The organism enters near the tip of the root hair and stimulates the latter to curl into the form of a shepherd's crook. It travels down the interior of the root-hair in the form of a homogeneous strand, as seen in fresh preparations. In sections of young galls this strand is seen branched through the tissues from its point of entrance from the root-hair. These strands pass through the cell walls by minute perforations and then enlarge again in the cell-lumen. Often the strand swells into a large body in the cell, with irregular pro jections, which led some to think that the bacteria like bodies found in abun dance at a later stage were budded off from these swel lings. These strands present in the young tubercles led a number of students to be lieve in the fungous nature of the organism, perhaps related to the smuts ; but especially by some it was considered to be one of the slime -molds similar to the Plasmodiophora brassicce, which causes the "clubfoot" of turnip, cabbage, radish and certain other crucif erous plants. For this rea son Schrmter, a German botanist, named it Phytsm yxa leguminosarum, and this seems to be the earliest sci entific name. More recent investigations seem to show that the organism is one of the bacteria. Many bacteria form gelatinous masses of individuals, which take on various shapes often char acteristic of the species. Especially on cultures on solidified artificial media are these colonies of various shapes very characteristic. These gelatinous masses are known as zoogIcea. These strands, then, which are so characteristic of the younger stage of the tubercles, are zolighea. Frank, another German botanist, was one of the first to demonstrate this feature of the organism, and it is now generally accepted, although different views are held as to the mor phology of the bacterium. He named the organism Rhizobium leguminosaruin.
The study of the organism in pure culture began with Beijerinck in 1888, who named it Bacillus radicieola, thus discarding the earlier specific name. He discovered, beside the rod-like form which is abundant in the old tubercles, and previously named " bacteroids" by Woronin, a very minute motile form. These two forms of the organism are now generally recognized. The minute motile form is about 1 e long by 0.2µ in width (µ is a micron or of a millimeter). This is the form which enters the root-hairs, multiplies and travels in the strand-like zooglcea into the root where the gall or nodule is stimulated. Because of this motile form, Moore has recently changed the name to Pseudomonas radicicola, though the relation of the cilia to the organism is not very clearly known, in consequence of which there may be some uncer tainty as to the appropriateness of this name. The larger rod-like form is 1.5µ to 5 A long by 0.6µ to 2.5 A in width. These are the "bacteroids." They are usually rod-like, but often branched forms occur which are Y- or X-shaped, or even sometimes more complicated in form. These bacteroids or rods which are found in such large numbers in the old tubercles are abnormal, or involution forms. It is thought by some that the Y and X forms are the result of branching, perhaps a false branching caused by division of the rods, several rods being held together within a gelatinous sheath. It is
well known that these "bacteroids," or dead invo lution forms of the organism, are rich in proteid matter. The host plant, which is the legume, has the power of dissolving these and of absorbing the nitrogenous matter from the tubercles and using it as food. When the tubercles die, some of them are emptied into the soil, and the minute motile form also escapes, thus keeping the soil inoculated with this organism where legumes are growing.
It7zy legumes are valuable in soil-enrichment.
It has long been known that certain leguminous crops like peas, clovers and alfalfa, were better crops for the enrichment of the land in nitrogenous food when plowed under than the cereals or grasses. A series of investigations, notable among which may be mentioned those of Hellriegel and Willfarth in Germany, Lawes and Gilbert in England, and Nobbe, Hiltner and others in Germany, led to the clear demonstration that (1) in a soil possessing all the constituents of plant-food except nitroge nous substances, if the soil were sterilized and then inoculated with a filtrate from garden soil, legumes would flourish and produce an abundance of seed, and the tubercles would be present on their roots ; (2) in similar sterilized soil, not inoculated with a filtrate from garden soil, legumes would develop no tubercles and the plants would develop only so far as the nitrogenous food stored in the seed per mitted them ; (3) in a similar soil, even if inocu lated with a filtrate from garden soil, the cereals and grasses would make only a feeble growth ; (4) in similar soil, inoculated with pure cultures of the legume tubercle organism, the tubercles are formed, which demonstrates that the tubercles are caused by the bacteria ; (5) there was an increase in ni trogen in the plants with tubercles over those with no tubercles ; the soil also increases in nitroge nous content where legumes with tubercles are grown ; (6) races of the bacterium occur, since inoculations from pure cultures of the bacterium from pea tubercles will not produce tubercles on cytisus, robinia, trifolium, serradella and others, while they will on the pea, lupine and others, and vice versa.
The fact that the nitrogen content of soils poor in nitrogenous plant-food is increased by the growth of leguminous plants, was used in support of the early theory that green plants assimilate the free nitrogen of the air, a theory which was shown to be unfounded by Boussingault more than sixty years ago. The fact that all other green plants except the legumes could not fix the free nitrogen of the air, and the latter could fix it only when the tubercles were present, led Frank to assert that the presence of the bacteria in the tubercles stimulated the legumes to assimilate the free nitrogen from the air through their leaves. It has since been shown that this is not the case, that when the tubercles are present on the roots, and the roots are supplied with air deprived of free nitrogen, no nitrogen is fixed by the legumes.