26 THE LOWER FUNGI— PHYCOMYCETES 



homologous with them, and differ only in failing to develop 

 endogenous spores. These cells function at maturity in their 

 entirety as single spores and germinate by one or more germ 

 tubes. Such reproductive cells correspond morphologically to 

 the conidia of the higher fungi and this term is often appHed 

 to them. 



The conidium of the higher fungi is an asexual spore. It is 

 usually cut off from a definite conidiophore, is deciduous, 

 is typically wind borne, and germinates by one or more germ 

 tubes, never by endogenous spores. In many of the higher 

 Phycomycetes (e.g., Peronospora, Cunninghamella, Empusa) 

 the asexual reproductive cells have all of these features. In 

 other forms (e.g., Saprolegnia, Mucor) they have the charac- 

 teristics of the typical sporangium, being persistent instead of 

 deciduous and germinating by endogenous spores instead of by 

 germ tubes. In intermediate types they are deciduous but 

 germinate by endogenous spores (e.g., Phytophthora, Piloholus), 

 or are persistent but germinate by germ tube, as in some cases 

 in Pythium. Furthermore, forms exist (e.g., Phijiophthora, 

 Plasmopara) in which the type of germination is unstable and 

 dependent on environmental factors. In such forms two asexual 

 cells identical in aspect and borne on the same hypha may be 

 induced to germinate differently (one by germ tube, the other 

 by endogenous spores) in an experiment in which different 

 temperatures are provided. Consideration of these facts make 

 it evident that precision in the application of the terms sporan- 

 gium and conidium in the Phycomycetes is extremely difficult 

 of attainment. In species in which the type of germination is 

 fixed the terms may be used without ambiguity, if their Hmits 

 be first arbitrarily defined, though all writers will not agree as to 

 their limits. In cases in which the method of germination is 

 dependent on environmental factors selection of the name must 

 be deferred until germination has actually occurred. Even then 

 it is sometimes necessary to designate either as sporangiophore 

 or conidiophore, a structure on which both sporangia and 

 conidia have been borne. 



In the present volume, the writer has attempted to avoid 

 confusion by restricting application of the term conidium in 

 the Phycomycetes to the Zygomycetes. This procedure is 

 purely arbitrary. The border Hne conditions which exist in 

 the Zygomycetes are less confusing than those in the lower groups. 



