specific for the groups as determined by cross-inoculation. By
cultural characteristics also the organisms from different legumes
show marked differentiation. Three distinct groups can be made
with reference to the rate of growth on artificial media, stickiness
of the culture and opacity of the colonies. All these facts form per-
haps a legitimate basis for the belief that distinct species exist among
the nodule-producing bacteria. In numerous other characteristics,
however, these organisms are so much alike, and as a whole they
differ so widely from any other species of bacteria, that it seems more
consistent to regard the adapted forms as varieties of tl\e single
species Pseudomonas radicicola.
Symbiotic nitrogen fixation has been found to occur in plants other than those of the Leguminosae; glands in the leaves of species of the Rubiaceae and Myrsinaceae, which were formerly believed to contain protein crystals, have been shown in reality to consist- of colonies of bacteria living symbiotically with the plant cells, re- ceiving their necessary supply of carbohydrates and salts from the surrounding green tissue of the leaves, and in return giving up their nitrogenous by-products to the plant. These organisms have been shown to fix atmospheric nitrogen when grown in artificial culture solutions devoid of any form of combined nitrogen. Their rela- tionship, if any exist, to Pseudomonas radicicola has not yet been determined. These bacterial glands have been found in a number of plants, including Pavetta, Psychotria, Kraussia and Ardisia, and seem to be as closely wrapped up with the well-being of the plants as are the root nodules of the Leguminosae; the organisms are present in the slime between the young leaves before the opening of the leaf buds, and have been found in the seed between the scutellum and the embryo. Their introduction to the seed takes place at the time of fertilization, the pollen tube conveying them from the stigma to the ovule. The infection of the leaves occurs immediately after the opening of the buds, the ordinary water pores of the leaf usually functioning as the ports of entry. In the cases of Pavetta and Psychotria, however, where the glands appear on the lamina of the leaf, a special stoma has been described as an extraordinary adapta- tion of the plant for the reception of the bacteria. This pore is of exceptional size as compared with the ordinary stomata of the leaf, and is said to be filled in by growth of the surrounding tissue after its function has been fulfilled.
The benefit derived by the host plants from the presence of their guests has been clearly demonstrated by seedlings raised from bacteria-free seed obtained by careful hot-water treatment of the seed in sterile and inoculated sand cultures fertilized with pot- ash and phosphorus but no nitrogen compounds. The plants grown in the inoculated cultures flourished and possessed typically green leaves, while those in the sterile sand showed all the signs of nitrogen starvation and soon died off.
Cellulose Fermentation. The classical investigations of Omelianski showed that the cellulose of plant remains was decomposed under anaerobic conditions giving rise to marsh gas and hydrogen. This knowledge, however, does not help towards an explanation of the rapid destruction of plant residues in ordinary cultivated soils where conditions are mainly aerobic. It is well recognized that the looser the soil the more rapid is the destruction of carbohydrate material. It is generally supposed that fungi play an important part in these processes and many species of moulds and actinomyces have been shown to possess the power of attacking cellulose. The American workers have invented cellulose media upon which bacteria can be cultivated, and have succeeded in isolating several species, Bacillus rossica, B. Amylolyticus, Bacterium flavigena and some fifteen others which are capable of using pure cellulose as their only source of carbon. All these organisms are morphologically and physiologically distinct from Omelianski's hydrogen ana methane organisms and grow well on ordinary gelatine media. The most powerful oxidizer of cellulose, however, is an organism discovered at Rothamsted in 1919. It is a peculiar organism exhibiting two distinct morphological characters at different stages in its life history, a long sinuous thread- like form and a large round " sporoid " form; it seems rather to be related to the spirochaetes than to the true bacteria and has re- ceived the name Spirochaeta cytophaga. It is an obligate aerobe and rapidly attacks cellulose, though it has no power of fermenting other carbohydrates; in fact, the presence of sugars, especially of the reducing sugars, strongly inhibits its action upon cellulose. Like the nitrifying bacteria it cannot be cultivated upon ordinary nu- trient media containing proteins, 0-25 % of peptone being sufficient to prevent growth. The products of decomposition of cellulose con- sist of a mucilaginous substance, small quantities of fatty acids and a yellow pigment allied to carotin. The discovery of this organism helped materially towards the production of an artificial substitute for farmyard manure, a great achievement in these days when motor traction has so reduced the available supply of this universal fer- tilizer. Moreover, the substitute has a considerable advantage over the natural product since the carbon-nitrogen ratio can be perfectly controlled.
Sulphur and Phosphorus Cycles. Considerable attention has recently been paid to the conversion of the sulphur and phosphorus present in the proteins of plant and animal residues in the soil; by series of bacterial reactions, forming complete cycles, these ele- ments pass from their combination in the protein molecule into the forms of sulphates and phosphates, and so become taken up and,
once more, elaborated into the organic constitution of the plant. It has long been assumed that the supply of sulphates in all soils was sufficient for the optimum growth of crops. This assumption was based upon the low sulphur content of plant ash; recent in- vestigations have shown, however, that as much as 90 % of the sul- phur of the plant may be lost in the process of ignition. The amount of sulphur removed by the crop from the soil is now a factor to be considered, and it has been shown experimentally that sulphur may become the limiting factor for crop production.
Further, the sulphur and phosphorus relations in the soil are con- sidered to be interrelated to the extent that the insoluble rock phosphate is rendered soluble by the action of sulphuric acid pro- duced in the oxidation by bacteria of the hydrogen sulphide from decomposing proteins. Pot experiments have shown that the application of sulphur as a fertilizer together with rock phosphate tends to increase the availability of the phosphate : the evidence at present, however, is insufficient to show whether any material profit is to be gained by this method of fertilization.
Sewage Disposal. The purification of sewage by the aerobic bacteria which are normally contained in it is so slow, requiring many days for completion, that sewage disposal by this means alone has long been regarded as impracticable. A method of hastening the process was, however, discovered in 1913, and since 1916 the " Ac- tivated Sludge Process " has actually been in successful operation. When sewage is well aerated the colloidal suspended matter grad- ually disappears, being acted upon by aerobic bacteria, and gives place to a granular brown mass which rapidly settles, leaving a clear solution of the inorganic salts, such as chlorides and nitrates, with only quite small amounts of soluble organic matter. It was dis- covered that this brown sediment added to a fresh supply of sewage and aerated by a blast of very fine air bubbles considerably hastens the oxidation process. On repetition, each increase in the amount of the sediment in relation to the volume of sewage is accompanied by an increase in the rate of oxidation, so that, when the relative amount of sediment approaches 30 % of the total volume, oxidation is complete in the space of a few hours. This brown sediment forms the so-called " activated sludge," and consists very largely of a mass of living organisms, bacteria and protozoa.
In practice two tanks are employed: (i) the aeration tank in which the sewage and activated sludge are blown with air forced through porous material so that it reaches the sewage in a finely divided state, and (2) the settling tank in which the sludge is de- posited and from which an effluent requiring no filtration is run away. Any excess of sludge over and above that required to main- tain the necessary quantity of 25 % to 30 % in the aeration tank is spread out to dry by evaporation and forms a valuable soil fertilizer.
The percentage of nitrogen in the activated sludge is considerably higher than that of the sludge from the sedimentation and septic tanks of the older and more usually employed method of sewage treatment. The results obtained from the activated sludge process in operation at Manchester show a yield of nitrogen per annum approximately equal to the total faecal nitrogen of the sewage treated, whereas in the older method much of this and all the urine nitrogen passes away in the effluent in the form of nitrates.
It has been stated that fixation of atmospheric nitrogen actually occurs in the process; from what is now known of the energy rela- tions of the nitrogenrfixing bacteria, however, any considerable amount of nitrogen fixation in a medium where the quantity of soluble nitrogenous compounds is large in relation to the quantity of carbohydrate material seems very doubtful. It is more probable that the conservation of nitrogen results from the fixation of ammonia which in the older process of sewage disposal becomes converted into soluble nitrates.
As at present produced the amount of nitrogen in the dry sludge is about seven per cent. If by any means this can be increased to about 10% and if economical methods of drying the sludge can be found there is a great commercial future for the process. As it is, around Worcester, England, where by the activated sludge process something like a million gallons of sewage are treated daily', the fruit growers take away the sludge in a semi-dry condition and pay about 303. a ton for it as it lies on the works.
Bacteriosis in Plants. The study of bacteria in relation to plant diseases may be said to have been in its infancy in 1910. At that time mainly through the researches of American bacteriologists it had been shown that bacteria could enter healthy plants through wounds and stomata and produce epidemics of disease so serious in nature that the crops over wide areas were partially, and in some instances entirely, destroyed. The subsequent decade saw consider- able activity in the field of plant pathology, and the pathogenicity of certain bacteria for plants has been fully established. In fact the number of species of bacteria now known to produce disease in plants is rapidly approaching that of the human pathogenes.
In comparing the disease-producing organisms in animals and plants one finds bacilli freely represented in both groups, but whereas the coccoid types, Streptococcus, Micrococcus and Staphylo- coccus, are frequently responsible for disease in animals, they have so far never been found to be associated with a disease in plants; on the other hand the genus Pseudomonas of Migula is strongly represented amongst the plant pathogenes while having no place, so far as is known, amongst the organisms pathogenic for
