February, 1921. 



SCIENTIFIC AGRICUUTURE 



59 



Symbiotic Nitrogen -Fixation by Leguminous Plants 

 with Special Reference to the Bacteria Concerned 



By ROY HANSEN 

 Professor of Soils, University of Saskatchewan. 



(Read before the Western Canadian Society of Ag'ronomy and published thi-ough the courtesy of tliat Society.) 



Historical. 



The use of leguminous crops for the pui'pose of re- 

 newing the fertility of the soil dates back before the 

 birth of Christ. The early Roman literature contains 

 a number of references to the use and benefits of 

 these crops. The following quotation from Columella, 

 written in the first century, A. I), will serve as a good 

 illustration: "Some of the leguminous plants manure 

 the land, according to Sasema, and make it fruitful, 

 whilst other crops exhaust it, and make it barren. Lu- 

 pines, beans, peas, lentils and vetches are reported to 

 manure the land. Where no kind of manure is to be 

 had, I think the cultivation of lupines will be found 

 the readiest and best substitute. If they are grown 

 about the middle of September in a poor soil, and then 

 plowed in (when well grown), they will answer as 

 well as the best manure." 



In addition to the crops mentioned b.v Columella, 

 lucerne or alfalfa and fenugreek are mentioned in the 

 early literature as having the same effect as manure. 



Another bit of interesting historical information was 

 added by Schultz-Lupitz, a German farmer with a 

 somewhat scientific turn of mind who published an 

 account of his experiences in 1881. .lie was growing 

 cereals on poor sand.v land with steadily decreasing 

 crop yields. He then began growing legumes and 

 continued until he had grown 15 consecutive crops on 

 the same land. To his surprise instead of the yields 

 growing smaller they increased steadily. Furthermore 

 he found that when he sowed cereals on this land 

 which had grown lupines, the .vield of cereals was two 

 or three times the yield of grain where no lupines had 

 been grown. Further he noted that the nitrogen-con- 

 tent of the soil which had borne lupines was consider- 

 ably greater than where they had not been grown. In 

 other words, whereas the yields of cereals on his land 

 steadily decreased, the yields of lupines increased; the 

 lupines in some way benefited the cereal crop follow- 

 ing; and they left the soil richer in nitrogen than 

 before. 



Neither the Romans nor Schultz-Lupitz nor the 

 scientists of their times understood the reasons for this 

 peculiar behavior of leguminous plants, and it remain- 

 ed for Hellriegel and Wilfarth to demonstrate in 

 1886 that legumes were able to utilize the free nitrogen 

 of the air, that this power to utilize the free nitrogen 

 of the air was associated with the presence of nodules 

 on the roots, and that the presence of certain micro- 

 organisms was necessary to the process. Their experi- 

 ments were conclusive, their results far-reaching. The 

 work stands as a model in agricultural research. 



In 1889 Beyerinck, a Dutch investigator, isolated 

 and described the organisms which inhabited the root 

 nodules. 



The Nodule. 

 Perhaps a few words about the structure and 

 functioning of the nodule might be of interest. The 



bacteria gain entrance to the host plant by dissolving 

 the cell wall of a root-hair or by attacking the root 

 epidermis. Once within the plant root, they multiplj' 

 and make their wa.y into the root cortex, just outside 

 the central cylinder. We must bear in mind that in 

 dichot.yledenous plants the fibro-vaseular system is 

 centrally located. In the root cortex an irritation or 

 stimulus is set up resulting in rapid cell division ; a 

 meristem is formed, and the young nodule emerges 

 from the side of the root in much the same fashion 

 as does a lateral root. But whereas in a normal lateral 

 root the fibro-vascular system is centrally located, in 



Plate I. — Nodules on the roots of young cowpea plants. 

 These nodules are caused by bacteria which enable the plants 

 to take nitrogen from the air. 



the nodule the strands are separated by the parasitized 

 parenchymal cells which oceupj- the central part of 

 the nodule. The divided strands pass from the central 

 cylinder of the root around the bacterial tissue and 

 connect with the tip or meristem end of the nodule. 

 AJl growth of course occurs in the meristem and these 

 fibro-vascular strands allow the interchange of food- 

 materials; thus the nodule is fed and thus a channel is 

 provided for suppljnng the plant with nitrogen-com- 

 pounds prepared by the bacteria. As the nodule grows 

 in length, the meristem is further removed from its 

 base of supplies, the bacteria appropriate completely 

 the cell contents, the interchange through the fibro- 

 vascular strands is interrupted and the nodule dies, 

 decays and finally sloughs off. As disease bacteria 



