528 PATHOGENIC FUNGI 



in this case the walled spores are not swarm -spores, but are adapted for 

 dispersal through the air. 



Some of the Phycomycetes can produce spores asexually in an entirely 

 different manner, namely, externally by abstriction from the end of a 

 hypha. Such asexual spores externally cut off are termed conidia, and 

 the special hypha bearing the conidia, if different in form from the 

 vegetative hyphfe, is termed a conidiophore. Each conidium can emit one 

 or more hyphse and thus give rise to a new plant. 



Other forms of asexual spores occurring in these simple fungi include 

 oidia, in which a hypha undergoes cross septation into a number of short 

 segments, each of which acts as an asexual spore. A hypha in this oidial 

 condition has a resemblance to a greatly magnified row of bacteria ; indeed 

 according to one theory bacteria represent merely oidial conditions of 

 very degenerate fungi. 



Finally, as opposed to the thin-walled asexual spores so far mentioned, 

 thick- walled asexual spores (often termed chlamydospores) occur in some 

 of these simple fungi, and are endowed with greater powers of resistance 

 to hostile external conditions and act as resting spores. 



Phycomycetes also reproduce sexually. In the simplest case, as repre- 

 sented by Mucor Mucedo, the ends of two hyphae come into contact 

 and the terminal parts of the hyphse are segmented off by a transverse 

 wall. The wall at the region of contact of the two hyphse is dissolved, 

 and the protoplasmic contents of the two terminal compartments fuse 

 and produce around the resultant mass a thick wall. This thick-walled 

 structure is capable of growing out to produce a new plant. As it is pro- 

 duced by the fusion of two similar sexual cells it is termed a zygospore. 

 Those Phycomycetes that have no marked structural distinction between 

 male and female cells or organs, and whose sexually produced cells are 

 therefore zygospores, are grouped together to form the class Zygomycetes. 



In other Phycomycetes there is a very clear distinction between, on the 

 one hand, the large female organ, which encloses one or more female cells, 

 the ova or oospheres, and, on the other hand, the usually smaller but 

 differently shaped male organ, which contains the equivalent of a number 

 of male cells. The union of some of the protoplasm of the male organ 

 with an oosphere results in the production of a fertilised egg-cell or 

 oospore. Those Phycomycetes having this mode of sexual reproduction 

 are grouped together to form the class Oomycetes. 



Sexually produced cells, zygospore and oospore, germinate vegeta- 

 tively to produce a new mycelium or in a fructificative manner to pro- 

 duce a sporangium. Now the number of spores inside a sporangium of 

 a Phycomycete is not only great but is at least often variable in the same 

 species. Thus if a plant of Mucor Mucedo is starved, the number of spores 

 produced in each sporangium is greatly reduced. Similarly in the 

 Phycomycetes the number of conidia produced on a conidiophore is 

 considerable and variable. Sporangia and conidiophores, then, are in- 

 definite in type in these simple fungi. 



The more highly organised fungi, the Mycomycetes, differ from the 

 Phycomycetes in that (1) their sporangia or conidiophores are definite ; 

 (2) the hyphse are septate, with numerous cross partitions ; (3) the sexual 

 process, organs, and cells are so modified as to be more or less difficult of 

 recognition, or even perhaps unrecognisable as such. In any case, the 

 Mycomycetes never have a sexually produced zygospore or oospore 

 capable of developing into an independent vegetating fungus. 



Two main series are recognisable in the Mycomycetes. In one series 



