632 ETCHING OF MARBLE BY ROOTS 



comes of greater significance when we consider the observations of 

 Hoffman^'^ upon soil productivity. In twenty-seven out of thirty- 

 two cases examined, it was noted that a greater number of bacteria 

 were to be found adjacent to, rather than away from, the roots. 

 The question therefore arises: Are the bacteria to any considerable 

 extent responsible for the degree to which etching takes place? The 

 results of controlled experiments have shown that roots, when grow- 

 ing in soil containing bacteria, possess a greater etching power upon 

 marble than when growing in the complete absence of bacteria. Gen- 

 eral experiments were made using the apparatus described by Fred^^ 

 for growing higher plants free of bacteria, as well as large Pyrex glass 

 tubes. 



Slabs of marble 4.2 cm. wide, 15 cm. long, and 2 cm. thick, were 

 polished by rubbing the marble on cloth soaked with an aqueous solu- 

 tion of oxalic acid, and quickly removing any adhering acid by 

 means of running water. It was arranged to have the polished sur- 

 face of the marble slabs extend diagonally down the lower part of the 

 culture vessel. 



The same weight (250 gm.) of Miami silt loam soil from Madison 

 was placed in each glass tube after the original soil sample was thor- 

 oughly mixed. The soil was shaken somewhat, in order to fill all the 

 space about the marble. The tubes were plugged with cotton and 

 sterilized in the autoclave at 15 pounds pressure for 2 hours on 3 

 successive days. The tubes were tested as to their sterihty by ob- 

 serving whether or not growth took place after several days, when 

 some of the soil was placed in beef peptone broth. Water was then 

 added to each tube by means of a pipette, until the water content of 

 the soil was 20 per cent by weight. 



Canada field peas, that had been sterilized^^ with a 0.25 per cent 

 mercuric chloride solution with reduced pressure, were placed in 

 Petri dishes and covered with Ashby's agar. When the agar had 

 sohdified, the plates were inverted and incubated at 27°C. for 2 

 days. At the end of this period the seeds had germinated and short 

 radicles were projecting through the seed-coats. This treatment 



^^ Hoffman, C, Kansas Univ. Sc. Bull., 1914, ix, 81. 

 11 Fred, E. B., /. Gen. Physiol., 1918-19, i, 623. 



