78 THE FUNGI WHICH CAUSE PLANT DISEASE 



strictly parasitic habit. The myceUal threads in most genera 

 wander between the host cells and draw nutriment from them by 

 short branches, sucking organs (haustoria), (Fig. 49) of various 

 forms, which penetrate into the victimized cell. In one genus 

 only, Phytophthora, does the mycelium grow directly through cells. 

 Two kinds of spores are produced, sexual and asexual. The 

 sexual spores result from the union of two 

 unlike gametes, the egg (oosphere) and 

 sperm, borne respectively in the oogonium 

 and antheridium. Each oogonium bears a 

 solitary oosphere. Fertilization is accom- 

 plished by means of a tube from the anther- 

 idium and penetrating into the oogonium. 

 The sexual spores are thick walled, re- 



FiG. 49. — ^Haustoria of a . . , , ,, . , , . . 



Peronospora. After sistant, and usually require a long time to 

 ^°'*^" reach maturity. They are, therefore, often 



called "resting spores." In germinating the sexual spores pro- 

 duce either germ tubes or develop directly into zoosporangia. 

 The asexual spores are conidia. They are borne on conidio- 

 phores which arise from the mycelium and which may be short 

 or long, simple or branched, subepidermal or superficial accord- 

 ing to the habit of the species. The conidia in various genera 

 germinate by three methods, (1) a germ tube is sent out by 

 the conidium, (2) the entire protoplasmic contents of the spore 

 passes outside the spore wall and then forms a germ tube, or 

 (3) the conidium by internal division breaks up into zoospores. 



Key to Families of Peronosporales 



Conidiophores, short, thick, subepidermal, 



conidia catenulate 1. Albuginaces, p. 78. 



Conidiophores, longer, superficial, simple or 



branched, conidia not catenulate 2. PeTonosporaces, p. 82. 



Albuginacese 



There is a single genus. Albugo (Persoon) Roussell. This genus 

 of about fifteen species is entirely parasitic upon flowering plants. 



