540 IOWA ACADEMY OF SCIENCE 



KEY TO TSUGA. 

 1. Leaves flat. 



2. Five to six rows of stomata on each side of midrib 



T. canadensis 



2. Seven to eight rows of stomata on each side of midrib. 

 Cells of upper palisade not deep. Cells of parenchyma 

 along lower surface irregular, or in shallow palisade 

 arrangement. Secretory cells of resin duct not con- 

 spicuously one-layered T. caroliniana 



2. Seven to nine rows of stomata on each side of midrib. 

 Deep and narrow palisade on upper surface. Cells of 

 parenchyma on lower surface having greatest length 

 laterally. Secretory cells of resin duct conspicuously 



one-layered T. heterophylia 



1. Leaves concave above. 



Stomata on both surfaces. Eight rows on each side of 

 midrib on ventral surface T. mertensiana 



TSUGA CANADENSIS Carr. Hemlock. 



Leaves flat, grooved on top, 1/3 to 2/3 inch long, edges tend to 

 turn up. 



Resin duct 1, large, central, on ventral side touching epidermis, 

 lined with 1 layer of thin flat lignified cells. 



Vascular bundles enclosed in an inconspicuous sheath of large, 

 round, loosely joined cells. Xylem bundles very small, 

 scarcely separated. Xo other cells except those of xylem lig- 

 nified. 



Hypodermal cells sometimes present along dorsal groove. Epi- 

 dermis not lignified, guard cells and those cells adjacent lig- 

 nified. 



Stomata on lower surface, 5 to 6 rows each side of midrib. 



Material from Campus of Iowa State College. 



Plate XL VIII. 



Figure 1. — Section of leaf of Tsuga canadensis. 



Figure 2. — Section through epidermis and stomata of leaf of 

 T. canadensis. 



Figure 4. — Section through vascular cylinder and resin duct 

 of leaf of T. canadensis. 



