The upright ray cells are ray parenchyma cells turned on end so that their long axis is vertical (Figure 2.17D).Upright ray cells are generally shorter and sometimes nearly square. Rays that have only one type of ray cell, typically only procumbent cells, are called homocellular rays. Those that have procumbent and upright cells are called heterocellular rays. The number of rows of upright ray cells
varies from one to more than five.The great diversity of hardwood anatomy is treated in many sources throughout the literature (Metcalfe and Chalk 1950, Metcalfe and Chalk 1979, Panshin and deZeeuw 1980, Metcalfe and Chalk 1987, IAWA Committee 1989, Gregory 1994, Cutler and Gregory 1998, Dickison 2000,Carlquist 2001).