1893.] Upon the Action of Gravity on Bacterium Zopfii. 49 



negatively geotropic, then it ought also to assume a centripetal direc- 

 tion if rapidly rerolved. 



Action of Centrifugal Force. — "We repeated Knight's experiment, 

 communicated to the Royal Society in 1806, and revolved the culture 

 tubes in the horizontal at the rate of three to five revolutions per second, 

 and we obtained most perfect pinnate growths, similar to those ob- 

 tained by growing in the vertical. 



The Cause of the Pinnate Qroicth. — The growth is very rarely ortho- 

 geotropic ; there is a force hindering this, which we believe to be the 

 resistance of the gelatine. Although the pinnate growth appears super- 

 ficial, this is not so ; the filaments tend to dip slightly or deeply into 

 the substance of the gelatine. It is observed that where they penetrate 

 deeply into the gelatine they are almost horizontal at the bottom of the 

 gelatine, whilst nearly vertical at the top of the gelatine column. 

 These differences are still more accentuated when the tubes are 

 centrifugalised. 



Circumstances which Favour or Retard the Symmetrical Growth. — It 

 is essential that the gelatine be not too stiff ; if it is hard, an irregular 

 superficial or an imperfect pinnate growth obtains. We employ a 

 freshly prepared 8 to 10 per cent, nutrient gelatine. A slight 

 variation in the reaction on the acid or alkaline side of neutrality 

 makes no appreciable difference. Temperature has a very important 

 effect ; symmetrical growths are with difficulty obtained at low tem- 

 peratures ; the most suitable is that between 20° C. and 21° C. 

 Carbonic acid hinders the symmetrical growth ; oxygen favours it. 

 Thus, if a streak culture on gelatine is placed in an atmosphere of 

 C0 2 , at the end of three days there is only a thick streak along the 

 line of inoculation ; if the tube is then transferred to oxygen, a 

 symmetrical growth develops in the course of twelve to twenty-four 

 hours. We have not yet observed any difference in the growth in the 

 colours of the spectrum. 



Xot only has the hardness of the gelatine a marked effect upon the 

 pinnate growth, but in addition the thickness of the gelatine sub- 

 stratum is important. Thus, we never obtained pinnate growths in 

 our method of plate cultures on microscope slides. Even in Petri 

 boxes, with a thickness of gelatine varying from \ inch to 1 inch, the 

 groivth upon the surface often tended to be irregular, but in these cases 

 the filaments which grew deeply into the gelatine assumed the pinnate 

 form, with the exception, as before mentioned, that the lowermost 

 branches were more horizontal than those at the top. Thus, to obtain 

 a pinnate growth upon the surface, the test-tube is the best culture 

 glass. 



Microscopic Characters of the Growth. — The organism is pleomorphic ; 

 filamentous, bacillary, baoteroid, spiral, and coccal form phases being 

 met with. There is also a motile stage, in which bacillary, bacteroid, 



VOL. liii. e 



