from the observations of Maze
that the bacterium can even absorb free nitrogen when grown in cultures
[v.03 p.0166] outside the plant. We have here a very interesting case of
symbiosis as mentioned above. The green plant, however, always keeps the
upper hand, restricting the development of the bacteria to the nodules and
later absorbing them for its own use. It should be mentioned that different
genera require different races of the bacterium for the production of
nodules.

The important part that these bacteria play in agriculture led to the
introduction in Germany of a commercial product (the so-called "nitragin")
consisting of a pure culture of the bacteria, which is to be sprayed over
the soil or applied to the seeds before sowing. This material was found at
first to have a very uncertain effect, but later experiments in America,
and the use of a modified preparation in England, under the direction of
Professor Bottomley, have had successful results; it is possible that in
the future a preparation of this sort will be widely used.

The apparent specialization of these bacteria to the leguminous plants has
always been a very striking fact, for similar bacterial nodules are known
only in two or three cases outside this particular group. However,
Professor Bottomley announced at the meeting of the British Association for
the Advancement of Science in 1907 that he had succeeded in breaking down
this specialization and by a suitable treatment had caused bacteria from
leguminous nodules to infect other plants such as cereals, tomato, rose,
with a marked effect on their growth. If these results are confirmed and
the treatment can be worked commercially, the importance to agriculture of
the discovery cannot be overestimated; each plant will provide, like the
bean and vetch, its own nitrogenous manure, and larger crops will be
produced at a decreased cost.

[Illustration: FIG. 17.--A plate-culture of a bacillus which had been
exposed for a period of four hours behind a zinc stencil-plate, in which
the letters C and B were cut. The light had to traverse a screen of water
before passing through the C, and one of aesculin (which filters out the
blue and violet rays) before passing the B. The plate was then incubated,
and, as the figure shows, the bacteria on the C-shaped area were all
killed, whereas they developed elsewhere on the plate (traces of the B are
just visible to the right) and covered it with an opaque growth. (H.