The First Land Plants 



were added to the soil (per hectare, that is, 2I acres) by 

 common vetch, 205 kilos, (about 4 cwt.) by peas, and 

 151 kilos. (3 cwt.) by lupines, in some recent experi- 

 ments.^ 



Now on the roots of all these leguminosae, one 

 finds odd little gall-like swellings or tubercles, which 

 have been investigated in many famous and classical 

 researches. The first to explain them clearly seems 

 to have been Schindler in 1884, who suggested that 

 they were due to bacterial <' friends." Then in 1888, 

 Hellriegel and Willfarth showed satisfactorily that 

 bacteria formed the swellings, and that these bacteria 

 obtained nitrogen from the air, and so handed it on as 

 nitrates to the plant. 



Pliny, Varro, and theother practical agriculturists were 

 thus shown to have anticipated our modern discoveries. 



The next point was to domesticate and utilise these 

 germs in ordinary garden and field work. But the 

 difficulties were great. 



These microbes have peculiar tastes. A vetch tubercle 

 bacillus (Pseudomonas radicicola is its proper style) 

 dislikes growing on beans or on anything except vetches. 

 The vigour or virulence of the microbe varies. Some- 

 times, when in captivity in a test-tube or on cotton- wool, 

 it becomes weak and languid, and refuses to work or 

 suddenly dies. And sometimes it gets far too strong, 

 and robs its vetch or bean-root of the nitrate obtained 

 by the roots. 



In good garden soil there are probably millions of 

 pseudomonas germs, and the question at once occurs, 

 Why then go to great expense in introducing it ? But 

 at first sight one would think that on poor or inferior 

 soil, especially where no leguminosae had been grown 

 for some years, there might be some advantage in using 

 artificially bred bacteria. 



46 



