4 INTRODUCTION [CH. 



In the lower fungi, spore formation may be associated with the meiotic 

 phase, but the spores produced resemble those concerned in the accessory- 

 methods of reproduction. 



Accessory spores. The accessory or non-sexual methods of repro- 

 duction have no relation to any sexual process either normal or reduced, 

 and therefore no significance in the alternation of generations; they are 

 devices for rapid multiplication comparable with the gemmae in Marchantia 

 or the arrangements for vegetative propagation in higher plants. The spores 

 concerned may be borne either on the sporophyte (rusts, etc.) or, as in the 

 majority of cases, on the gametophyte. 



In many of the lower fungi zoospores are developed in spherical, ovoid 

 or tubular zoosporangia ; this is the case especially in aquatic forms. In 

 relation to the change from aquatic to subaerial conditions the contents of 

 the sporangium may come to be shed as walled non-motile spores, or the 

 sporangium may itself be set free without division of its contents. Such a 

 structure, borne externally on its parent hypha, is termed a conidium, and 

 is the characteristic accessory reproductive unit of the fungi. In the large 

 majority of cases the conidium germinates by means of a germ-tube, but 

 where the fungus has not completely abandoned its aquatic habit the 

 conidium, if it falls in wet conditions, may give rise to zoospores either in- 

 ternally or in a vesicle borne on a short hypha. The conidia are developed 

 either singly or in groups on conidiophores; these may be free, they may 

 be gathered into a sheaf or coremium, or they may be formed inside a 

 special flask-shaped receptacle known as a pycnidium ; they show an almost 

 endless variety in form and arrangement. 



A less common reproductive cell is the chlamydospore; these are borne 

 either singly or in chains in the course of the ordinary vegetative hyphae or 

 at the ends of special branches; they are characterized, as their name im- 

 plies, by an exceptionally thick wall. 



In certain species and under certain conditions whole hyphae may break 

 up into series of separate cylindrical cells or spores. Such a spore is termed an 

 oidium. Oidium-formation appears to be a rapid and efficient method of 

 multiplication and is the only one found in the fungi of such diseases as 

 favus {Achorion Schoenleinii), pityriasis versicolor {Microsporon furfur^ 

 and thrush {Monilia albicans). 



In these cases attempts to cultivate any more characteristic fructification 

 have failed, and the fungus cannot therefore be assigned to any particular 

 group. 



Morphology of the spore. The individual spore whether belonging to 

 the principal or accessory fructification is, when first formed, a ihyaline, 

 colourless cell ; in the course of development it may divide to produce 

 a row or a mass of cells and in the latter case is described as muriform ; 



