120 BACTERIA IN RELATION TO PLANT DISEASES. 
cytoplasmic strands. The quantity of the bacteria may vary in infected cells, and there is a corre- 
sponding variation in the appearance of these cells. Normal cells contain starch-grains. Cells with 
no bacteria contain many starch-grains. Those containing a small number of bacteria also contain 
“some starch-grains, the number being in inverse proportion to the number of bacteria. 
Peirce’s study of the appearance of the tissue indicates beyond doubt that the nucleus and 
cytoplasm are seriously injured by the presence of the bacteria and finally destroyed (fig. 31). He 
looks upon the organism as beyond question a parasite. He says that the direction of the growth 
of the infection threads can not be determined by the oxygen or the nitrogen of the air, for if this 
were the case, the strands of bacteria would be found extending toward the periphery of the tubercle 
in all directions, which is not the case. ‘Not only do the infection threads run definitely toward 
the growing-point of the tubercle; they also grow toward the nucleus of each cell (fig. 22) which 
they enter.” Even where at first they seemed not to grow toward the nucleus a study of serial 
sections showed that such was the case, branches in sections above or below being given off toward 
the nucleus. 
““Microtome sections, differentially stained, as before described, of carefully fixed growing 
tubercles of the species of leguminous plants which I have especially studied, show that in most 
cases the infection threads run definitely toward the nuclei of the tubercle cells.” * * * ‘When 
infected cells contain any considerable number of bacteria, they cease to be able to divide.” * * * 
“The presence of the tubercle bacteria is not beneficial to the cells which contain the bacteria.” 
* * * “One point more needs to be made clear. Miss Dawson says that it is difficult to conceive 
how such strictly aérobic bacteria as these can flourish in the cells of such compact tissue as composes 
the tubercle. This difficulty is of her own conceiving, for do not the cells of the tubercles respire and 
are they not necessarily supplied with oxygen for respiration?’ * * * “Unless we are to imagine 
anaerobic respiration for these cells, it is unnecessary to assume it for the bacteria which infest them.” 
The following is a synopsis of the long paper by Hiltner and Stérmer, published in 1903: 
The “‘nitragin’’ inoculations having discouraged growers because very often the results were not 
what had been anticipated, additional seed and soil inoculations were undertaken and good results 
were often obtained, care being taken to use bacteria which were more active than those already 
in the soil. Experiments since 1900 have shown, however, that failure may be due not only to poor 
virulence but also to conditions which prevent the penetration of the bacteria. In inoculations of 
pure cultures mixed with soil, success depends on chemical and physical soil conditions favorable 
to the development of the bacteria. In inoculations by moistening the seed, failure is probably often 
due to the fact, discovered by the authors, that a substance dissolved out of the seed-coat during 
germination exerts an injurious influence on the bacteria. 
In spite of many failures, and in spite of the widespread opinion, expressed by Gerlach, that 
pure culture inoculations are of no value since sufficient bacteria already exist in the soil, the authors 
are convinced that soil inoculation has a future, since nitragin has given results safely beyond the 
limit of error in a sufficient number of instances to refute the opposing theory. One of these favorable 
experiments was that of Loges who obtained an increase in crop of 124 per cent with field beans, 
46.7 per cent with peas, and 4oo per cent with vetches. In this case the sandy field used had not 
borne any legumes, except lupins, for a number of years. Seed inoculation was used. The favorable 
result here was probably due to the fact that the seeds were soaked 24 hours before they were inocu- 
lated, and hence the bacteria did not have to encounter the injurious substance in the seed-coat. The 
inoculated seeds germinated more quickly than the uninoculated ones, which had also been soaked. 
Although this was undoubtedly independent of inoculation it gave opportunity for early and success- 
ful penetration of the bacteria into the roots. 
When rainy weather follows seed-inoculation, the bacteria may be washed from the seeds to 
places where they will not be affected by the injurious substance in the seed-coat and yet where they 
can reach the roots. When sufficient moisture is present, similarly, favorable results may be obtained 
by the otherwise uncertain method of direct inoculation of the soil. 
Dietrich obtained good results with blue lupin by inoculating the soil 12 days after sowing, when 
the plants were well started. Rainy weather favored the spread of the nitragin through the soil. 
His crop from inoculated soil was 65 per cent greater in green substance and 92 per cent richer in 
nitrogen than that from uninoculated soil. 
The inoculation of the field by strewing inoculated soil can not be recommended universally 
because results are too dependent upon the weather. If continued dry weather follows the sowing, 
the probability that the bacteria will reach the roots diminishes daily. 
In some cases where the inoculated fields showed an increase in crop of from 10 to 24 per cent 
over uninoculated ones, the difference in stand was scarcely noticeable. This fact may explain many 
seeming failures where the results have been judged only by appearances. 
se 
