ASEXUAL SPORES 7 



pressure or it is dissolved by secreted enzymes, and so the sporangio- 

 spores are set free. If sporangiospores are ciliated they are called 

 zoospores. See Figs. 23 and 31. 



Exogenous spores, either unicellular or multicellular, may be 

 formed from the mycelium in several ways, but in all cases they are 

 born free, not contained within membranes. Collectively they are 

 called conidia and the stalks of mycelium which bear them are 

 conidiophores. The term sporophore means a differentiated portion 

 of mycelium which bears spores. It is a more general term than 

 sporangiophore, conidiophore, etc. In recent years many mycol- 

 ogists have distinguished more carefully between the different sorts 

 of conidia, especially in the class of Fungi Imperfecta Such distinc- 

 tions were first clearly made by Vuillemin, who introduced new 

 names to indicate the various sorts. 



Vuillemin divided exogenous spores into two main divisions, thal- 

 lospores and "conidia vera." The true conidia, in contrast to the 

 thallospores, are produced by an abstriction of the hyphae. The 

 thallospores are further divided into arthrospores, produced by the 

 disarticulation of a filament of septate mycelium into its component 

 cells, and blastospores which are produced by budding from the ends 

 or sides of the filaments of mycelium (Fig. 118). These two sorts 

 of thallospores thus correspond to the two sorts of unicellular growth 

 forms previously described as oidia and yeast-like cells. The distinc- 

 tion between thallospores and unicellular growth forms is not clear, 

 but in general the spores are formed on aerial mycelium, are dry, 

 and are capable of remaining dormant and of being distributed by 

 the wind, whereas the unicellular growth forms are produced by the 

 submerged mycelium, are moist, and are capable of continued growth 

 as unicellular bodies. But one may find in a single culture of, for 

 instance, Geotrichum candidum all transitions from submerged oidia 

 to aerial arthrospores, and recent authors have tended to avoid such 

 distinctions, referring for example to the yeast-like growth forms of 

 Candida albicans as blastospores and all the reproductive bodies of 

 Geotrichum as arthrospores or oidia. We shall, in general, follow 

 this usage. 



True conidia were subdivided by Vuillemin according to whether 

 they were borne on what he interpreted as undifferentiated hyphae 

 as in Sporotrichum, or on well-defined conidiophores as in most Fungi 

 Imperfecta See Figs. 45 and 52. Among the latter he distinguished 

 those which were borne on terminal, differentiated, bottle-shaped 

 cells of the conidiophores, the phialides or sterigmata. Conidia borne 

 on phialides have occasionally been called phialospores. See Fig. 52. 



