SOIL ACTINOMYCES 293 



Another characteristic of the growth of some species is the formation 

 of "fairy rings" consisting of concentric spore-bearing rings and spore- 

 free rings disposed in zones; the zones are also formed in the spore-free 

 colony. It has been suggested 20 that ring formation by fungi is a result 

 of diffusion of injurious substances present or formed in the medium or 

 due to the action of light, which produces a change in transpiration and 

 temperature. No ring formation by actinomyces could take place as 

 a result of changing periodically the environmental conditions. 



Vegetative mycelium. The vegetative growth consists of a mycelium 

 composed of profusely branching hyphae (Nos. 121-122, PL XVI), 

 the terminal growing portions of which are densely filled with pro- 

 toplasm. The vacuoles increase in size towards the center of the 

 thallus. They are possibly associated with the presence of metachro- 

 matic granules which are often mistaken for micrococci or bacterial en- 

 dospores. The actinomyces may be divided into two groups on the 

 basis of the length of the hyphae: (1) those forming long, abundantly 

 branching hyphae and (2) those which show on the slide only short 

 unbranching pieces of mycelium or rods. According to Lieske, the 

 aerobic forms, both spore-forming and non-spore forming, growing fast 

 into the substratum, belong to the first group, while the anaerobic 

 pathogenic forms are included in the second. The composition of the 

 medium greatly influences the length of the hyphae so that, with 20 per 

 cent of cane sugar in the medium or in strongly alkaline media, a long 

 mycelial species was found to form a very short mycelium. 



The vegetative mycelium is usually infrequently and irregularly 

 septated. While in some forms transverse walls appear with somewhat 

 greater frequency, there are none in which septation approaches any 

 pronounced degree of regularity or closeness. The branches are formed 

 by the elongation of lateral buds arising some distance back from the 

 growing point of an axial filament. The branch thus produced gives 

 rise to secondary branches by lateral proliferation. 21 In addition to the 

 typical monopodial branching, true dichotomy may occasionally occur. 22 

 In old cultures, certain swellings of the terminal ends of the hyphae are 

 observed; these may also be formed under abnormal growth conditions, 

 as in concentrated media or in the presence of substances like caffeine. 



20 Alunk, M. Bedingungen der Hexenringbildung bei Schimmelpilzen. 

 Centrbl. Bakt. II, 32: 353-375. 1912. Biol. Centrbl., 34: 621. 1914. 



21 Mace, E. Sur les caracteres de culture du Cladothrix dicholoma. Compt. 

 Rend. Acad. Sci., 106: 1622. 1888. 



22 Neukirch, H. Uber Strahlenpilze. II, 1902. 



