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P, pith; PB, primary bast; SB, secondary bast; C, cambium; PR, pith ray; PW, primary wood; SW, secondary wood; PS, procambium strands.

Isolated Fibers and Cells. a, four cells of wood parenchyma; b, two cells from a pith ray; c, a single cell or joint of a vessel, the openings, x, x, leading into its upper and lower neighbors; d, tracheid; e, wood fiber proper.

Microscopically the white pine can be distinguished by having usually only one large pit, while spruce shows three to five very small pits in the parenchyma cells of the pith ray communicating with the tracheid.

The following convenient and useful classification of pines into four groups, proposed by Dr. H. Mayr, is based on the appearance of the pith ray as seen in a radial section of the spring wood of any ring: Section I. Walls of the tracheids of the pith ray with dentate projections. a.

Walls of tracheids of pith ray smooth, without dentate projections. a.