ROOT-NODULES OF LEGUMINOSAE. 



105 



Beyerinck found swarmers in minute nodules which were still inclosed in the mother root. He 

 divides the root-nodule organisms into groups and varieties as follows : 



Group I. This contains the larger more hyaline colonies. Growth absent or difficult on meat 

 peptone gelatin. Growth is favored by cane-sugar or grape-sugar. Swarmers are very minute. The 

 bacteroids are two-armed, globose, or pear-shaped. Meristem is always present in the nodules. The 

 primary bark of the nodule is closed. Slime threads are distinct. The following forms belong here : 

 B. radicicola, vars.fabae, mcia-hirsutae, trifoliorum, pisi, lathyri. 



Group II. Colonies more cloudy white. Growth better on meat peptone gelatin. Swarmers 

 more rod-shaped, somewhat longer. Bacteroids like the bacteria, that is, seldom branched. Slime 

 threads absent or little developed. Mostly no meristem in the nodules (Robinia an exception). 

 Three types occur: (i) Phaseolus type; (2) B. radicicola, var. lupini; (3) Robinia type. 



In Vicia faba, as the bacteroids are exhausted the color of the cytoplasm changes from reddish 

 to intense green. The bacilli from this plant when grown in Faba stem gelatin in a cool place (cellar) 

 were alive and motile at the end of a year. Active cultures can be obtained from all parts of the 

 nodules which have been exhausted by the bacteria. They are present in a living condition therein 

 in great numbers. The result is 

 quite different when the host 

 empties out the contents of the 

 bacteroids. Then it is more and 

 more difficult to get any bacterial 

 growth from the meristem. The 

 longer the bacteria remain in the 

 nodules the more bacteroids occur. 



Beyerinck found saprophytes 

 in the nodule tissues mixed in 

 with B. radicicola and named at 

 least two B. luteo albus and B. 

 agglomerans. Another green 

 fluorescent form thought certainly 

 to come from the nodule was 

 identified as B. fluorescens putidus. 

 A form resembling B. radicicola 

 and found in certain nodules was 

 first named B. radicicola lique- 

 faciens, but subsequently Beyer- 

 inck came to regard this as an 

 intruder having nothing to do 

 with their formation. This lique- 

 fying organism was afterwards 

 called Bacterium beyerinckii by 

 Trevisan. 



The bacteroids are found in 

 other parts of the roots than the 

 nodule, but less well developed, 

 e. g., in the root-hairs and epi- 

 dermis cells. Beyerinck never 



found them in parts above ground, except once in a stem of Vicia faba where inoculated by hypo- 

 dermic injection. 



The bacteroids are always derived from the bacteria. They occur in old cultures as well as 

 in the nodules. The swarmers easily pass through the walls of the Chamberland filter. 



When fresh nodules are put into water at room temperature this water clouds first with a mixture 

 of bacteria, of which B. radicicola is the chief. Later, when the nodules decay, other bacteria appear. 



The tissues of legumes have a strong attraction for this organism, as is shown by the fact that 

 in such roots placed in the water any little cracks or wounds are immediately occupied by this 

 organism and the intercellular spaces flooded with it. These roots may be considered as a bacterial 

 trap apparatus. 



The infection of the living pericambium of the root must take place through pores, possibly 



Fig. 35.* 



*FlG. 35. -Poured-agar plates of Bad. leguminosarum from bean, introduced to show effect of repeated freezings: 

 a. Contents of a loop before freezing several hundred colonies per square centimeter; b. Contents of a similar loop 

 of culture fluid after 7 freezings less than one colony per each 2 square centimeters. Each freezing lasted half an 

 hour; time between freezing short, i. e., only long enough to thaw out tube in cool water and make necessary plates. 

 Round colonies are on surface; spindle-shaped ones are buried. 



