222 A TEXTBOOK OF THEORETICAL BOTANY 



Relationships 



The chief interest of the Monoblepharidales lies in the fact that they are 

 the only group of Phycomycetes, or in fact of the Fungi, in which a relatively 

 large non-flagellate oosphere is fertilized by a small motile, flagellated 

 antherozoid. By those who would relate the Phycomycetes directly with the 

 Algae, this group is used to point out a close similarity to the Chlorophyceae, 

 and in particular to Vaucheria and Oedogonium. On the other hand, those 

 who maintain that the higher Phycomycetes are derived from simpler 

 isogamous Chytridiales see in the antherozoid a similarity to the isogamous 

 gamete of a type like Rhizophidium, pointing out at the same time that its 

 amoeboid character is also a primitive feature. Similarly they consider that 

 in the oogonium and oosphere there is a connecting link with the Saprolegniales 

 and Peronosporales. While the possibility of an algal relationship is certainly 

 attractive, it must be admitted that most mycologists now consider the latter 

 view is more probable. 



Peronosporales 



The Peronosporales are Phycomycetes in which the vegetative mycelium 

 IS composed of branched coenocytic hyphae, septa occurring rarely in older 

 hyphae or in the formation of the sex organs. Sexual reproduction is 

 oogamous, but only one oosphere is formed in each oogonium. Fertilization 

 is effected by a fertilization tube. Asexual reproduction may be by sporangia, 

 which either give rise to zoospores or produce spores which germinate by a 

 germ tube. In the higher members conidiophores are produced which 

 abstrict conidiospores, which also germinate directly by a germ tube. 

 Intermediate stages between the two types are known. Where zoosporangia 

 are produced the contents pass out into a vesicle in an incompletely differ- 

 entiated state and then divide into reniform zoospores with lateral flagella. 



The species are characteristically parasitic on higher plants, though a few 

 occur as parasites of Thallophytes. Some can live as saprophytes after the 

 death of the host, but in the higher members this faculty is absent, nor can 

 they be cultivated on artificial media. 



We shall consider three examples of this order, Pythium, Cystopiis and 

 Peronospora, each of which represents a separate family and shows an apparent 

 advance on the previous one in adaptation to a land habit. 



Pythium debaryanum (The " Damping-off " Fungus) 



Unlike Saprolegnia, Pythium deharyamim can live as a parasite within the 

 tissues of seedlings, causing them to collapse and producing a disease which 

 is frequently spoken of as " damping-off" (Fig. 213). During the early 

 stages in the growth of the Fungus no external signs of its presence are notice- 

 able on the host, and it is only after it has gained a firm hold that the hypocotyl 

 collapses, with the appearance of having been pinched off near the base. 

 With the death of the host the Fungus continues to live saprophytically upon 



