78 



Occurrence of Microorganisms in Soil 



0.5-0.8 fx in diameter. The mycelium develops either in the sub- 

 strate or on the surface of the substrate as aerial growth. The 

 mycelium breaks up into short fragments, which may look like bac- 

 terial rods and resemble true bacteria in their protoplasmic proper- 

 ties. When examined directly under the microscope, the aerial my- 

 celium is found to consist of ^'ery fine, characteristic, long or short 



branching hyphae, with distinct 





4^k Hf^> 1/ 



spore-bearing hyphae. 



The reproductive conidia, which 

 are characteristic of the genus 

 Streptomijces, are produced by a 

 simultaneous division of the proto- 

 plasm in the sporogenoiis hyphae, 

 progressing from the tip toward 

 the base. The spores possess a 

 somewhat greater power of re- 

 sistance to environmental factors 

 than the vegetative hyphae. They 

 resemble bacteria in size, shape, 

 and staining properties, are 0.5- 

 1.5 fx in diameter, 1-2 jx long, oval 

 to rod-shaped. 



All actinomycetes, particularly 

 in young preparations, are Gram- 

 positive. In stationary liquid 

 media, they never cause trn-bidity, 

 but grow either on the surface of 

 the medium or in the form of 

 flakes or small colonies through- 

 out the medium; they may sink to 

 the bottom of the container or adhere to the glass. The surface 

 colonies may grow together to form a smooth or wrinkled surface 

 membrane. The colonies on solid media are usually tough, leathery, 

 smooth or wrinkled, often growing high above the surface of the 

 medium, and are broken up only when appreciable effort is applied. 

 When transferred to suitable media, the spores germinate readily. 

 The older the mycelium, the more reduced is the germinating power 

 of the individual fragments. In shaken cultures, they grow in the 

 form of "clumps" or "colonics" throughout the medium. This mass of 

 growth can easily be removed by filtration, leaving a clear fluid. 



Fig. 34. Streptomijces griseus, with 

 a short mycelium and abmidant 

 branching: a, b, c, portions of aerial 

 mycelimn; d, f, spores germinating 

 with one and two germ tubes, re- 

 spectively (from Drechsler). 



