74 Henderson — Comparative Study of Pyrolaceae and 



In comparing Monotropa, Pterospora, Pleuricospora, and Sar- 

 codes, these have been considered in order according to their 

 relative fleshiness and size. The series, given in order of their 

 relative structure from the least simplified to the most simpli- 

 fied, taking as a basis of comparison the relative amount of 

 phloem and wood produced, would be Pleuricospora, with a 

 continuous ring of well developed wood equal in width to the 

 phloem, Sarcodes and Pterospora, with the wood not continuous 

 and not so strongly developed but equal in width to the phloem, 

 then Monotropa, with the wood only present in small patches 

 of about five to eight cells in a group and the phloem very well 

 developed. 



Sections of the axes of the youngest shoots of some typical 

 ericaceous plants were examined. These resemble closely 

 those of Chimaphila and Pyrola. In Ledum groenlandicum, 

 Gaultheria procumbens, Rhododendron lapponicum, the epidermis 

 and one to two outer layers of the cortex are thickened just as 

 in Chimaphila and Pyrola. The inner cortex is less regular, 

 there being wider and more irregular intercellular spaces than 

 in Chimaphila and Pyrola. There is a ring of hard bast present 

 as in nearly all the Pyrolaceae and Monotropaceae. The wood 

 is generally wider than in Chimaphila. The pith is slightly 

 different, there being present trabeculae of thicker walled cells. 

 No comparison can be made between Chimaphila and Pyrola 

 on the one hand, and older sections of ericaceous plants, as 

 none of the former lives over four years and many of the latter 

 live for many years becoming very woody. 



As regards the structure of the epidermis, cortex and the 

 development of cork tissue there is a complete series from 

 shrubby Ericaceae to the most simplified soft fleshy types of 

 Monotropaceae. In Rhododendron lapponicum no cork cam- 

 bium is developed until the third or fourth year when one de- 

 velops out of phloem tissue. This origin of the cork cambium 

 has been reported by Vesque (82). Layers of cork are laid 

 down and all of the external cortex and epidermis fall off. In 

 the more woody Pyrolaceae, Chimaphila and Pyrola, there is a 

 tendency for the outer thicker-walled layers of the cortex to 

 separate from the rest during the second and following years. 

 As the plant only lives for 3-4 years no opportunity for the 

 formation of a cork cambium takes place. In the saprophytes 



