268 Professor J. Beynolds Green [Feb. 9, 



and rod-like, at others formed like a V or a Y. The latter have been 

 seen to arise from the branching of originally straight ones. As the 

 tubercle still grows, there goes on an enormous coincident develop- 

 ment of these minute bodies which, from their resemblance to bacteria, 

 have been called bacteroids. When the root perishes at the end of 

 the summer, most of the plants being annuals, these bacteroids are 

 liberated from the cells in which they are formed, the latter decaying. 

 The soil, consequently, in which these plants have been growing, 

 contains them in large numbers. The original infection of the plant 

 is no doubt brought about by one of these organisms in the soil coming 

 into contact with the root-hair, into which it makes its way as already 

 described. 



We know but little so far of the steps by which the nitrogen is 

 made to enter into combination. It depends upon the bacteroid or 

 the tubular structure to which it gives rise, for if the soil is carefully 

 sterilised no such appropriation takes place. If such a soil is after- 

 wards watered with an extract of an uusterilised soil in which some 

 of the plants have been growing, the tubercles will be speedily pro- 

 duced and the gain of nitrogen made evident. The same result can 

 be obtained by inoculating a root with a bacteroid obtained from a 

 pure culture of the organisms. 



There is some evidence which points to the fungus or schizo- 

 phyte as having the power to fix the nitrogen. In some cases this 

 appears to be done in the sheath of the tube as it penetrates the 

 tissue of the root. In some cases these tubes have not been observed, 

 so that this cannot be the only locality. It seems probable that it goes 

 on also in the cells which we have seen are filled with the bacteroids. 

 In any case the appropriation seems to be done by the lowly partner 

 in the symbiosis, which thus provides valuable nutrient matter for the 

 leguminous plant. The latter cannot independently fix the nitrogen, 

 whether it is in symbiosis or not. 



The advantages which the green plant affords to its fellow sym- 

 biont are such as have been described in other cases already. 



These cases of symbiosis are on the whole not very difficult to 

 explain, and we can trace more or less fully the influence of the one 

 plant upon the other. Associations of organisms are found also lower 

 down in the scale of life and their relations are much more obscure. 

 The symbiosis seems in many cases to be shared by several organisms, 

 but most probably in nearly all cases some of these are only casual 

 intruders which have nothing to do with the true association of the 

 others. 



We find many instances of a relation of this kind among the 

 fungi and schizomycetes which set up various fermentations. It is 

 necessary here to define carefully what we mean by the terms sym- 

 biosis and symbiotic fermentation when used in connection with these 

 organisms. The idea of mutual co-operation for the common benefit, 

 which has been traceable through all the relationships so far con- 

 sidered, cannot be seen so completely in these cases, no doubt because 



