50 



MYCOLOGY 



or sporangium, they may be termed sporangiospores (Fig. 13,/)- Fre- 

 qucntlyspores arc formed by a modification of certain cells of tlic hyphal 

 branch. These spores arc usually thick- walled, as in the smuts, and 

 become known as chlamydospores. Where the whole hypha is divided 

 up into a chain of spores one after the other in close order, such spores 

 are called oidiospores. Special receptacles are associated with the 

 formation of the non-sexual spores. These are found in the sac fungi, 

 ASCOMYCETALES, where the depressed conceptacle becomes a pycnidium, 

 or conidial fruit, and the spores which it contains are pycnidiospores, 

 pycnospores, pycnoconidia or the stylospores of Tulasne. This form of 

 conidial fruit is surrounded by a firm wall or peridium. The pycnidia 

 may be depressed in the tissues of a host plant or elevated above its 

 surface, as the case may be. In some fungi the conidiophores, in- 

 stead of being separate, are arranged in parallel order, side by side, 

 at an early stage, and thus are united into a fascicle to which the name 

 coremium has been applied. 



The principal sexually produced spores in the fungi are zygospores, 

 oospores and ascospores. The first two forms are found in the sub- 

 class PHYCOMYCETES. 



Their formation proceeds in such a manner that the zygospores are 

 produced isogamously, that is, by the union of two similar cells, while 

 the oospores are heterogamous, that is, they are produced by a union of 

 an egg cell and a sperm cell. Hence, we distinguish two orders of 

 the PHYCOMYCETES, namely, the ZYGOMYCETALES and the 

 OOMYCETALES, thefirst showing isogamy and the latter heterogamy. 

 Details will be given when these orders are considered in detail. Until 

 recently, it was beheved that sexuality did not exist in the sac fungi, 

 ASCOMYCETALES, but recent research has shown that the nuclei 

 of two adjoining cells unite and this is followed by the formation of a 

 spore sac, or ascus, containing sac spores, or ascospores. The formation 

 of the asci is usually associated with the production of definite fruit 

 bodies. It is doubtful whether sexuality is found in any of the other 

 groups of fungi. Curious nuclear fusions in the rusts have been sug- 

 gested as a sexual union, but it is safer to await future discoveries 

 before adopting such a position. However, there are fungi in which 

 sexual organs seem to be lost entirely and many of these belong to the 

 most highly developed forms where the thallus and fructifications are 

 of a complex type. The whole trend of evolution in the fungi is for 



