SOIL ACTINOMYCES 295 



3. A unicellular delicate branching mycelium, at the extreme tips of which 

 single oval spores are formed. There is growth in liquid media in the form of 

 small firm grains at the bottom and along the glass. The name Micromonospora 

 is suggested for this group. 



A classification found very convenient for the study of soil action- 

 myces is tentatively suggested here. It is based on the formation of 

 sporogenous hyphae. 



1. Presence of substrate mycelium alone and no aerial mycelium formed on 

 synthetic or organic media. 



2. The aerial mycelium consisting of very long filaments, rarely branching 

 and mostly sporogenous almost to the point of origin in the nutritive mycelium, 

 without any coiling. This group can be subdivided into two sub-groups: (a) 

 long mycelium and little branching (No. 123, Pi. XVI); (b) short mycelium and 

 abundant branching (No. 124, PI. XVI). 



3. Maturation of the sporogenous hyphae associated with the formation of 

 characteristic spirals. A flexuous habit of the young filament early manifesting 

 the tendency towards the coiling condition which becomes more definite with 

 continued elongation. This group can be also divided into sub-groups on the 

 basis of the obliquity of the spiral, diameter of turns, construction with ref- 

 erence to the dextrorse and sinistrorse condition (constant characters, according 

 to Drechsler). The spirals range from open (No. 127, PI. XVI), barely per- 

 ceptible turns to strongly compressed spirals (No. 126), so that the adjacent 

 turns are in continuous contact. The number of turns ranges from two or three 

 to twenty or more. 



4. Sporogenous hyphae formed in knot-like groups of three or four along a 

 central hypha (No. 125, PI. XVI). 



Group 3 is most abundant in the soil and is, therefore, described in most 

 detail. The representatives of this group form sporogenous hyphae 

 ranging from the straight mycelium with a barely perceptible waviness 

 of group 2 to the strongly compressed spirals, resembling a closed fist. 

 The diameter of the spirals is usually somewhat in an inverse ratio to 

 the number of turns characteristic of the species. It must be noted here, 

 however, that the nature and composition of the medium greatly influ- 

 ence the morphology of the organisms. Species belonging to group 4 

 (Act. reticuli) when grown on Czapek's agar, are found in group 3 when 

 grown on nutrient agar or even certain inorganic media. 



By comparing the relation of the sporogenous branches to each other 

 and the axial filaments, Drechsler noted two main types approaching 

 each other in apparently intermediate forms, but quite distinct at 

 the extremes : (a) erect dendroidic type in which the sequence of develop- 

 ment of the sporogenous hyphae is successive; fructification starts from 

 a single erect hypha with a spiral termination; sporogenesis commences 



