78 Henderson — Comparative Study of Pyrolaceae and 



Pyrola the leaves become less leathery until in P. (Moneses) uni- 

 flora the leaf has a characteristic deciduous structure but is still 

 evergreen. In P. aphylla, the foliage leaves are deciduous, and 

 the ascending axis, which bears the flower, produces only scale 

 leaves along its course. This species is a connecting link be- 

 tween the evergreen type and that of all the members of the 

 Monotropaceae, where no foliage leaves are produced, their 

 place being taken by expanded fleshy scale leaves. 



The question arises as to whether the scale leaves in the 

 Monotropaceae are the homologues of the scale leaves, or of the 

 foliage leaves in Ericaceae and Pyrolaceae. In the writer's 

 opinion, they represent both scale leaves and foliage leaves. 

 The deciduous scale leaves of the Ericaceae have gradually 

 become green and persistent, then expanded, enlarged and much 

 more numerous. The green leaves have become smaller and 

 all have changed in color from green in the Ericaceae and Pyrol- 

 aceae to brownish in Pterospora, crimson in Sarcodes, pinkish 

 yellow to yellow in M. hypopitys, to pinkish white or white in 

 Monotropa. 



The Leaf — Microscopic Structure 



The microscopic structure of the leaf of C. umbellata has been 

 described and figured by H. E. Petersen (60). His material 

 was collected in Denmark, but corresponds according to his 

 description and figures almost exactly with the material exam- 

 ined by the writer. The epidermis, both upper and lower, is 

 heavily cuticularized, and, on the surface of this, a layer of wax 

 is present. (The latter is not mentioned by Petersen.) The 

 epidermal cells are rectangular on transverse section, wavy 

 walled on surface view. There is a three-layered palisade tissue 

 beneath this, and a spongy mesophyll composed of thin-walled, 

 branched cells with large intercellular spaces. Stomata are 

 present on the lower epidermis on the same level as the other 

 epidermal cells. Petersen reports the presence of starch in 

 both the upper and lower epidermis, and the absence of hairs 

 and hydathodes. Starch is very plentiful in both palisade and 

 spongy mesophyll, though he does not make note of this. He 

 also does not mention the presence of conglomerate crystals of 

 calcium oxalate, which the writer finds occur in large numbers 

 throughout the spongy mesophyll. These occur as small con- 



