LEGUMES 



LEGUMES 



393 



form. These two forms of the organism are now 

 generally recognized. The minute motile form is 

 about 1 M long by 0.2 n in width (/i is a micron 

 or -nnnr of a millimeter). This is the form which 

 enters the root-hairs, multiplies and travels in 

 the strand-like zoogloea into the root where the 

 gall or nodule is stimulated. Because of this motile 



Root nodules. Black medic {Medieago lupulina). 

 Two and one-half times natural size. 



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 1.5 /u to 5 /» long by 0.6 /* to 

 2.5 yn 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. 



Why 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 Prank 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. 

 On the other 

 hand, it has been 

 shown by Maz6 

 and others that 

 under proper 

 cultural condi- 

 tions the tuber- 

 cle bacteria on 

 artificial media 

 fix (by assimila- 

 tion) free nitro- 

 gen from the air. 

 That they do fix 

 free nitrogen 

 from the air, 

 when in the tu- 

 bercles of the 

 legumes under 

 normal condi- 

 tions, is abun- 

 dantly proved, 

 thus confirming the results of empirical observa- 

 tions, that leguminous plants grown in soils poor 

 in nitrogen flourish and sometimes have a larger 

 content of nitrogenous substance at maturity than 

 they could have obtained from the poor soil; that 



Fig. 592. Root nodules. Vetch (.Tida 

 viilosa). Two-thirds natural size. 



