THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 241 



The Fungus is not a serious disease of cultivated crucifers, for though it 

 frequently attacks them, it is rarely that the host plant is seriously affected. 

 The oospore can apparently retain its vitality for as much as four years. The 

 infection of the host takes place only through the cotyledons of the seedling 

 or through such delicate tissue as the flower buds. The parasite cannot 

 penetrate the mature epidermis, consequently an epidemic spread of the 

 disease among crucifers is not very likely. 



Despite the fact that Cystopus was the first name given to this fungus, 

 and therefore undoubtedly the correct one, American workers have for many 

 years used the name Albugo. The confusion is increased by the fact that the 

 genus is now included in a separate family of the Peronosporales under the 

 name Albuginaceae, in which the type genus is, according to European 

 workers, Cystopus. 



Mucorales 



The ]\lucorales are Phycomycetes in which the sex organs are relatively 

 simple, and in which sexual reproduction may only occur at very infrequent 

 intervals. When it takes place it consists of the fusion of two non-motile 

 gametes. In most forms, including Mucor, these are morphologically identical, 

 but in some other types they differ in size. Asexual reproduction is by means 

 of aerially distributed spores, which may either be enclosed in a sporangium 

 or may be abstricted in chains from conidiophores. 



We shall consider only one example of this order, Mucor. 



Mucor mucedo (The Common Bread Mould or Pin Mould) 



This Fungus lives exclusively as a saprophyte and is frequently found 

 producing a whitish-grey down on bread, cheese, jam, and other food 

 material. It is also very widely distributed in the humus of the soil. It 

 may be cultivated by keeping damp horse-dung or bread under a bell-jar 

 for a few days (Fig. 233). 



The mycelium of Mucor consists of long, slender hyphae, generally 

 white or colourless, which ramify over the surface of the food material and 

 send down absorptive hyphae into it, for it is by this means that the essential 

 nourishment is taken into the mycelium. These hyphae are coenocytic, 

 though cross walls may occur at inter\'als in old hyphae.* 



The Fungus grows very rapidly, covering the substance on which it is 

 feeding. Concurrently with growth there is abundant provision for repro- 

 duction. Normally this is effected asexually by means of spores, though 

 sexual reproduction occurs very occasionally and under special conditions, 

 which we shall discuss later. 



* In Rhizopus nigricans (Mucor stolonifer), a very common species, the mycelium gives 

 rise to arched, aerial stolons, which do not branch but grow rapidly in all directions. At 

 intervals these stolons produce tufts of brown rhizoids which penetrate the substratum, and 

 opposite each tuft of rhizoids there arise one or more pairs of upright sporangiophores. 



