68 MISSOURI BOTANICAL GARDEN. 



It may be noted that the greatest number of tubercles 

 either branched or unbranched are attached by a small root 

 to a larger one, it seldom happening that the tubercles or 

 clusters are attached close to a large root. Often a cluster is 

 formed by a number of rootlets, which are closely packed to- 

 gether, each of which may have several tubercles, so that 

 the irregular masses composed of unbranched individual 

 tubercles are probably caused by the infection of many ad- 



jacent rootlets. 



est 



may 



lg prob- 

 ably taken place through an epidermal cell or a root hair. 

 Instances are very rare where a tubercle, or cluster of them, 

 terminates a root so as to result in such massive structures 

 as on the alder. At first the young tubercle is merely a bulg- 

 ing of the epidermal tissue, subsequently the vascular cylin- 

 der sends off a branch toward the infected region and the 

 young tubercle becomes spherical, then ovoid and finally 

 elongates into a cylinder as the vascular tissue increases in 

 growth. The average diameter of an ordinary tubercle is 

 from 1 to 1.5 mm. and in one year's growth it may attain 

 from 3-6 mm. in length. The growth in length continues 

 from year to year so that some may become 11-14 mm. long, 

 but they still retain their original diameter. Through di- and 

 tri-chotomous branching the loose cluster is formed on a 

 single rootlet, and one of these may ultimately result in a 

 mass 4-6 cm. in diameter, all of which originated from a sin- 

 gle tubercle (pi. 6) . There is some variation in the color of 

 the tubercle, due to the nature of the soil in which these 

 plants grow. As a rule the youngest tubercles are light gray, 

 some are almost perfectly white, becoming pinkish as they 

 grow older, while the older ones are flesh colored, becoming 

 darker with age. Atkinson (1) describes the very youngest 

 as having a flesh color, probably due to his not having found 

 the very earliest stages of the tubercles. There is no pigment 

 in the outer layer of cells, such as is abundantly present in 

 the alder tubercles. 



