46 AGRICULTURAL BACTERIOLOGY 



fectly clear except for the growth at the surface or bottom. 

 The culture growth may vary widely in its profuseness, 

 from that which is invisible to the eye to one that may 

 occupy a considerable part of the volume of the medium. 

 The profuseness of growth is a reflection of the morphology 

 and physiology of the organism concerned. If pronounced 

 capsules are formed, the mass of growth will be large. 

 Again, if the by-products are extremely toxic to the organ- 

 ism, growth can continue but a short time ; while, if they are 

 not toxic, the growth may be limited only by lack of food. 

 The consistency of the growth, whether moist or dry, 

 friable or slimy, is also noted, as is the color. The great 

 majority of the bacteria produce a grayish white growth, 

 which is more or less opaque. Some produce pigments, and 

 are called chromogenic organisms. The most common col- 

 ors noted are yellows, varying from a pale lemon to a deep 

 orange. Blue, violet, green, red, brown, and black are less 

 common. In some instances the pigment is a by-product, 

 produced under certain conditions of growth and not under 

 others. Its production is thus not an essential part of the 

 physiology of the cell. In other cases it is a constant prop- 

 erty, and undoubtedly is a part of the vital processes of the 

 cell. If the pigment is soluble in water, the medium in 

 which the organism is growing will be colored. If it is in- 

 soluble, the growth alone is colored. Among the manifes- 

 tations of the chromogenic bacteria that have attracted 

 especial attention are green pus, blue milk, and bright red 

 spots on bread, the latter due to B. prodigiosus. 



The relation of the organism to air will determine the 

 location of its growth in culture media, and thus affect the 

 appearance of the culture. The by-products of the organ- 

 ism will also aid in determining the cultural characteristics. 

 The formation of gas is an example. 

 Physiological characteristics. In the detailed studv of 



