THE FUNGI WHICH CAUSE PLANT DISEASE 65 



Class I. Phycomycetes, Alga-like Fungi (p. 64) 



The Phycomycetes are characterized by the absence of septa in 

 the mycelium except in sporing branches, where they occur to 

 cut off the spore-bearing cells or the gametangia, and in old fila- 

 ments. The body is multi-nucleate and sexual spores as well as 

 asexual ones are usually, though not always, produced. Some of 

 the Phycomycetes live in water and possess zoospores, others are 

 parasitic on land plants and bear conidia or sporangia. These 

 may germinate either by germ tubes or by zoospores. The char- 

 acteristic fertilization consists of a union of two gametes which 

 may be hke in character (isogamy) or unlike (heterogamy). If 

 the sexual organs are unlike the receptacle which bears the sexual 

 spores is called the oogonium, its eggs before fertilization oospheres, 

 and the spores oospores. The receptacle bearing the fertiliz- 

 ing gamete is the antheridium, and the fertilizing" elements 

 are the sperms. The sperms may be motile and swim or creep 

 into the oogonium or the antheridium may develop a tube leading 

 into the oogonium through which the fertilizing nuclei pass. In 

 some forms which, by their sexual or asexual spores, show relation 

 to the Phycomycetes the mycelium is wanting and the vegetative 

 body is reduced to a single spherical or amoeboid cell, which fre- 

 quently lives in a purely parasitic manner entirely imbedded in 

 the protoplasts of its host. This mode of life constitutes the 

 strictest kind of parasitism inasmuch as the fungus derives its 

 nourishment from the still living host cell. 



Key to Orders of Phycomycetes 



Sexual spores when present heteroga- 



mous Subclass I. Oomycetes, p. 66. 



Conidia absent; sexual spores and zoo- 

 sporangia only 

 Mycelium poorly developed, frequently 

 reduced to a single cell 

 Fruiting mycelium a single cell, or a 

 group of cells in a sonis, forming 

 either asexual resting spores or 

 sporangia from the entire proto- 

 plasmic mass 1. Chytridiales, p. 66. 



