9 8 



BACTERIA IN RELATION TO PLANT DISEASES. 



growth has, however, been obtained in some instances, it would seem, on soils practically 

 destitute of combined nitrogen, and also, I believe, in an atmosphere destitute of all traces 

 of combined nitrogen, i. e., of ammonia and nitric acid. 



Figure 32 shows a pea grown to maturity in a closed space on nitrogen-free sand (on 

 which oats and buckwheat starved) by adding soil extract containing the nitrogen root- 

 nodule organism. This water extract contained only 0.15 mg. of nitrogen, and the hermeti- 

 cally sealed carboy was opened to the date of the photograph (fiftieth day) but three times 

 and then for a few moments only to introduce a measured quantity of pure washed carbon 

 dioxide (6 liters) necessary for the growth of the plants. The capacity of the carboy was 



44 liters and while the contents of the air 



in combined nitrogen was not deter- 

 mined it could not have been over a 

 small fraction of a milligram. The pea 

 grew for a period of 4 months and 

 fruited, yielding a total dry weight of 

 IO -359 grams, of which 233.5 m g s - were 

 nitrogen. Some nitrogen was also re- 

 covered from the sand, making (after 

 proper deductions for nitrogen in the 

 seeds, etc.) 248 mgs. of fixed nitrogen. 

 The oats and buckwheat made only a 

 very starved growth and finally perished 

 without fruiting. 



It will be remembered that Boussin- 

 gault got no growth and no nitrogen 

 assimilation in a closed space under 

 sterile conditions. This is exactly 

 Boussingault's experiment plus the addi- 

 tion of soil extract containing root- 

 nodule bacteria, the result being entirely 

 different. 



The bacteriologist finds that in 

 pure cultures the root-nodule organism 

 appears to be able to grow on substrata 

 in which there is absence of nitrogen 

 compounds or at least great paucity of 

 such compounds. Maze states that the 

 organism requires a minimum of com- 

 bined nitrogen to make a good growth 

 and store nitrogen. He says it can not 

 do so if all initial combined nitrogen is 

 withheld. Others (e. g., Moore) state 

 that it can make a decided growth on 



media in which there is no combined nitrogen. According to these observers it makes a 

 good growth on media believed to be free from all nitrogen compounds, and hence the 

 assumption is that it must be able to obtain its nitrogen from the uncombined nitrogen in 

 the air. It is, however, a difficult matter for the ordinary bacteriologist to be assured that 

 all traces of nitrogen compounds are absent from a given culture medium and from the air 



*FlG. 32. Hellriegel and Wilfarth's pea No. 384 grown from germination time in a closed space alongwith an oat 

 plant and a buckwheat plant on nitrogen-free quartz sand. The sand which had been previously heated to redness was 

 mixed with nitrogen-free nutrient salts and enough twice-distilled water added to approximate 70 per cent of soil 

 saturation. The sand was then inoculated with a little soil extract from earth adapted to peas. 



Buc-liweiwn. Hnfrr. 



Erbse No. 384. 



Fig. 32.* 



