni- 

 nucleate antheridium. The oogonial nucleus is rather larger than those of 

 the vegetative cells, the antheridial nucleus decidedly smaller. 



The investigations of Harper, and subsequently of Blackman and Eraser, / 

 show that the wall between the oogonium and antheridium now breaks down 

 (fig. 41 d), the male nucleus passes into the oogonium, travels to the female 

 nucleus and fuses with it (fig. 41 c, d). The pore between the sexual cells 

 soon closes and the cytoplasm left behind in the antheridium degenerates, 

 sometimes forming a densely staining cap on the oogonium. The fertilized 

 oogonium elongates, the fusion nucleus divides (fig. 41/) and a wall separates 

 the daughter nuclei so that two uninucleate cells are formed. The upper 

 divides again and eventually a row of cells is produced, the penultimate con- 

 taining two nuclei while the others are uninucleate (fig. 41 ^). The formation 

 of the first wall is sometimes delayed, so that the undivided oogonium may 

 for a time show two or three nuclei. 



At about the time when the sexual organs reach maturity the develop- 

 ment of the sheath begins, branches grow out from the stalk of the oogonium 

 and enclose the sexual organs in a 

 single layer of cells. These give off 

 branches towards the interior, and 

 thus a second zone is formed whose 

 cells become rich in cytoplasm and 

 contain two or three nuclei each. The 

 nutritive and protective envelopes are 

 thus laid down and their further de- 

 velopment produces the typical sheath 

 already described. 



Meantime the penultimate cell of 

 the septate oogonium enlarges to form 

 the single ascus characteristic of 

 Sphaerotheca, the terminal cell and 

 the cells below the ascus are pushed 

 aside and disappear, the two nuclei 



fuse, the usual three successive nuclear Fig. 42. Sphaerotheca Humuli (DC.) Burr.; de- 



,. . . . , 1 J velopmeiit ofarchicarp; in c. two nuclei, re- 



dlVlSlons take place, and ascospores gardedastheproductofdivision, are shown in 



are produced. ^^^ oogonium, while a ceirSf the top of the 



i,, , ' 1 r 1 1 c oogonium, regarded as the antheridium, still 



The only record of the number Ot contains a nucleus; after Winge. 



