2s8 THE BIOLOGY OF FLOWERING PLANTS 



§ 4. Bacterial Symbiosis 



Leguminosse. — It has been known from antiquity that 

 the growth of vetches and lupins enriches the soil. Pliny, 

 in the seventeenth book of his " Natural History," wrote : 

 " Every one agrees that nothing is better for manuring 

 the fields than green lupins ploughed or dug into the ground 

 before the pods are formed ; and that there is nothing better 

 for trees or for the vine than to bury, at the foot, handfuls of 

 this plant." In the eighteenth book he says of the lupin : 

 " It thrives in dry sandy places and requires no cultiva- 

 tion. . . . We have observed elsewhere that it enriches the 

 fields and vineyards where it is sown. Far from requiring 

 manure for its cultivation it itself takes the place of excellent 

 dung. . . . Vetches, too, enrich the soil." Theophrastus 

 wrote of the bean : " Beans are in other ways not a burden- 

 some crop to the ground, they even seem to manure it . . . 

 wherefore the people of Macedonia and Thessaly turn over 

 the ground when it is in flower." 



The use of these and of other leguminous crops as green 

 manure has long been an agricultural practice ; but only in 

 the 'eighties of last century was the reason for this improve- 

 ment of the soil satisfactorily cleared up and connected with 

 a well-known case of bacterial infection of the nodules or 

 tubercles on the roots of the Leguminosae. This was due in 

 the first place to the work of Hellriegel and Willfarth (1889), 

 who showed (i) that in sand lacking nitrogen salts, lupins 

 and peas could make satisfactory growth, while cereals 

 could not ; (2) that this satisfactory growth did not take 

 place in sterilised sand, but set in if the sand was watered 

 with an extract of arable soil ; and (3) that this was linked 

 with the formation on the roots of the well-known bacterial 

 tubercles which could evidently be formed when the 

 sterilised soil was infected with the necessary organism from 

 arable land. Where nodules were formed a definite and 

 considerable gain in combined nitrogen was registered. 

 The bacterium was isolated — a matter of difficulty, as it 

 does not grow under the conditions suitable for most 



