164 C.-E. A. WiNSLOw AND Anne F, Rogers 



more translucent growth of indefinite color and somewhat shiny 

 appearance; but there was no sharp boundary to separate the types. 

 For the present we have omitted this character, akhough it may prove 

 to be of importance in more detailed work. 



We have therefore noted, as cultural characters on the agar streak, 

 only the color production and the vigor of surface growth. The 

 method of studying the former character will be described under 

 " Chromogenesis." Under "Vigor of Surface Growth" we found 

 it possible to distinguish five different types. Grade i includes 

 forms of the Streptococcus type which form only a very faint, veil- 

 like, growth, or a few translucent dotted colonies on the surface. 

 Grade 2 is reserved for a somewhat more abundant, but still meager, 

 growth. Grade 3 corresponds to a good, but not abundant, streak; 

 Grade 4, to an abundant growth; and Grade 5, to a very heavy 

 surface development. 



The relation to free oxygen is distinctly involved in the vigor 

 of surface growth, and the agar streak also served for the study of 

 various other biochemical reactions. Inhibition of growth by acid- 

 ity and alkalinity of media, temperature relations, and pigment for- 

 mation were all recorded on this medium under conditions to 

 be described below. 



5. Biochemical Reactions. 



Action upon milk. — Milk is a favorable nutrient medium for bacte- 

 rial growth because of its rich food properties, and in many groups 

 it gives important information, but it has no specific diagnostic 

 value for the Coccaceae, as all the changes it undergoes are corre- 

 lated with those which occur in sugar broths and with the general 

 activity of the organism. No coagulating enzymes and casein- 

 digesting enzymes are found in this group, so far as we are aware, 

 and no gas or odor is produced. The only changes which the cocci 

 effect in milk are therefore the production of acid or alkali, coagu- 

 lation and decolorization of the litmus. 



Decolorization has no significance, except that it indicates the gene- 

 ral activity of the organism. When the organism is most active, it uses 

 up the oxygen and reduces the litmus, which is accordingly decol- 

 orized, and, conversely, when activity grows less, oxygen diffuses 

 from the surface making the litmus pink again. 



