LEGUMINOUS FORAGE CROPS 127 



cultivated legumes, the presence of tubercles is, in most soils, 

 essential to a growth that is economically successful. This 

 has been demonstrated many times with alfalfa when introduced 

 into regions not hitherto growing alfalfa. 



134. Character of the Tubercles. The tubercles vary in shape 

 and size with the species of plant, each species bearing tubercles 

 that are fairly characteristic in size and form. They may be 

 round, oval, pear-shaped, or variously lobed. They may vary 

 in size from 0.06 inch or less to 0.31 inch or more in diameter. 

 They are generally rather loosely attached and are readily 

 detached when roots are pulled from the soil. They are a 

 modification of the cell structure of the roots, caused by the 

 presence of the micro-organism. They are softer and more 

 juicy than the ordinary root structures. The organisms enter 

 the plant through the root hairs, hyphae being formed which 

 enable the bacteria to pass from cell to cell. 1 



Tubercles occur on the roots of other than leguminous plants. 

 Some are due to the larvae of insects; others to fungi proper 

 (mycelium-bearing cryptogams) ; while others may be due to 

 bacteria. Hiltner has shown that some plants other than leg- 

 umes fix nitrogen with the aid of bacteria residing in the 

 tubercles of their roots. 



135. Form of the Organism. It is not the tubercle that aids 

 the plant in the acquirement of the free nitrogen, but the or- 

 ganisms for which the tubercle constitutes a home. The 

 organism (Bacillus radicicola Beyerinck) which occupies the 

 tubercle is generally considered to be a bacterium, although it 

 contains some exceptional features of structure and develop- 

 ment. The process by which these organisms find their condi- 

 tions of growth in the host plant, and at the same time promote 

 the growth of the host, has been called symbiosis. 2 The nature 



1 See G. F. Atkinson: Cont. Biol. Org. Causing Leguminous Tubercles; in 

 Bot. Gaz. Vol. XVIII, pp. 157, 226, 257. 



2 The Chinese word for this process means friendship. 



