GENERIC CHARACTERS IN THE COCCACEAE 169 



an orange pigment of much deeper tint in the stab than on the sur- 

 face. In comparing color at different temperatures we have found, 

 in general, a much better pigment formation at 20 than at 37. A 

 deep orange growth at the lower temperature may often correspond 

 to a white one at 20. This effect has been recorded in our routine 

 studies, and will be more fully discussed later with their results as a 

 basis. Besides these temporary modifications of the chromogenic 

 power, the actual color of cultures may be indirectly affected by 

 certain other factors. The general vigor of growth is naturally 

 correlated with apparent depth of color, and the dryness of the atmos- 

 phere increases its intensity by evaporating moisture and concen- 

 trating the pigment. Both these factors, increase in the total amount 

 of pigment and concentration by evaporation, produce a pro- 

 gressive 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 pre- 

 vious history. In thermal death-point observations we have found 

 interesting cases of this sort. Some streaks made from broth cul- 

 tures which had been exposed to a temperature of 50 or 55 were 

 deeper in color than was the normal for the organism, but in most 

 cases they were much lighter. Sometimes streaks made from a yel- 

 low or an orange chromogen after such treatment were almost color- 

 less, although successive transfers generally restored the normal 

 properties. Finally, we have noticed in our work spontaneous 

 variations in chromogenesis 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 selections of the extremes 

 will produce 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 attribute the first, and, more doubt- 

 fully, the second, to variation rather than contamination. 



In spite of all these facts it is clear that, as the cocci normally 

 occur in nature, chromogenesis is one of their most distinct and sig- 

 nificant differences. In any series of plates sown with washings 

 from the skin four well-marked types red, yellow, orange, and 



