320 



BACTERIA IN RELATION TO PLANT DISEASES. 



were 4 to 7 mm. in diameter and a few of them had fused. These colonies were circular, 

 smooth, flat, and distinct on the margin. The buried colonies were small and elliptical. 

 The agar was that ordinarily used in the laboratory (see Vol. I, p. 195). 



Under low powers of the microscope Harding found colonies on standard nutrient 

 agar plates to consist usually of a central circular area homogeneous in texture and embrac- 

 ing a darker spot as nucleus. This portion of the colony was surrounded by a coarsely 

 granular zone usually darker in color than the center. The periphery of the colony formed 

 a third zone either homogeneous or very finely granular. 



The surface of old cultures on agar, potato, etc., frequently becomes a substratum for 

 the development of new colonies. These are usually not very large and are generally 

 circular in outline. They are readily distinguished from the older growth by a difference 

 in color, which gives to the surface a spotted appearance. 



Growth in stab cultures is always or nearly always best near the surface. In streak 

 cultures the surface growth is smooth and wet-shining. 



It is an organism easy to isolate and cultivate, growing readily on a variety of media. 

 The organism is not specially sensitive to dry air or to its own decomposition products. 



*FiG. 120. — Longitudinal section through root of a turnip-plant (No. 53) parasitized by Bacterium campestre. 

 The section passes through a reticulated vessel. Three non-lignified cells to the left of the vessel are also occupied by 

 the bacteria; the rest of the tissue is entirely free. Drawn from a photomicrograph. Circa x 500. 



