314 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



shriveled. They have grown fast, attained maturity early, 

 and they die young. The reasons for this are probably 

 two : First, in the upper layers of soil, which are certainly 

 best aerated, the bacteria in the tubercles obtain the uncom- 

 bined nitrogen which they absorb and fix (Maze, 1897) 

 more readily and more abundantly than those in tubercles 

 farther down; hence, second, they grow and multiply more 

 rapidly, the tubercle-cells are irritated proportionally. Be- 

 cause the rapid growth and multiplication of the bacteria, 

 and, probably as a consequence, of the tubercle-meristem 

 cells also, are not uniform, branching occurs as a result of 

 some parts of the tubercle growing faster than others. The 

 greater activity of the bacteria and of the host-cells in these 

 branched tubercles is not accompanied by adequate, much 

 less proportional, growth of the base, and of the conducting 

 tissues in the base, of the tubercle. These tubercles sooner 

 cease to receive as much food from the leguminous plant as 

 they need, and hence are the first to die. Since these 

 branched tubercles are the largest and contain most bac- 

 teria, one would suppose they would benefit the plant more 

 than the smaller ones (if any tubercles are beneficial), and' 

 that they would be best supplied with afferent and efferent 

 conducting tissues, as indicated by the proportional size of 

 their bases. This is not the case. 



The vertical distribution of tubercles on the roots has 

 been reported by Frank (1890, pp. 22-3). The greatest 

 number and the largest tubercles occur on the lupine within 

 seven centimeters of the surface of the soil, and there is 

 a rapid decrease in both number and size as the depth 

 increases, till below fifty-three centimeters none was found. 

 The distribution on Bur Clover roots corresponds. The 

 strictly aerobic character of the tubercle bacteria, as shown 

 by artificial cultures, accounts for this, but the relations of 

 these organisms to the uncombined nitrogen of the air as 

 well as to the oxygen should be borne in mind when these 

 bacteria are said to be aerobic. The distribution of air in 

 a soil varies with the nature of the soil — a well drained 

 gravelly soil being well aerated to a greater depth than a 



