392 



LEGUMES 



LEGUMES 



/ 



Fig. 588. Root nodules. 



'veriTHfoliumpratense). One 

 and one-fourtli times natural 

 size. 



Red clo- 



Legume Root-tubercles. (Figs. 588-592.) 



By George F. Atkinson. 



The legume root-tubercles, or "nodules," are 

 small galls on the roots of leguminous plants, which 

 are caused by the activities of minute bacteria 



present in the soil 

 wherever leguminous 

 plants grow. The galls 

 vary in form on dif- 

 erent species or gen- 

 era, being oval on the 

 red clover, rounded 

 and slightly lobed on 

 the soybean, cylindri- 

 cal or club-shaped, 

 simple or branched 

 once or twice, on the 

 vetch (Vieia sativa), 

 or many times dichot- 

 omously branched 

 into a rounded mass, 

 as in Medieago den- 

 tieulata. They are 

 whitish or of a pale 

 flesh-color, sometimes 

 sordid brown in age. 

 They occur on the 

 roots of nearly all 

 leguminous plants, 

 but are absent on 

 some, as, for example, 

 on the honey locust (Gleditschia triacanthos). 



History of the study of root-tubercles. 



While the history of the study of these root- 

 tubercles of leguminous plants is extremely inter- 

 esting, reference can be made here only to a few 

 of the diverse views which have been entertained 

 as to their nature, origin and significance. Some of 

 the early observers thought that they were galls 

 produced by insects, or by eel-worms. By others 

 they were regarded as lateral roots with dwarf 

 growth, or swollen lateral root organs for the 



purpose of absorbing 

 food, while others 

 held that they were 

 lenticels which played 

 some physiological 

 role in the life of the 

 plant. They were also 

 thought by others to 

 be imperfect buds 

 which could repro- 

 duce the plant. They 

 were classed as fungi 

 of the genus Sclero- 

 tium by some, or as 

 pathological out- 

 growths. Since Wor- 

 onin, in 1866, discov- 

 ered in the nodules bacteria-like bodies, which he 

 thought to be the cause of their formation, the 

 theory has been generally accepted that they are 

 galls produced by the presence of fungi or bac- 



Fig. 589. Root nodules of alfalfa 

 (clustered on smaU side root- 

 lets in this case) . Two-thirds 

 natural size. 



teria, which enter through root-hairs and stimulate 

 the tissues of the root to the production of an 

 abnormal rootlet, which is called the tubercle or 

 nodule. 



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 brassiece, 

 which causes the "clubfoot " 

 of turnip, cabbage, radish 

 and certain other crucif- 

 erous plants. For this rea- 

 son Schroeter, a German 

 botanist, named it Phytom- 

 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 zooglcea. These 

 strands, then, which are so 

 characteristic of the younger stage of the tubercles, 

 are zoogloea. 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 

 RMzobium leguminosarum. 



The study of the organism in pure culture began 

 with Beijerinck in 1888, who named it BacUlus 

 radieicola, 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 



Fig. 590. Root nodules. 

 Soybean {Glycine his- 

 pida). One -half nat- 

 ural size. 



