BACTERIA IN RELATION TO PLANT DISEASES. 



The effect of the soil-inoculation on legumes differed from the effect of nitrate of soda in that, 

 in the former case after the period of germination, a peculiar and very characteristic hunger-stage 

 supervened which was followed by very energetic and rapid development of the plants. 



In two experiments under sterile conditions, the peas grew well in the nitrogen-free sand until 

 the food stored in the seed was exhausted and then dwindled, dying after about 6 leaves had been 

 formed. On these plants not a trace of root-nodules could be found. The same negative result was 

 obtained when the soil-extract was boiled or heated to 70 C. before adding it to the pots. 



They concluded, therefore, that the nitrogen assimilation of legumes was in some way connected 

 with root-nodules and the bacteria present therein. 



In other words, as expressed in their final report : 



There are, therefore, for the Leguminosae two sources of nitrogen, viz., the combined nitrogen 

 of the soil and the elementary nitrogen of the air, the latter being made available to them through 

 the agency of micro-organisms which, to be effective, must enter into a symbiosis with the plant. 



Numerous experiments with lupins failed : No successful second growth could be obtained with 

 pea soil and the conclusion was reached that the nodule organism of lupins must be different from 

 that of peas. Only when inoculations were made with soil from a field where lupins grew well 

 did the experimental plants overcome their nitrogen-hunger and do well. On this experiment two 

 check rows of pots were held, one untreated and one inoculated with extract from pea soil. In all 

 three the plants germinated and grew well at first. Then followed a period of starvation, each of the 

 three rows showing equal nitrogen-hunger at the end of a month. Then the first row became green 

 and grew well, while the other two rows continued feeble and red-brown in color. The roots of the 

 first row (inoculated with lupin soil) bore numerous large nodules. The roots of the second row bore 

 none whatever. The roots of the third row (inoculated with pea soil) bore none whatever, except 

 one plant on which asingle small nodule wasfound. Serradella behaved like the lupin. Peas, vetches, 

 and beans grew best in the third row. 



In Hellriegel's own words: 



" Leguminosenknollchen und Wachsthum der Papillionaceen in stickstofffreiem Boden lassen 

 sich willkiirlich hervorrufen durch Zusatz von geringen Mengen Kulturboden und Verhindern durch 

 Ausschluss von Mikroorganismen. Bei verschiedenen Papillionaceenarten wirkt nur der Zusatz von 

 gewissen Bodenarten Knollchen bildend und Wachsthum fordernd." 



Lawes and Gilbert sum up these experiments very well in the following paragraph : 



"The negative result with the Gramineae, the negative result with the peas when everything 

 was sterilized, or when the sand was not seeded by the soil-extract, the positive result with the peas 

 when the sand was seeded by the humus soil extract, the negative result with the lupins when their 

 soils were not seeded, or when they were seeded with the same extract as the peas, and the positive 

 result when seeded with the extract from the sandy soil where lupins were growing, seem to exclude 

 any other conclusion than that the micro-organisms supplied by the soil-extracts were essential agents 

 in the process of fixation. Further, the development of nodules on the roots was, to say the least, 

 a coincident of the fixation." 



The following year (1887) Dr. Wilfarth stated at the Naturforscher Versammlung in Wiesbaden 

 that they had repeated and extended their experiments with wholly confirmatory results (plate 12). 

 From this time on the scientific world generally accepted their views as may be seen from the fol- 

 lowing comments of Lawes and Gilbert: 



"Thus it may be considered established that the Papillionaceae can take the whole of their 

 nitrogen from the air. 



" It will be seen that the results are not only confirmatory of those given by Hellriegel the year 

 before, but that they are even much more definite and striking. Thus, taking no account of the 

 fraction of a milligram of combined nitrogen supplied in the soil-extract, the amount of dry matter 

 produced is nearly 50 times, and the amount of nitrogen assimilated is nearly 100 times, as much 

 with, as without, the soil-extract. ' ' 



The full account of their experiments was first published in 1888. No figures since published 

 are any more striking or convincing than the six plates which appeared in this epoch-making pub- 

 lication, two of which are here reproduced (plate 1 2 and fig. 32). 



In their experiments with serradella, Series C, 1897 (plate 12), each jar contained 4,000 grams 

 of sterilized nitrogen-free quartz sand to which was added the necessary nitrogen-free nutrient 

 salts (monopotassium phosphate, potassium chloride, calcium chloride, and magnesium sulphate). 

 Eight seeds were germinated in each jar, the number of plants being reduced soon after to four. 

 The plants were watered with distilled nitrogen-free water. 



To some jars additional fertilizers were added as follows: Nos. 264 and 265 each a small amount 

 of calcium nitrate (41 mgs.); the two end pots of each row (right side) received each 40 grams of 

 calcium carbonate which was sterilized by heat and mixed with the sand previous to planting; No. 

 250 received some potassium carbonate. 



