80 Henderson — Comparative Study of Pyrolaceae and 



filled with starch grains; small and large conglomerate crystals 

 of calcium oxalate, exactly similar to those of C. umbellata, 

 occur in the spongy mesophyll. 



At the midrib the laminar halves form an angle of 135 - 

 180 , the leaves being almost entirely flat, but with a ridge 

 on the lower epidermis, similar to that of C. umbellata. The 

 upper epidermis is of the same appearance as that over the 

 lamina. The palisade still consists of two layers, but the cells 

 are shorter and wider. Beneath this are one to two layers of 

 spongy mesophyll. The midrib bundle is very similar in ap- 

 pearance to that of C. umbellata. Under the bundle are three 

 layers of spongy mesophyll cells, closely packed together and 

 with slightly thickened walls similar to those of C. umbellata, 

 as is also the layer just next to the lower epidermis, with its 

 rounded thick-walled cells, closely packed together and pressed 

 against the lower epidermis. The cells of the latter have be- 

 come shorter, more isodiametric, and have their outer walls 

 curved outward slightly. 



The microscopic structure of the leaf of P. rotundifolia has 

 been described by Rommel (65), and of P. rotundifolia var. 

 grandiflora by Petersen (60). The material examined corre- 

 sponds fairly well with both of these descriptions except that 

 the writer finds stomata, not hydathodes, on the upper epi- 

 dermis; chlorophyll grains in the upper epidermal cells; crystals, 

 both small and large, in the mesophyll (Petersen does not 

 mention them, and Rommel states that they are absent); and 

 no differentiation into palisade and spongy mesophyll. The 

 epidermis is covered by a thin cuticle (much less than in Chima- 

 phila) and a layer of wax. Both upper and lower epidermis 

 consist of wavy-walled cells on surface section, rectangular in 

 transverse section, and contain chloroplasts. Numerous sto- 

 mata are present on the lower epidermis, projecting slightly 

 from the level of the other epidermal cells. They are fairly 

 frequent on the upper epidermis, especially toward the margin 

 of the leaf. The mesophyll consists of five to six layers of 

 closely packed thin-walled cells. Rommel describes the meso- 

 phyll as having smaller intercellular spaces near the upper 

 epidermis, and larger intercellular spaces near the lower epi- 

 dermis. Petersen's figure (p. 81) also illustrates this, but the 

 writer's material shows no difference between the upper and 



