SYMBIOSIS. 



97 



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within the scope of this treatise. Is there such a thing? We will run over some of the 

 alleged sorts and let the reader decide for himself. The most discussed case in recent years 

 is the relationship existing between root-nodule bacteria and legumes. 



ROOT-NODULES OF LEGUMINOSAE. 



Should the root-nodules of Leguminosae be cited as examples of symbiosis? The 

 plant submits to distortions and enlargements and final destruction of portions of its roots, 

 giving water, mineral foods, and carbon compounds in exchange for which it receives 

 nitrogen compounds, at least this is the current view. The agricultural chemists appear 

 to be satisfied that the host-plant actually receives the nitrogen and that it is from this 

 source, and is not simply combined nitrogen drawn again to the surface of the earth by the 

 deep feeding roots of legumes, as would be our first thought, considering how readily 

 nitrogen compounds leach out of agricultural soils into the deeper substrata where they 

 can not be reached by surface feeding roots. 



The pathologist sees a nodular growth stimulated by the presence of a foreign organism 

 and various phenomena not unlike those of genuine parasitism as Peirce and 

 others have pointed out. We might have, however, a local injury and yet 

 general advantage to the plant if the bacteria really store nitrogen available 

 to the legume. 



The organism appears to be able to infect only through very young roots 

 or root-hairs (fig. 21). As soon as the cells of the roots have passed out of a 

 rapidly dividing condition the nodule takes on a definite form and ceases to 

 grow. Subsequently it passes through the same stages of disorganization 

 as other overgrowths in which there is no suggestion of symbiosis. That the 

 micro-organisms infest the interior of the cells, rather than the intercellular 

 spaces, does not alter the case materially. In the end they destroy plasma and 

 nucleus (fig. 31), and the nodule decays. So far then as the morbid anatomy 

 goes we must look upon Bad. leguminosarum as a restricted parasite. 



Does the host receive something in return ? The agriculturist has observed 

 a more luxuriant growth when the nodules are present on the roots of the plants 

 than when they are absent. This, however, by itself might mean only that 

 infections are most abundant on rapidly growing plants, i. e., on plants in a 

 good soil capable of inducing a rapid growth. In recent years, however, in 

 experiments on poor soils, marked increase of growth over that in untreated 

 check plants has been obtained sometimes, by infecting the soil or seeds with 

 this organism at planting time (plate 12). This experiment has yielded the 

 same result, it is said, on a large scale in field practice (see statements by 

 Hiltner and by Moore) , and that not once, but many times in widely different 

 localities. On many fields, however, no marked difference has been observed between the 

 check plots and those inoculated with this organism, and sometimes the check plots have 

 given the best returns, even when nodules have been abundant on the roots of the inoculated 

 plants. Sometimes these failures have been on fields already well stocked with nitrogen 

 compounds, but apparently such has not been the case always. Moreover, granting the 

 increased growth associated with an increased number of nodules on the roots it does not 

 necessarily mean free nitrogen assimilated by the bacteria and turned over to the host- 

 plant, unless it can be shown that combined nitrogen is absent from the soil and air, since 

 plants often make an increased growth under the stimulus of weak poisons. This extra 



*Fio. 31. Normal and shriveled nuclei from cells in root-nodule of soy-bean. The three lower nuclei are from 

 cells fully occupied by Bad. leguminosarum they are distorted, flattened, have taken the stain deeply, and apparently 

 little is left except skin of nucleus. The two at top of figure are globose and faintly stained except the peripheral 

 chromatin they are from uninfected cells distributed sparingly among cells destroyed by the bacteria. 



'/too mm 



Fig. 31.* 



