VII.] THE NODULE ORGANISMS 133 



of nitrogen took place in the plant as a whole. The seat 

 of this fixation was then demonstrated to lie in the 

 bacteria themselves — these bacteria in fact live in 

 partnership (symbiosis) with the clover or other plants. 

 They receive from its leaves a certain amount of sugar 

 which they oxidise in order to obtain the energy 

 necessary to effect the combination of free nitrogen ; in 

 their turn they give up to the plant the nitrogen com- 

 pounds it requires for its life cycle. It has since been 

 found possible to cultivate these bacteria apart from the 

 leguminous plant with which they are usually associated 

 and to get them to fix nitrogen when fed with sugar, 

 but the amount so fixed is small compared with that 

 obtainable by them for the living plant. The organism 

 has also been shown to exist in several different forms ; 

 in the soil it takes a motile form very much smaller 

 than the rod-shaped and even branched forms which 

 may be seen in the nodules, and in this state it is 

 capable of passing through the fine root hairs of a 

 seedling leguminous plant, an infection which soon 

 becomes evident in the growth of a nodule swarming 

 with the transition forms of the bacterium. The 

 laboratory evidence for the fixation of nitrogen by these 

 bacteria was thus rendered very complete ; indeed it is a 

 simple matter to arrange an experiment to demonstrate 

 the process. All that is necessary is a series of small 

 pots, preferably of glazed earthenware, filled with sand 

 mixed with i per cent of calcium carbonate, the pots 

 and their contents being sterilised before starting by 

 heating in an oven for an hour or so. The seeds of 

 some leguminous plant — vetches, sweet peas, or lupins are 

 convenient for the purpose — are sterilised by washing in 

 alcohol and recently heated distilled water, and are then 

 germinated on sterile sand or filter paper until the 

 seedlings are large enough to handle. Meantime the 



