372 YOUNGKEN— ON THE MYRICACEAE 



black walnut. Each tubercle is a short cyhndrical blunt-ended root 

 hke structure which branches di- or trichotomously after attaining 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 maximum length of a tubercle is 5 mm. The average length of the 

 branches being from 2-3 mm. 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. 



Histology 



The tubercles when studied microscopically exhibit the following 

 structural detail: 



A cork, constituted of from 2 to 4 layers of suberous cells, whose outer 

 ones are dead, filled with gummy lignin, and in the process of exfoliation, 

 forms the external bounding layer. The cork tissue is derived from the 

 outer layer of pericambium of the host root which functions as a phellogen 

 during the development of the tubercle. Beneath the cork lies a very 

 broad cortex, which, instead of being formed as in normal roots of 5-12 

 layers of cells separated by large intercellular air spaces, is constituted 

 of 15-24 layers of very closely united parenchyma cells. The outer 3-5 

 layers of this region are composed of rounded to tangentially elongated 

 cells, some of which contain starch grains, others tannin, a few gummy 

 lignin. Underneath this lies a zone usually 2 to 3 cells broad of radially 

 elongated cells and a few smaller rounded cells which are separated by 

 small air-spaces. The radially elongated cells are hypertrophied and 

 contain the Actinomyces parasite (Plate 88, Fig. 24), which may or may 

 not be enveloped by gummy lignin. Many of the abutting smaller 

 cells are rich in tannin and show no evidence of the parasite. Beneath 

 this zone of infested cells is found a usually broader zone of smaller 

 isodiametric cells intermingled with a few oblong cells. In M. Carolinen- 

 sis as noted by Chevalier, in M. Macfarlanei, M. Gale and Comptonia 

 asplenifolia as noted by the writer, it frequently happens that other 

 cells scattered without order throughout the cortex are also infested by 

 Actinomyces. These hke those of the infested radially elongated zone 

 are also hypertrophied. All of the infested cells are united by means of 

 Actinomyces threads which run through the cell walls from cell to cell 

 as well as the intercellular air-spaces. The endodermis or innermost 

 layer of the cortex is composed of small oval thick-walled cells which 

 contain a yellowish-brown substance (gummy lignin). The walls of 



