LEAVES 267 



substitute for insect flowers. These leaves, when powdered, 

 show the following structure (Plate 112): 



Both the upper and lower epidermis have stomata (Figs, 

 i and 2), but they differ in that the surrounding cells of the 

 upper epidermis are wavy, while the corresponding cells of the 

 under epidermis are similar, though the under epidermis has 

 many attached hairs (Figs. 3 and 4). The T-shaped hairs form 

 the most abundant element of the powder. They are similar 

 in structure to those found on the scales and stem. Fragments 

 of the mesophyll have round cells and contain chlorophyll 

 (Fig. 6). The conducting cells are spiral or reticulate. 



The different cells of the leaf differ greatly in structure, in 

 amount, and in arrangement. In uva-ursi, boldus, pilocarpus, 

 eucalyptus, and chimaphila leaves the outer walls of the epidermal 

 cell is very thick. In uva-ursi leaves this thick wall appears 

 bluish green when viewed under low power of the microscope. 



In belladonna, stramonium, henbane, peppermint, spear- 

 mint, digitalis, and horehound, the outer wall of the epidermal 

 cells is thin. 



In witch-hazel, stramonium, coca, phytolacca, and pepper- 

 mint there is a single layer of palisade parenchyma on the 

 upper surface only of the leaf. 



In senna there is one layer of palisade parenchyma on 

 the upper and one layer on the under side of the leaf. In 

 matico and tea leaves there are two layers of spongy parenchyma 

 on the upper side of the leaf. 



In chestnut leaves there are three layers of palisade paren- 

 chyma on the upper side of the leaf. 



In eucalyptus leaves the entire central part of the leaf, 

 with the exception of the secretion cells and nbro-vascular 

 bundle, is made up of the palisade parenchyma. 



In some leaves no palisade parenchyma occurs. Trailing 

 arbutus (Plate in) is an example of such a leaf. 



In stramonium leaves the spongy parenchyma is strongly 

 branched; in mountain laurel the spongy parenchyma is 

 mostly non-branched and circular in form, as in trailing arbutus 

 (Plate in, Fig. 3), and as occurs in the midrib portion of most 

 leaves. 



In stramonium and chestnut are found rosette crys- 



