62 Henderson — Comparative Study of Pyrolaceae and 



subepidermal cells divide radially to form a wider cortex. ' The 

 central cylinder of Sarcodes and Pterospora is least reduced 

 and its development may be traced in them with some certainty. 

 Here the 5-6 xylem bundles alternate with the simple phloem, 

 enclosing a well marked medulla. The first step in the sec- 

 ondary growth is the sclerotization of the medulla, and is fol- 

 lowed by the lignification of this tissue in Sarcodes. Next the 

 phloem gives rise to a cambium which develops wood inter- 

 nally, and bast on the outside. The latter consists for the 

 greater part of elongated elements of narrow lumen which do 

 not undergo any marked thickening of the walls. The wood 

 formed by the cambium joins directly on to the lignified medulla. 

 The inner ends of the primary medullary rays also undergo 

 sclerotization to some extent, but the outer portions show as 

 broad bands one or two layers in thickness with the character- 

 istic appearance of being compressed tangentially. The advance 

 of the cambium is at first fairly regular, as the cambium zone 

 moves outwardly beyond the first ring formed, the transforma- 

 tion into vessels is accomplished with such disturbance or vari- 

 ance from the customary manner that it is not possible to draw 

 a line separating the two regions. Furthermore, some of the 

 cambium cells of great size remain as great thin-walled elements 

 in the wood, or these may be arranged in radial lines simulating 

 tertiary rays. A region of cambiform elements, four to six 

 layers in thickness, may be seen entirely surrounding the xylem. 

 The structure formed by this behavior of the cambium resembles 

 that of a stem, and indicates that the roots of the two genera 

 in question may attain an age of two years or more." 



In Monotropa hypopitys, the oldest part of the root is the 

 horizontal portion that gives off secondary roots and buds, 

 which lengthen to form flowering shoots. The structure of 

 this root has been described by Drude (11), Kamienski (39), 

 and Queva (62). The outer epidermal cells become completely 

 filled with hyphae and fall off. The cortex consists of 8-10 

 layers of cells that are much larger than the epidermal ones. 

 The endodermis is composed of small cells with thickenings 

 on the radial walls in the younger roots, but is indistinguishable 

 in the older ones. The fibrovascular cylinder is composed 

 mainly of thin-walled cells, there being only 3-4 patches of 

 xylem (Kamienski). Material that the writer examined showed 



