CHAPTER VII 



MUCORALES 



In the Mucorales or Zygomycetes, the thallus is usually coenocytic, al- 

 though in some of the higher forms it is secondarily divided into cells. Under 

 certain conditions the hyphae may fragment into hyphal bodies, etc., which 

 occasionally develop further as sprout mycelia. In an extremely unfavorable 

 environment, small portions of hyphae develop into thick-walled chlamydo- 

 spores. 



In most forms the reproductive structures are aerial. The haploid my- 

 celium produces sporangia or conidia, the diploid mycelium produces zygo- 

 spores. Sporangia are typical of the Mucoraceae and the Endogonaceae, 

 conidia of the Entomophthoraceae, a group of parasites upon insects. The 

 sporangia form nonmotile, endogenous sporangiospores. In the successively 

 higher genera there is a tendency for reduction in the number of sporangio- 

 spores produced by a sporangium until the sporangium is practically reduced 

 to the level of a conidium which produces mycelium directly without the 

 medium of a spore formation. 



The sexual act is essentially isogamous in spite of heterothallism. The 

 higher Mucorales tend more and more toward heterogamy. The products of 

 the sexual act, the zygospores, are primarily resting spores and have never 

 been reported in many species. 



The relationships of the Mucorales are wholly obscure. Vuillemin (1886, 

 1912) and Lotsy (1907) connect this order through Basidioholus, a genus 

 dividing its life cycle between the digestive tracts of beetles and frogs, to 

 the Conjugales of the green algae and derive the other Entomophthoraceae 

 and Mucoraceae from this genus. Davis (1903) considers in general terms 

 the green algae, especially the isogamous forms, such as Cladophora or the 

 Siphonales. As intermediate forms are wholly lacking and apparently such 

 simple structures as the Mucoraceous sporangium are still unknown in other 

 groups, phylogenetic speculation is unfruitful. The present tendency is to 

 connect the Mucorales with the more primitive Phycomycetes. It is quite 

 po.ssible that the Mucorales include phylogenetically heterogeneous organisms, 

 which are only similar in the copulation of their coenocytic gametangia. 



In their classification the Mucorales are divided into two groups, one of 

 which forms sporangia, the other conidia. The sporangia! group includes the 

 large family of Mucoraceae and a small family, the Endogonaceae, consisting 

 of small, truffle-like fungi usually growing under leaf mold or in the soil. 

 The conidial group contains the Entomophthoraceae consisting of two tribes, 

 the Basidioboleae found in the intestinal tracts of beetles and amphibia and 

 the Entomophthoreae mostly parasitic on living insects, although capable of 



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