THE FUNGI WHICH CAUSE PLANT DISEASE 173 



This binucleate cell after fusion of its nuclei develops into the 

 one ascus characteristic of the genus. The ascus nucleus by- 

 division gives rise to the spore nuclei and the spores are cut out of 

 the periplasm by reflexion of the astral rays. 



In Erysiphe** the oogonium and antheridium arise in a very 

 similar way, the oogonium being somewhat curved. Feri;ilization 

 is also similar consisting of the union of two gametic nuclei. After 

 fertilization the oospore nucleus divides and the oogonium de- 

 velops into a short bent tube, which contains from five to eight 

 nuclei. Septa now appear cut- 

 ting off cells, some uninucleate, 

 some with two or more nuclei. 

 The ascogenous hyphae develop 

 a knot and soon divide into 

 two or three cells each and 

 give rise to the asci which are 



in the beginning binucleate. 



T T>i. II J.- ■ Si ii_ F'"- 125. — Phyllactinia, male and female 



In fnyliactmia °* the OOgO- branches; uninucleate oogonium and 



nium, antheridium and fertili- "■^theridium. After Harper. 



zations are as in Erysiphe, though the oogonium may be quite 

 curved so as to make almost a complete turn around the anther- 

 idium. Fig. 125. 



After fertilization the antheridium degenerates and enveloping 

 protective hyphae arise both from the oogonium and the antheridium 

 stalk cells. The oogonium becomes three to five nucleate and 

 develops to a row of cells of which the penultinate cell has more 

 than one nucleus. The ascigerous hyphae arise from this binu- 

 cleate cell, perhaps also from other cells of the series, become 

 septate and form the asci either terminally, laterally or inter- 

 calary. The young ascus is binucleate, fusion follows and the 

 spores develop as in the preceding genera. 



The family contains, according to Salmon, forty-nine species 

 and eleven varieties, according to Saccardo more than one hun- 

 dred species. These are parasitic on some one thousand five 

 hundred hosts, some of them upon economic plants and of 

 serious harmfulness. 



The matter of delimiting species and even genera is often diffi- 

 cult, owing to intergrading forms. This question is complicated 



