6 A Monograph of the Erysiphaceae 



The second phase in the life-history of the Erysiphaceae occurs 

 later in the summer, or in autumn, usually when the vitality of the 

 host-plant begins to diminish. The formation of conidia is then 

 gradually superseded by that of perithecia, containing ascospores, 

 or resting-spores, whose function is to carry the fungus in a dor- 

 mant condition through the winter, when host-plants are not 

 available. 



The history of the development of the perithecium was first 



investigated by de Bary (98 and 99). At the crossing point of 



two hyphae, or at the place where two neighboring hyphae touch, 



each develops a small upright branch, which is soon cut off by a 



septum from the parent hypha. One of these branches swells to 



an oval-oblong shape, and becomes the oogonium. The other 



lengthens slightly, and applies itself closely to the side of the 



oogonium, curving above so that its end lies on the apex. The 



uppermost part is then cut off by a septum, and forms the anthe- 

 ridium. 



I 

 I 



De Bary observed no breaking down of the wall between the 

 antheridium and the o5gonium, and so supposed that no conju- 

 gation took place, but nevertheless considered that these organs 

 represented a true sexual apparatus, and that the perithecium sub- 

 sequently formed was to be regarded as the result of a sexual act. 

 This last conclusion has lately been strikingly verified by the 

 work of Harper. The following details are taken from the two 

 important papers of this author (160 and 161). 



The o5gonium and antheridium each contain a single nucleus. 



At the time of ff^rtiHyptinn tlip f-pn_„r'.1I k^<-„.^„„ ii, „ i^ 



dissolved, and the nucleus of the antheridium enters the o5gonium, 

 and unites with its nucleus. At this point the protoplasn^of the 

 antheridium is in direct contact with that of the o5gonium ; soon, 

 however, after the passing over of the nucleus of the antheridium 

 into the o5gonium, a fresh wall is formed between the two organs, 

 and then only a small quantity of protoplasm is found in the an- 

 theridium. At the time when the union of the two nuclei takes 

 place, the development of the walls of the future perithecium 

 begms. From the stalk cell of the o5gonium a number of hyphal 

 branches spring, closely pressed side by side, and grow upwards 

 formmg a single layer round the o5gonium. The stalk cell of the 



