3l6 CALIFORNIA ACADEMY OF SCIENCES. [Proc. 3D Ser. 



shows that below a depth of twenty centimeters from the 

 surface the number of tubercles decreases rapidly. The 

 roots of this plant are perennial, and the new roots each 

 season are most of them formed so" far below the levels in 

 which tubercles ordinarily occur on the roots of leguminous 

 plants that this plant should form a good test object of the 

 vertical distribution of tubercle bacteria in the soil. Com- 

 pared with such annuals as Bur Clover, a member of the 

 same genus, there were far fewer tubercles on Alfalfa roots 

 than on Bur Clover at the time that I dug around Alfalfa 

 (December). There were young roots near the surface as 

 well as further down, but the greater number of young 

 roots must have been formed far below where I reached by 

 digging, for there were not enough young roots above 

 to meet the needs of the plant. Alfalfa and many other 

 perennial Legnminosis, may therefore form the majority of 

 their new roots each year so deep in the soil that they can- 

 not become infected. As the tubercles are not perennial, 

 whatever advantage may accrue to the perennial legumi- 

 nous plant by its association with bacteria would be limited 

 in time and quantity to the early life of the individual, when 

 its roots were all in the layers of soil containing active 

 tubercle bacteria. The question is well worth study. 



IV. The; Structure of Root-tubercles. 



The structure of a tubercle is shown somewhat diagram- 

 matically in fig. 7. This is a sketch, at a magnification of 

 thirty-fiv^e diameters, of a section of a young and still grow- 

 ing tubercle. The section is parallel with the long axis of 

 the tubercle and at right angles with the root. The cam- 

 bium of the tubercle lies between a and b, parallel with and 

 in the broken curved line. This meristem is composed of 

 two or three layers of cells. Those toward the periphery 

 of the tubercle as well as those toward the center divide, 

 the cells toward the periphery differentiating rapidly into 

 cells which round off from one another and form the pow- 

 dery, cap-like tissue which wears away but protects 

 the meristem within just as the root-cap protects the 



