58 KANSAS UNIVERSITY SCIENCE BULLETIN. 



and it was from these plants, in the early part of September, 

 1913, that the material for this study was secured. 



LEAF. 



There are very distinct external differences between the leaf 

 of A", globosum and of X. pennsylvanicum. . 



The leaves of X. globosum are dark green and have a much 

 crinkled surface, while those of X. pennsylvanicum are lighter 

 in color and have a comparatively even surface. The leaf of 

 A", americanum is in its superficial aspect very much like that 

 of X. globosum. There are no very striking differences in leaf 

 structure shown in either cross or tangential sections (figs. 



I, 2, 3, 4, 9), but some minor differences occur which are 

 constant. 



The leaf of each species is petiolate, with about ten to four- 

 teen vascular bundles entering from the stem. The leaf is 

 entered by three large veins, each containing about the same 

 number of vascular bundles (fig. 12). In the apex of the leaf, 

 and in the apex of each deeply cut tooth on the margin, the 

 veins end in clusters of tracheids (fig. 10, W). The midrib 

 and other large veins of the leaf contain a very large amount 

 of thin-walled mesophyll cells, scanty in cell contents (figs. 



II, B; and 12, J). The vascular bundles contain a relatively 

 large amount of phlem (fig. 21, X) . The epidermis of the 

 midrib is uniform in shape and size (fig. 15, N) , with a heavier 

 cuticle layer than on other parts of the leaf (fig. 11) . Beneath 

 the epidermis of the midrib there is a layer of collenchyma 

 cells from three to six cells in thickness (fig. 11, C) . 



Rather large border parenchyma cells, which are almost 

 empty of cell contents, surround the vascular bundles of the 

 veins (figs. 1, A; 2, D; 3, G), and extend in plates to the 

 margin above and below (fig. 4, L). Resin ducts, in cross 

 section, surrounded by, on the average, six secreting cells, 

 occur in the parenchyma in the midrib and surrounding the 

 larger veins (fig. 12, G) . 



The leaf of each species has the dorsi-ventral differentiation 

 shown in figures 4 and 9. In X. pennsylvanicum the palisade 

 tissue has a length in cross section of 3 cells, on the average 

 (fig. 9), while the average in X. globosum is 2 cells, and in 

 X. americanum 2.5 cells (fig. 4). 



The ratio of the area, in cross section, of the palisade tissue 

 to the area of spongy parenchyma is in X. pennsylvanicum 



