304 Prof. E. Boyce and Mr. A. E. Evans. 



to the back of the gelatine was painted black, growth was still faster 

 and might cover the surface in 12 hours. On the other hand we 

 found accidentally that a reflecting surface retarded the growth. 

 These latter phenomena we attributed to differences of radiation. 



When next we directed our attention to the gelatine cultures on 

 glass plates and in Petri boxes, we encountered a still greater number 

 of irregularities. Upon the glass slides covered with a thin layer of 

 gelatine we never obtained a good negatively geotropic growth ; the 

 usual condition was the irregularly branched appearance seen in 

 figs. 15 and 16 under a low power. For the reasons of this we are 

 still at a loss, unless it be the increased resistance of the gelatine. In 

 the Petri boxes with a thickness of gelatine varying from J to 

 1 inch, or even more, we obtained some instructive results. We 

 never succeeded in getting symmetrical pinnate growths on the 

 surface comparable to those in test- tubes ; but that we obtained 

 evidence of geotropism is manifest from the photographs, figs. 2, 3, 4, 

 5. In figs. 2 and 4 vertical streaks had been made; in fig. 3, a 

 circular streak ; and in fig. 5, a cross. The time occupied in the 

 formation of these growths was relatively very much longer than in 

 the case of test-tubes ; moreover the filaments upon the surface rarely 

 possessed the delicate character of that obtained in test-tube 

 cultures; they had, in fact, as we shall subsequently see, the 

 character of zoogleaform threads. In fig. 6 a horizontal streak has 

 been made, and although the glass dish was kept vertical, an irre- 

 gular growth has extended for an equal distance both above and 

 lelow the horizontal line; Bacterium Zopfii may, in fact, readily 

 extend downwards upon a surface, but it always does so in irregular 

 masses. These results disappointed us at first, until on closer ex- 

 amination we found that in every case parallel threads passed into the 

 gelatine almost invariably at the common angle of 45. These geo- 

 tropic threads had formed just as quickly as those found on the 

 surface of the gelatine in the case of test-tube cultures ; indeed, 

 so rapid is their growth into the gelatine that they may extend a 

 distance of f to 1 inch in 12 hours. The symmetrical ingrowing 

 into the gelatine we had also observed in the case of test-tube 

 cultures where the growth upon the surface had been retarded by 

 lowness of temperature, whilst painting the test-tube black in the 

 manner previously referred to appeared to still further increase the 

 inward growth. Fig. 7 is a view of the back of an early growth to 

 show the regular ingrowing tendency ; but photographs of the more 

 perfect cases are impossible owing to the extreme delicacy of the 

 filaments ; fig. 5 is a later stage of 7. In fig. 6 the ingrowing is 

 partially seen, and, as fast as the growth extended upon the surface, 

 fresh rami projected symmetrically into the gelatine. We noticed in 

 some cases that the filaments, after having penetrated the gelatine 



