The Action of Gravity upon Bacterium Zopfii. 309 



phenomenon may be observed in test-tubes, especially when the 

 growth upon the surface tends to be irregular. If the Petri boxes 

 are kept horizontal, the surface of the gelatine is soon covered with 

 a quite uniform growth, in which, if rami can be distinguished, they 

 take no particular direction. If, instead of these boxes, glass plate 

 cultures are made, and in which, therefore, the coating of gelatine 

 is necessarily thin, a pinnate growth with rami at the angle of about 

 45 is never obtained. The chance of getting a symmetrical growth 

 upon a large flat surface is therefore much less than in the case of a 

 test-tube. The explanation is difficult. In the test-tube, the surface 

 of the gelatine has a slight curvature, and one often notices that 

 towards the edge of the Petri dishes, where the gelatine is also 

 slightly curved, the growth is more regular. Inequalities on the 

 surface of the gelatine do cause alterations in the direction of the 

 rami, and in some cases a few of the branches on this account appear 

 to take a positive geotropic position. Thus, in addition to tempera- 

 ture and consistence of substratum, there are other circumstances 

 which may favour or retard a symmetrical growth. 



When test-tubes containing streak cultures are placed upon the 

 vertical disc of the clinostat, and revolved at rates between one 

 revolution in 2 minutes to 1 in 1 hour, there is in the first place 

 a partial absence of the delicate rami seen in vertically sloped 

 tubes, and when rami are formed they grow in various directions. 

 Fig. 13 shows the irregular growth which first started, and grew over 

 the middle area of the gelatine, and then the formation at the 

 periphery of delicate rami, which in this case are completely hori- 

 zontal. If a Petri dish is substituted for the test-tube, the result 

 depicted in fig. 8 is obtained, a mode of growth which offers a 

 striking comparison to figs. 2 6. Our results with the clinostat 

 have been invariably the same, and, therefore, accord with what 

 Sachs* obtained with the higher plants when the disc of the clinostat 

 revolved slowly. In his case the direction of growth was neither that 

 induced by gravity nor by centrifugal force. Having repeated these 

 experiments very many times, we next tried Knight's experiment 

 upon the action of centrifugal force upon growing plants. In his 

 paper which was communicated to the Royal Society in 1806, he 

 showed that, if young plants were rotated at rates varying between 

 80 to 250 times in a minute, that the ascending axis of the plant was 

 centripetal, and the radicle centrifugal. Great was our satisfaction 

 when we found likewise that the rami of Bacterium Zopfii, which, 

 from the preceding experiments, we considered negatively geotropic, 

 took a marked centripetal course when rotated in the horizontal at 

 the rate of about 240 revolutions per minute. We obtained, in fact, 

 as fig. 14 shows, much more symmetrical and beautiful results than 

 * Wiirzburg, Med.-Phys. Gesellschaft, March, 1872. 



