RELATIONSHIP TO HOST 305 



surrounding cells, and as growth takes place the cortical parenchyma 

 and epidermis are forced outward, thus forming a nodule. The 

 growth of the nodule is apical. The various tissues common to the 

 plant are present. In the central portion of the nodule is the so- 

 called bacteroidal tissue, which is ochre, flesh, or gray in color, 

 according to the age of the nodule, and in this portion the infecting 

 strand (Infektion-schlauche) is distinguished in the young nodule. 

 It ramifies throughout the cells, causing those which it enters to 

 lose their power of cell division but not of growth. Later, or in 

 older nodules, the infecting strand is not visible, and the bacteroidal 

 tissue loses its firmness. At the period when seed formation is at 

 its height, most of the nodules are soft, and the internal tissues slough 

 off, leaving the more resistant epidermal tissue a mere shell, which 

 later decays. The endurance of the nodule depends upon several 

 factors, chiefly, however, upon the kind of legume plant on which 

 it is produced and the need of nitrogen by that plant. 



"Pierce considers the nodules as originating endogenously from 

 the same layer of cells as the lateral roots, and as being morpho- 

 logically similar to them; however, as the lateral roots rupture the 

 epidermis the above statement is not entirely in accord with what 

 actually takes place. 



" The nodules are largest and most numerous where " aeration is 

 best in the soil. In saturated soils they occur at the surface and 

 are often found colored green, very similar to sunburned potatoes. 

 Nodules form in solutions, and exceptionally well in certain nutrient 

 solutions. Several interesting instances have been brought to the 

 attention of the Experiment Station, in which the observers believed 

 that the nodules had grown above the ground. These peculiarities 

 were undoubtedly caused by unobserved physical conditions occur- 

 ring at the time of infection or afterward." 



Relationship to Host. Even today the relationship between Ps. 

 radicicola and its host is a mooted question. Some authors claim that 

 they are true parasites and that the relationship between the 

 tubercle organisms and their host plants is that of .two contending 

 parties and the bacteria draw on the nitrogen of the air in their 

 endeavor to make up the deficiency of nitrogenous substances which 

 have been taken from them by the plant. Moreover, inoculation 

 experiments have demonstrated that Ps. radicicola causes a certain 

 resistance similar to that produced by an organism in combating a 

 true parasite. 



Hiltner has given the six following conditions as instances in which 

 immunity demonstrates itself: 



1. The organisms cannot get into the plant. 



2. The organisms gain admission into the plant, but do not pro- 

 duce nodules because the plant, by its greater resistance, absorbs 

 the bacteria. 



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