ZYGOMYCETES 529 



the sporangium has become definite in type, as it produces inside it a 

 number of spores that is definite and constant to the species. The number 

 of spores is usually eight, but a few species produce other multiples of 

 two. This definite sporangium is termed an ascus, the spores are asco- 

 spores, and the group of fungi having asci is the Ascomycetes. In some 

 of the Ascomycetes the asci are grouped together and form a kind of fructi- 

 fication (ascocarp), which, to give an example, is a closed spherical body 

 in Aspergillus and Penicillium (vide infra). 



In the other series of Mycomycetes it is the conidiophore that has 

 become definite in type, being constant and defined in form and numbers 

 of conidia produced. The conidiophore usually bears four conidia or, in 

 a few species, two or a multiple of two. Such a conidiophore is termed a 

 basidium, and characterises the class Basidiomycetes, of which the common 

 toadstools are examples. 



There are some groups of fungi whose characters are sufficiently well 

 known and defined as to be capable of diagnosis, and yet do not accord in 

 characters with any of the classes already mentioned. One of these groups 

 that of the true rust-fungi, Ustilaginacece belongs to the Mycomycetes : 

 among the salient features belonging to the members is their capacity to 

 produce thick- walled asexual resting-spores, which in germination give 

 rise to a minute plant that buds off indefinite numbers of conidia. The 

 other group, the Chytridiales, on the contrary is a collection of minute 

 fungal parasites so exceedingly low in organisation as to have feebly 

 denoted or no filamentous hyphse. 



The life-histories of some fungi placed in the groups already enumerated 

 are incompletely known, yet certain characteristic stages are known, so that 

 it is possible to refer these types to their correct systematic position and 

 class. But there still remain many kinds of fungi that are known only in 

 their conidial stage, and the conidiophores are indefinite in type (not 

 basidia). These imperfectly known fungi cannot be placed in their 

 natural classes and have to be empirically grouped according to the 

 arrangement and form of their conidiophores, structure and colour of their 

 conidia, and so forth. They form the large unnatural group Fungi 

 Imperfecti. Finally, there remain a few parasitic fungi known only in a 

 sterile mycelial condition. 



We now give examples of common non-pathogenic types. 



Zygomycetes : Mucor Mucedo (and other species of Mucor). This 

 form occurs on damp bread, horse dung, and other organic substrata. 

 To the naked eye it appears as a white or smoky mould composed of fine 

 filamentous usually non-septate hyphse spreading over the substratum. 

 Here and there arise erect hyphse which in a saturated atmosphere may 

 attain a length of several inches, but which are very much shorter in 

 ordinary air. Each erect hypha ends in a spherical sporangium whose proto- 

 plasm is separated off from that of the supporting hypha by a transverse 

 wall, which bulges greatly into the cavity of the sporangium and forms 

 the so-called columella. The protoplasm of the sporangium divides into 

 many masses, each of which acquires a cell- wall and is then a spore. 

 The spores escape by the rupture of the wall of the sporangium. (The 

 needle-like bodies often seen outside the wall of the sporangium are 

 crystals of calcium oxalate.) The less frequent sexual method of repro- 

 duction and the formation of the zygospore has already been described. 

 The infrequency of the sexual mode of reproduction is due partly to 

 the fact that the individual plants are sexually differentiated and 

 might be termed male and female. Zygospore and asexual spore alike 



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