OF THE EASTERN UNITED STATES 373 



these cells become suberized very early. Underneath the endodermis 

 is found the vascular cyUnder, which is quite reduced in size as compared 

 with that of the normal root. In the young tubercle it is constituted of a 

 a radial tetrarch fibrovascular bundle which surrounds a small pith. The 

 phloem elements of the bundle become inactive very early. The xylem 

 is composed mostly of wood fibres intermingled with a few tracheae. 

 Secondary development is of very short duration. 



In the younger tubercles the vascular cylinder extends only part way 

 into the apex, while in older ones the cylinder, with some cortical paren- 

 chyma cells surrounding it, grows out into a slender thread from which 

 lateral branches are then sent off. 



Actinomyces living in the tubercles is best observed in its various 

 relations, in a radial longitudinal section. There the youngest stages 

 may be observed in the meristematic region of the apex, while the older 

 stages may be traced back toward the base of the tubercle. As ob- 

 served by Shibata in M. rubra tubercles, the writer has likewise noted in 

 the case of the tubercles of M. cerifera, M. Carolinensis, M. Macfarlanei, 

 M. Gale and Comptonia asplenifoUa that the differentiation of the ring 

 of infested cells starts in the meristem near the growing apex, thus indi- 

 cating that infection takes place acropetally. Since tubercles are found 

 on the seedling roots of 5-6 months' growth, it would indicate that 

 infection takes place very early in the life of the young seedUngs. 



These infested meristematic cells become radially elongated and are 

 found to contain several extremely fine non septate and branched thread- 

 Uke structures. The Actinomyces (Streptothrix) threads extend through 

 the transverse and longitudinal walls of these cells aided evidently by 

 the secretion of a ferment which dissolves the cell wall in the line of the 

 organism's progress. They then invade neighboring cells where they 

 run between the starch grains toward the nucleus around which the 

 organism seems to derive its greatest benefit. Shortly after the appear- 

 ance of parasite threads within the invaded cells, the starch grains become 

 dissolved and in this form are appropriated as food by the organism. 

 The nucleus becomes hypertrophied and finally perishes. The endophyte 

 by this time has grown very rapidly into a dense thready reticulum. 

 Gummy lignin appears at first of a clear yellow color but later becoming 

 yellowish brown. The dense thready web of the organism sends out 

 clusters of radial thread branches forming an Actinomyces rosette, which 

 in many cells completely fills up the whole cell lumen. These threads 

 frequently become club shaped at their extremities. Some of the 

 threads after piercing through the wall of a cell develop clavate ends. 



