368 YOUNGKEN— ON THE MYRICACEAE 



6. A cambium forming a very wavy zone of tangentially dividing 

 meristematic cells. 



7. A xylem which is quite porous and composed of numerous wood 

 wedges separated by medullary-rays, for the most part 1-2 rows of cells 

 wide. The tracheae of the metaxylem show broad transverse pits in 

 their walls as well as barred septa and Actinomyces forms. The woody 

 fibres are extremely lignified especially in the autumn wood. The 

 medullary-ray cells show many prominent protoplasmic connections. 



8. The pith, like that of M. Gale, is irregularly lobed. It is composed 

 of parenchyma cells which become lignified very early. These cells 

 are of variable size and appear mostly rounded in transverse section. 

 Many of them contain gummy lignin, while some possess rosette crystals 

 of calcium oxalate. 



Subterranean Branch of Comptonia Asplenifolia, (L.) Alton 

 The microscopical characteristics of an underground branch of this 

 plant are for the most part similar to those of an aerial stem. The 

 important differences are a diminution in the number of cork layers, 

 the tendency of the medullary ray cells to become broader, the presence 

 of more cells containing gummy lignin and a slight increase in the diame- 

 ter of the tracheae. 



Comparative Morphology of the Root Tubercles 



Within the past thirty years various investigations have been carried 

 on by Brunchorst, Moller, Shibata, Chevalier, Harshberger and Arzber- 

 ger in respect to an endophytic organism living in the tissues of the 

 Myricas and forming tubercles. 



Brunchorst" was the first to mention the tubercles on Myrica Gale 

 and named the fungus producing them — Frankia Subtilis — because 

 he considered this organism similar to that in the tubercles of Alnus. 



Moller" later found the organism to differ considerably from that 

 infesting Alnus and named it — Frankia Brunchorstii. 



Shibata^^ investigated the tubercles found on Myrica rubra, his ob- 

 servations being on both fresh and preserved material. He described 

 the morphology of the tubercle, showing that the fungus confines itself 

 to a ring of from 1-3 layers of cells beneath the cork thus differing from 

 the condition found in Alnus. He also pointed out that infection takes 

 place acropetally by means of fungal threads. He traced these threads 

 into the already differentiated meristematic cells where they grew rapidly 



