Monotropaceae with Reference to Ericaceae 81 



lower mesophyll. There is, however, the distinct transparent 

 median non-tannin-containing layer of the mesophyll described 

 by Petersen for his Danish specimens of P. rotundifolia. The 

 mesophyll cells, beside containing starch and tannin, contain 

 crystals similar to those of Chimaphila. At the midrib, the 

 leaf becomes prominently ridged on both surfaces. The upper 

 epidermal cells become enlarged and have a much thicker layer 

 of cuticle and wax than over the rest of the lamina. The first 

 layer of mesophyll cells become packed closely together and 

 against the upper epidermis. There are four to five layers of 

 rounded thin-walled cells, above and below the midrib bundle. 

 The latter closely resembles that of Chimaphila. The lower- 

 most layer of mesophyll also consists of rounded, thick- walled, 

 closely packed cells, tightly pressed against the lower epidermis. 

 The latter has its cells enlarged and has a thicker cuticle than 

 on the laminar region. 



In P. elliptica, the structure of the lamina is exactly similar 

 to that of P. rotundifolia, except that the cuticle with its cover- 

 ing of wax is much thinner and that on the upper epidermis 

 is slightly thicker than that on the lower epidermis. At the 

 midrib, the structure is again like that of P. rotundifolia, except 

 that the cuticle is thinner. 



The structure of the leaf of P. secunda has been described 

 by Petersen and Rommel and figured by the former, p. 85. 

 The type of leaf is very similar to that of P. rotundifolia — 

 there is no differentiation into a palisade and a spongy meso- 

 phyll. The cuticle, according to Petersen, is thicker than in 

 P. rotundifolia (this is not true of the writer's material) and 

 stomata are present also on the upper epidermis. Rommel 

 fails to note the latter, and also states that no crystals are pres- 

 ent. At the midrib, the leaf is ridged above, but not as much 

 as in P. rotundifolia. 



The structure of the leaves of P. media and P. picta has been 

 described by Rommel. These are similar to P. rotundifolia 

 for there is no palisade mesophyll. He states that crystals 

 are absent in these. 



The leaf of P. minor has been described and figured (p. 83) 

 by Petersen as having an undifferentiated mesophyll like P. 

 rotundifolia, but Rommel states that there is one layer of palisade 

 tissue. The writer finds the structure of the leaf to be similar 



