90 SYMBIONTICISM AND THE ORIGIN OF SPECIES 



The presence of bacteria in the root nodules of the 

 legumes was first established by Winogradsky. These 

 symbiotic bacteria {Bacillus radicicola) have been the ob- 

 ject of intensive research by a host of investigators. It 

 has been demonstrated that they have the faculty of fixing 

 atmospheric nitrogen, and rendering it available for use 

 by the host plant. The microorganism is present in the 

 soil. It enters the root hairs of the host plant and comes 

 to lodge in some of the root hair cells. Its presence stimu- 

 lates these cells to special growth, and an organ, — the root 

 nodule — is formed. The invading organisms are minute 

 rod-shaped structures. After a sojourn for some time in the 

 root nodule cells, they become transformed into rather 

 large bacilli and branched Y-shaped forms. In the older 

 cells of a nodule, the bacteria have become still more modi- 

 fied. In sections of the nodule stained by a mitochondrial 

 method, the author found large globular forms in the oldest 

 part of the nodule (Walhn, '22b). Lohnis ('21) had pre- 

 viously observed these forms, and beheved that they rep- 

 resent "regeneration bodies," but the true significance of 

 these globular forms is not known. In the young root 

 nodules no other organisms but the Bacillus radicicola are 

 present in the cells. As the nodules mature, other micro- 

 organisms begin to invade the cells of the nodule. In an 

 old nodule, very few if any Bacillus radicicola can be found 

 in the nodule. They appear to have been replaced by a 

 variety of parasitic microorganisms. 



In recent years botanists and bacteriologists have demon- 

 strated the presence of nodules containing bacteria on the 

 roots of a number of plants other than the legumes. The 

 physiological significance of these symbiotic relationships 

 is not known in most instances. There are also a number 

 of plants that contain extracellular symbiotic bacteria in 

 the leaves and other parts of the plant. Psychotria bac- 

 teriophila is an example of such a plant in which it has been 



