PLANT ORGANS AND ORGANISMS 
95 
The writer has found tubercles on Myrica cerifera, Myrica Car- 
oliniensis and Myrica Macfarlanei seedling primary roots of 5 to 6 
months’ growth, and from thence onward on the secondary roots 
inserted on the hypocotyl axis, on nearly all the adventitious roots of 
subterranean branches and on the subterranean branches of Myrica 
cerifera, M. Caroliniensis, M. Gale, M. Macfarlanei, and Comptonia 
asplenifolia. The inciting organism has been isolated by him in 
pure culture according to Koch’s postulates and named Actinomyces 
Myricarum Youngken. 
Fig. 39. — Ps. radicicola. i, From Melilotus alba; 2 and 3, from Medicago saliva ; 
4, from Vicia villosa. ( Marshall , after Harrison and Barlow from Lipman.) 
The tubercles occur either singly, as is frequently the case on 
subterranean branches, in small groups the size of a pea, or in larger 
coralloid loose or compact clusters which frequently attain the size 
of a black walnut. Each tubercle is a short cylindrical blunt-ended 
root-like structure which branches di- or trichotomously after attain- 
ing a certain length. The branches frequently rebranch at their tips 
which grow out into long thread-like structures from 1-3 cm. in 
length that may also branch and become entwined about the roots of 
other plants. The color of the youngest tubercles is a pinkish-gray 
brown. As the tubercles become older their color changes to brown, 
dark-brown and even black. (For a detailed description of the 
Myrica and Comptonia tubercles and their inciting organism, con- 
