H. Gr. Thornton 257 



limited motility is not dependent on the surface moisture and the 

 evaporation from the gel itself is too slow to produce visible effect. 



The experiment thus shows that slowing and cessation of spreading 

 are due normally to the disappearance of the surface moisture film, for 

 they take place after a longer period when the proportion of water to agar 

 is increased, and do not occur if the gel be so thin as to allow the motile 

 organisms to penetrate away from the surface. 



As described above, the cessation of motile spreading on nutrient 

 agar is accompanied by the formation of sporangia and endospores. It 

 appeared possible that the composition of the medium might be so 

 altered as to bring about spore formation and consequent loss of motility 

 and spreading, before the water film dried, thus shortening the period 

 of spreading. I therefore investigated the influence of various substances 

 on spore formation in the organisms. Tests were first made with different 

 organic nitrogen compounds. The medium used as a base in these experi- 

 ments had the following composition : 



To this medium, various nitrogen compounds were added in amounts 

 giving nitrogen equivalent to 0-05 gm. of asparagine. In each medium 

 to be tested, duplicate stab cultures of B. dendroides were made and 

 incubated for 14 days at 30° C, after which the growth was examined 

 for spores, both alive and by means of Ziehl Neelsen's spore stain. Media 

 containing the following sources of nitrogen were tested. 



1. KNO3 alone. 4. KNO3 + Tyrosine. 



2. KNO3 + Alanine. 5. KNO3 + Peptone. 



3. KNO3 + Asparagine. 6. KNO3 + Lenico. 



On the medium without organic matter and on media 2 and 3, to 

 which alanine and asparagine were added, no spores were produced. 

 On platings of such media, surface colonies cease to spread when the 

 surface water dries off, but the rods retain their flagella and active 

 motility is immediately resumed if the surface be wetted. When, however, 

 the stab cultures had been kept for two months at 30° C, it was found 

 that spore formation had taken place on all media. I also found that if 

 platings of B. dendroides on the KNOg-asparagine medium were dried 

 over H2SO4, spores were produced after 11 days. Thus spore formation 



Ann. Biol. IX 17 



