PSEUDOMONAS ^RUGINOSA. 305 



color, which becomes more and more marked as growth 

 advances, is not seen in the growth itself to any extent, 

 but is diffused through the medium on which the organ- 

 ism is developing. Ultimately this color becomes much 

 darker, and in very old agar-agar cultures may become 

 almost black (sometimes very dark-blue green, at others 

 brownish-black). 



NOTE. To a fresh agar culture of this organism, in 

 which the green coloration of the medium is especially 

 marked, add about 2 c.c. of chloroform. Shake gently, and 

 note that the chloroform extracts a blue coloring-matter 

 from the culture, leaving the latter more or less yellow. 



Its growth on gelatin in stab-cultures is accompanied 

 by liquefaction and the diffusion of a bright-green color 

 throughout the surrounding unliquefied medium. As 

 liquefaction continues, and the whole of the gelatin 

 ultimately becomes fluid, the green color is confined 

 to the superficial layers in contact with the air. The 

 form taken by the liquefying portion of the gelatin in 

 the earliest stages of development is somewhat that of 

 an irregular slender funnel. (See Fig. 57.) 



On gelatin plates the colonies develop rapidly ; they 

 are not sharply circumscribed, but usually present at 

 first a fringe of delicate filaments about their periphery. 

 (See Fig. 58.) As growth progresses and liquefaction 

 becomes more advanced the central mass of the colony 

 sinks into the liquid, while at the same time there is 

 an extension of the colony laterally. At this stage the 

 colony, when slightly magnified, may present various 

 appearances, the most common being that shown in 

 Fig. 59. 



The gelatin between the growing colonies takes on a 



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