Monotropaceae with Reference to Ericaceae 61 



but this is rare. The walls of the outer cells of the cortex also 

 become thickened and these also fall off. Meantime the 4-5 

 arch bundle system has undergone secondary thickening. New 

 xylem cells have developed so that there is formed a solid central 

 cylinder of wood enclosed by a few layers of thin-walled phloem 

 cells. These roots are perennial and annual rings of wood 

 are laid down. The oldest one examined was found to have 

 four years of growth. The largest part of the wood seems 

 to be composed of pitted vessels with a few spiral ones. 



C. maculata, P. rotundifolia, and P. elliptica showed prac- 

 tically the same appearance except that none of the mater- 

 ial examined was old enough to have formed annual rings 

 or to have had the epidermis entirely dead and sloughed off. 



Fig. 3. Transverse section root of Moneses uniflora showing central xylem 

 tissue X 300. 



In Moneses uniflora the epidermal cells become filled with 

 hyphae and fall off. Hyphae may penetrate into the outer 

 layers of the cortex. The wood is at first diarch — a few remain- 

 ing cells seem to indicate that this is a reduction from a tetrarch 

 condition (Fig. 3). Secondary wood is formed. 



MacDougal and Lloyd (50) have described the structure of 

 the old root of Sar codes and Pterospora. The epidermis becomes 

 completely filled by the fungus and falls off. After this the 



