68 RELATIONSHIPS OF THE COCCACE^E 



is naturally correlated with depth of color, and the dry- 

 ness of the atmosphere increases color intensity by evap- 

 orating moisture and concentrating the pigment. Both 

 these factors, — increase in the total amount of pigment 

 and concentration by evaporation, — produce a progres- 

 sive deepening of color in old cultures. 



Even if the temporary conditions of cultivation are 

 quite constant, the chromogenic power of an organism 

 may be modified by its previous history. In thermal 

 death-point observations we have found interesting 

 cases of this sort. Streaks made from broth cultures 

 which had been exposed to a temperature of 50 or 55 

 degrees were in a few instances deeper in color than was 

 normal for the organism; but in most cases they were 

 much lighter. Sometimes streaks made from a yellow 

 or an orange chromogen after such treatment were 

 almost colorless, altho successive transfers generally 

 restored the normal properties. Finally we have noticed 

 in our work apparent spontaneous variations in chro- 

 mogenesis such as have been recorded by Neumann 

 (1897), Conn (1900), and Sullivan (1905). The latter 

 authors note that on a plate sown from a single colony 

 there may develop colonies varying appreciably in 

 shade, from which selection of the extremes will pro- 

 duce quite, distinct types. Neumann records the sudden 

 appearance of widely different strains, as sectors in old 

 and carefully sealed stab cultures. We have observed 

 both phenomena in our cultures, and are inclined to 



