1900.] on Symbiosis and Symbiotic Fermentation. 267 



organism, whether vegetable or animal, is the gas nitrogen. The 

 source from whicb this element is derived by the plant has been 

 ascertained to be some compound present in the soil, either a nitrato 

 or nitrite, or a compound of ammonia. When we consider that the 

 atmosphere surrounding the leaves of a plant contains about 80 per 

 cent, of this gas, it seems surprising that this vast store should not 

 be utilised. The most careful experiments have shown, however, 

 that in the vast majority of instances atmospheric nitrogen is of no 

 use to a plant. 



During the past 10 or 15 years, however, various physiologists 

 have determined that in the case of tbe leguminous plants mentioned 

 just now, more nitrogen can be found in the body of the plant in the 

 course of very careful quantitative experiments, than can have been 

 derived from the medium in which it has been cultivated. Soil, plant 

 and manurial applications have been all most carefully analysed. Tbe 

 results have been so startling, and so contrary to received opinion, 

 that they have been very carefully checked by many distinguished 

 observers, but their absolute accuracy has been found indisputable. 

 Clearly then, these leguminous plants can in some way appropriate 

 the nitrogen of the atmosphere. A careful investigation of the phe- 

 nomena presented by these plants during their growth, led to the 

 discovery of the existence of a form of symbiosis, to which the 

 phenomenon is due. All the details of it are not yet clear, and no 

 doubt much time will elapse before the steps of appropriation are 

 fully known. When one of these plants, growing in ordinary soil, 

 is removed from the earth and its root-system is carefully washed, it 

 presents the appearance of much-branching roots, on which certain 

 tubercular outgrowths are visible. These occur, both upon the main 

 tap root and upon the branches, and are present in considerable 

 numbers, arising usually in those parts of the root where the root- 

 hairs occur. 



A tubercle cut across shows that its axial mass consists of large 

 polyhedral cells getting smaller towards the apex, where they form a 

 mass of rapidly-growing tissue. Several layers of compressed cells 

 surround them. 



In the formation of these bodies a tubular structure seems to 

 penetrate one of the root-hairs, and to make its way into the cells 

 just under the surface of the root, in which it branches somewhat 

 freely. It can be seen ramifying in the substance of the young 

 tubercle, the growth of which is apparently due to a hypertrophy of 

 the tissue caused by the stimulus of the irritation of its presence. 

 In the large-celled tissue the parasite becomes rampant, and indeed, 

 the tubes of which it at first consists, can be seen throughout the 

 tubercle. In older tubercles the tube can be seen to end, at first 

 blindly, in the centre of the cells, and from the blunt end by repeated 

 branching and constriction of the branches, an enormous number of 

 extremely small bodies are cut off, which accumulate in the proto- 

 plasm of the cells. They are of various shapes, sometimes straigbt 



t 2 



