CLIMBERS, PARASITES, SAPROPHYTES. 207 



leguminous plants through the root-hairs. In the root-hairs 

 they give rise to fine tubes which, making their way into the 

 cortical tissue, stimulate it to active growth and thus lead to 

 the formation of tubercles. The developing tubercles are rich 

 in starch, and later each receives a branch from the vascular 

 bundle of the root. The bacteroids found in the fully grown 

 tubercle are developed inside the invading tubes. 



Analysis shows that the tubercles are very rich in nitro- 

 genous substances, also in potash and phosphorus, and they 

 seem to develop best in soils which are poor in nitrogen 

 compounds. The tubercles do not develop in plants grown 

 in garden or field soil which has been heated so as to kill 

 any organisms present in it. On the other hand, they grow 

 on roots of plants which have been germinated in garden 

 soil and then placed in culture solution. 



There can be no doubt that we have here an example of 

 symbiosis. The fungus apparently makes use of the free 

 nitrogen of the air, and brings it into combination, just as 

 some true bacteria do ; and it is probable that, while the 

 leguminous plant gets the benefit of the nitrogen-compounds 

 (nitrates) formed, the fungus is supplied with carbohydrates 

 (sugar) manufactured by the green plant. The arrangement 

 may be regarded as a special kind of mycorhiza. 



Some striking results have been obtained recently by Professor 

 Bottomley, who has taken a leading part in the investigation of 

 nitrogen-fixing organisms. For instance, two lots of Tare ( Vicia saliva) 

 seeds were taken. One lot was inoculated with nodule-bacteria and 

 planted in sterilised sand to which the requisite potash and phosphorus 

 salts were added. The vmtreated seeds were sown in sand containing 

 nitrate of soda in addition to potash and phosphate. The seeds were 

 sown in May ; in July the Tares supplied with nitrate of soda yielded 

 1-9% of nitrogen, while the inoculated Tares gave 3% of nitrogen. 

 Three experimental field-plots of Tares gave the following results in 

 September : (1) No nitrogenous manure used, 3 '41% nitrogen ; 

 (2) nitrate of soda used, 3 '75% nitrogen ; (3) inoculated seeds 4*04% 

 nitrogen. In the case of field-crops the results are less striking, 

 because fertile farm-soil always contains nitrogen-fixing Bacteria. 



Professor Bottomley 's work has led to a still more important 

 result : plants other than Leguminosae can be inoculated with 

 nitrogen -fixing Bacteria. Tomatoes have been made to yield a 

 greatly increased crop by this means ; the Bacteria were first culti- 

 vated in Tomato-juice. In Wheat the Bacteria can be induced to 

 establish themselves in the rind (cortex) of the roots, though no 

 nodules are formed. 



