82 Henderson — Comparative Study of Pyrolaceae and 



to that of P. rotundifolia — no palisade tissue being present. 

 In some places, however, a few of the cells of the layer next 

 to the upper epidermis become slightly elongated, as if a palisade 

 was beginning to form, or rather as if these were the few remain- 

 ing cells of what was once a palisade layer. Rommel notes the 

 presence of crystals. Petersen mentions the presence of stomata 

 on the upper epidermis, a fact not stated by Rommel. The 

 writer finds stomata on both upper and lower epidermis, pro- 

 truding slightly from the level of the epidermis. At the midrib 

 the leaf is ridged above and below, but even less than in P. 

 secunda. The bundle is not as strongly developed as in P. 

 rotundifolia. 



In P. chlorantha, Rommel states that there is one layer of 

 palisade mesophyll, and that crystals are present. The writer's 

 material agrees with this. The epidermal cells are wavy-walled 

 on surface view, rectangular in transverse section. Stomata 

 are present on the lower surface only. There are one layer of 

 palisade and four layers of spongy mesophyll with larger inter- 

 cellular spaces than in P. rotundifolia. At the midrib there is 

 only a slight curving upward and a small downward ridge. 

 The bundle is similar to that of P. minor. 



In P. aphylla, Holm (30) describes the leaf as having a normal 

 epidermis, covered by a thick and wrinkled cuticle, stomata are 

 present on both upper and lower epidermis, but more numerous 

 on the lower epidermis. He reports the presence of two layers 

 of palisade tissue and a spongy mesophyll of loosely connected 

 cells with large intercellular spaces. 



In the genus Pyrola, most of the species examined, i.e., P. 

 elliptica, P. secunda, P. media, P. picla, conform to the type 

 of P. rotundifolia with non-differentiated mesophyll. P. minor 

 seems to show a transition toward the formation of a palisade. 

 P. chlorantha shows one layer of palisade tissue, and P. aphylla 

 shows two layers of palisade and a typical spongy mesophyll. 

 The leaves of Moneses uniflora have also been described 

 microscopically by Petersen and Rommel and figured by the 

 former (p. 86). The epidermal cells — wavy -walled in surface 

 section, rectangular in transverse section — have only slightly 

 cuticularized outer walls and contain chlorophyll. Stomata 

 are numerous on the lower epidermis, and are slightly project- 

 ing. The upper layer of the mesophyll consists of slightly 



