SYMBIOTIC NITROGEN FIXATION 117 



until the diameter is about doubled, but they still remain non- 

 motile. These cells become ellipsoidal and develop high motility 

 in which stage they are known as " swarmers." The swarmers 

 elongate to form the rod-shaped cells which remain motile for a 

 time, but gradually become non-motile as the food becomes con- 

 sumed. When the food material becomes exhausted or other such 

 unfavorable conditions develop, the rods appear as banded cells 

 with the protoplasmic contents concentrated in several locations. 

 Eventually many of these cells become branched, swollen, or other- 

 wise changed from the rod shapes into the characteristic bac- 

 teroids. The bands of cell material may give rise to small cocci 

 once more when placed under suitable conditions of adequate 

 food supply. 



This relationship between the nodule organism and the host 

 plant has been called symbiosis, since presumably both organisms 

 benefit from the association. The plant produces carbohydrates 

 through its photosynthetic activities and supplies such food and 

 other nutrients to the bacteria as enable them to develop exten- 

 sively. The bacteria, through some agency, enable the plant to 

 derive nitrogen for its development through the transformation 

 of gaseous nitrogen to fixed compounds. Under certain condi- 

 tions, however, the relationship may not be one of symbiosis but of 

 actual parasitism. If the plant is unable to make normal growth 

 on account of a lack of certain essential nutrients, providing that 

 the bacteria enter the roots, the plant tissue may not react to 

 develop a normal nodule, but will initiate development of nodular 

 tissue which soon becomes extensively invaded by the bacteria, 

 completely destroying the tissue. 



The mechanism of the fixation of nitrogen is little understood. 

 The bacteria related to the process appear to be quite unable to 

 fix nitrogen while growing independently of the host plants. Like- 

 wise the legume fails to fix nitrogen while growing free from the 

 bacterium. It is only in the associative development that appre- 

 ciable fixation of the nitrogen occurs, and then only when the vas- 

 cular system of the root develops into the nodule and furnishes 

 carbohydrates for the growth of bacterial and plant cells; that 

 system must also carry away the products of cell development, 

 including presumably nitrogenous compounds resulting from the 

 fixation. Whether the bacteria fix the nitrogen under the condi- 

 tions existing in the nodule, or whether the plant fixes the nitrogen 



