60 



S C I E N T I F I L- AGRICULTURE. 



February, 1921. 



Fig. 1. Fig. 2. 



Plate II. — The bacteria causing the root nodules on legu- 

 minous plants, y^ 1000. 



Fig. 1. — The normal vegetative form showing the whip-like 

 polar flagellum, the means of locomotion. (Cowpea bacteria-) 



Fig. 2. — The so-called "bacteroids." (Red clover bacteria). 



attack and destroy an animal thus defeating their own 

 ends, so the nodule bacteria destroy their own house 

 over their heads ; they are set free in the soil and must 

 rely upon chance providing a new opportunity. 

 The Bacteria. 



The bacteria, for such they were found to be, are 

 peculiar, that is if we may speak of any bacteria as 

 being peculiar. They are able to exist in several dif- 

 ferent forms, so different that one could scarcely 

 believe them to be the same organism. This char- 

 acteristic we term polymorphism. The common ve- 

 getative form is the small rod-shaped organism. Fre- 

 quently they exist, as small oval bodies much smaller 

 than the vegetative form called swarmers from the 

 German "schwarmer" as described in the early liter- 

 ature. The most characteristic forms and those which 

 identify the species are the branched X and Y forms 

 commonly called bacteroids, a misnomer since bac- 

 teroid means something resembling bacteria, but which 

 are not bacteria. Obviously the bacteroids are bac- 

 teria just as much as are the other forms. These 

 peculiar forms exist frequently as huge threads with 

 swollen end. sometimes branched to form an X or Y 

 and sometimes variously branched with no evident 

 scheme. In this form they are much larger than the 

 vegetative form and more resemble fungi or actinomy- 

 ces than they do bacteria. 



This polymorphism led to controversies in descrip- 

 tion, in classification and in naming. No less than 

 about twenty different names have been proposed for 

 this helpless, unsuspecting creature, such names as 

 Schinzia legvminosarum. Bacillus radicicola, Cladochi- 

 irium tuierculorum, Rhizobium lerjuminosannn, Psfu- 

 domonas radicicola, and .so on. Some seientLsts believed 

 the organism to be a fungus, some an actinomycetc, 

 and when finally agreed that it was a bacterium, they 

 disagreed on its morphological characteristics. And 

 so today the controver.sy goes merrily ou : different 

 scientists use different names. But in spite of the 

 controversies, in spite of the weight of these numer- 

 ous scientific appelations the little bug does its work 

 uncomplaining, and we of a practical turn of mind 

 call them nodule-bacteria and let it go at that. 



Another interesting trait of the nodule-bacteria is 

 their ability to fix nitrogen of the air without the 

 host plant, without the nodule. This mav be de- 



monstrated by growing the bacteria in culture solu- 

 tions, feeding them sugar and such mineral salts as 

 are essential. "We find, however, that the amount of 

 carbohydrate used up as food and energy for the bac- 

 teria is very great as compared to the amount of ni- 

 trogen fixed so that if we depended upon this process 

 under field conditions we should get very little ni- 

 trogen in return for the organic matter consumed. 

 I'\irthennore the soil contains other organisms equally 

 or more efficient in their ability to fix free nitrogen 

 of the air,- though not able to produce nodules. Among 

 these are Azotobaeter and Bacillus radiobacter. It is 

 only when working in S3-mbiotic relation in the root 

 nodule that the nodule bacteria are efficient. There- 

 fore this ability of the nodule bacteria to fix nitrogen 

 without the host is not peculiar to the nodule bacteria 

 alone and furthermore it does not explain the extra- 

 ordinary efficiency when working symbiotically. 

 Upmi this latter point there is no explanation. 

 Is Symbiosis Possible Between Nodule Bacteria and 

 Non-Leg^uminous Plants? 

 What would it mean to agriculture if we could in- 

 duce the nodule bacteria to work upon the crops other 





Plate ni. 



Fig. 1. — Longtidunal section of a nodule of red clover which 

 was well advanced but still growing, showing vacuolated bac- 

 terized cells, vascillar tissue, nodule corte.\, and meristem 

 X 100- 



Fig. 2. — Cross section of a similar nodule of red clover. 

 Mounted in iodine solution affer Flemming's triple stain to 

 show starch. The fibro-vascular bundles are especially 

 prominent. X 100. 



Courtesy Illinois Agr. E.xp. Sta. 



