82 



THE ACTINO.MYCETES, Vol. I 



of the order Actinomycetales as constantly 

 nonmotile. 



Aerial Mycelium 



Species of Streptomyces are characterized 

 by the production of a typical aerial my- 

 celium superimposed upon the vegetative 

 growth. The nature of the organisms, com- 

 position of medium, and conditions of incu- 

 l:)ation are of great importance in this con- 

 nection. Certain actinomycetes have the 

 capacity to form, on agar media, colonies 

 that show concentric rings of sporulating and 

 spore-free zones in their aerial mycelium. 

 One can frequently observe similar zonation 

 in the growth of nonsporulating colonies, the 

 zones being thinner or thicker in the sub- 

 strate growth of the organisms. Various 

 theories have been proposed to explain this 

 phenomenon, which can be observed also in 

 the growth of certain bacteria and fungi. 

 Among these theories, the following deserve 

 consideration: (a) that it is due to diffusion, 

 by the growing cultures, of metabolites, 

 which stimulate or inhibit the growth of the 

 zone next to that producing the metabolite; 



(b) that the phenomenon is hereditary in 

 nature. Lieske adhered to the second theory. 

 Too little experimental work has been done 

 to warrant a suitable explanation (Ilippel 

 and Witter) . 



The aerial hyphae may reach a diameter 

 of 1 n or even 1.4 )u. They vary considerably 

 in length and in structure. The aerial myce- 

 lium may cover the whole colony, in the 

 form of a cottony mass or as a powdery, 

 chalk-like surface, or as an almost granular 

 layer. 



The sporophores of Streptomijces differ 

 greatly in their structure. Some are straight, 

 long or short. The short hyphae correspond 

 to a powdery surface and the long hyphae 

 to a cottony surface. Some of the sporo- 

 phores are curved, with various degrees of 

 curvature, ranging from very slight at the 

 tip of the sporophore, through "open cork- 

 screw" shaped bodies, finally to compacted 

 spirals, giving a "fist-like" appearance. The 

 sporophores are arranged individually, mon- 

 opodially, at varying distances from one 

 another, or are close enough together to 

 give a broom-shaped appearance. 



FiGURK 42. Types of streptomyces spores (H(>pro(luce(l from: Flai^, W. (I id. Zeiitr. li;ikt(Miol. 

 Parisitenk. Abt. II. 108: 383, 1955). 



